//------------------------------------------------------------------------------
// <copyright file="XmlSchemaSet.cs" company="Microsoft">
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// </copyright>
// <owner current="true" primary="true">Microsoft</owner>                                                                 
//------------------------------------------------------------------------------
using System.Diagnostics;
using System.Collections;
using System.Threading;
using System.Collections.Generic;
using System.Runtime.Versioning;

namespace System.Xml.Schema {
#if SILVERLIGHT
    public class XmlSchemaSet
    {
        //Empty XmlSchemaSet class to enable backward compatibility of XmlSchemaProvideAttribute        
        //Add private ctor to prevent constructing of this class
        XmlSchemaSet() { }
    }
#else
    /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet"]/*' />
    /// <devdoc>
    ///    <para>The XmlSchemaSet contains a set of namespace URI's.
    ///       Each namespace also have an associated private data cache
    ///       corresponding to the XML-Data Schema or W3C XML Schema.
    ///       The XmlSchemaSet will able to load only XSD schemas,
    ///       and compile them into an internal "cooked schema representation".
    ///       The Validate method then uses this internal representation for
    ///       efficient runtime validation of any given subtree.</para>
    /// </devdoc>
    public class XmlSchemaSet {
        XmlNameTable nameTable;         
        SchemaNames schemaNames;
        SortedList schemas;              // List of source schemas 
        
        //Event handling
        ValidationEventHandler internalEventHandler;
        ValidationEventHandler eventHandler;

        bool isCompiled = false;

        //Dictionary<Uri, XmlSchema> schemaLocations;
        //Dictionary<ChameleonKey, XmlSchema> chameleonSchemas;
        Hashtable schemaLocations;
        Hashtable chameleonSchemas;

        Hashtable targetNamespaces;
        bool compileAll;
        
        //Cached Compiled Info
        SchemaInfo cachedCompiledInfo;
        
        //Reader settings to parse schema
        XmlReaderSettings readerSettings;
        XmlSchema schemaForSchema;  //Only one schema for schema per set
     
        //Schema compilation settings
        XmlSchemaCompilationSettings compilationSettings;

        internal XmlSchemaObjectTable elements;
        internal XmlSchemaObjectTable attributes;
        internal XmlSchemaObjectTable schemaTypes;
        internal XmlSchemaObjectTable substitutionGroups;
        private XmlSchemaObjectTable typeExtensions;

        //Thread safety
        private Object internalSyncObject;
        internal Object InternalSyncObject {
            get {
                if (internalSyncObject == null) {
                    Object o = new Object();
                    Interlocked.CompareExchange<Object>(ref internalSyncObject, o, null);
                }
                return internalSyncObject;
            }
        }

//Constructors

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.XmlSchemaSet"]/*' />
        /// <devdoc>
        ///    <para>Construct a new empty schema schemas.</para>
        /// </devdoc>
        public XmlSchemaSet() : this(new NameTable()) {
        }
 
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.XmlSchemaSet1"]/*' />
        /// <devdoc>
        ///    <para>Construct a new empty schema schemas with associated XmlNameTable.
        ///       The XmlNameTable is used when loading schemas</para>
        /// </devdoc>
        public XmlSchemaSet(XmlNameTable nameTable) {
            if (nameTable == null) {
                throw new ArgumentNullException("nameTable");
            }
            this.nameTable = nameTable;
            schemas = new SortedList();

            /*schemaLocations = new Dictionary<Uri, XmlSchema>();
            chameleonSchemas = new Dictionary<ChameleonKey, XmlSchema>();*/
            schemaLocations = new Hashtable();
            chameleonSchemas = new Hashtable();
            targetNamespaces = new Hashtable();
            internalEventHandler = new ValidationEventHandler(InternalValidationCallback);
            eventHandler = internalEventHandler;
            
            readerSettings = new XmlReaderSettings();

            // we don't have to check XmlReaderSettings.EnableLegacyXmlSettings() here because the following
            // code will return same result either we are running on v4.5 or later
            if (readerSettings.GetXmlResolver() == null)
            {
                // The created resolver will be used in the schema validation only
                readerSettings.XmlResolver = new XmlUrlResolver();
                readerSettings.IsXmlResolverSet = false;
            }
            
            readerSettings.NameTable = nameTable;
            readerSettings.DtdProcessing = DtdProcessing.Prohibit;
    
            compilationSettings = new XmlSchemaCompilationSettings();
            cachedCompiledInfo = new SchemaInfo();
            compileAll = true;
        }


//Public Properties       
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.NameTable"]/*' />
        /// <devdoc>
        ///    <para>The default XmlNameTable used by the XmlSchemaSet when loading new schemas.</para>
        /// </devdoc>
        public XmlNameTable NameTable {
            get { return nameTable;}
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.ValidationEventHandler"]/*' />
        public  event ValidationEventHandler ValidationEventHandler {
            add {
                eventHandler -= internalEventHandler;
                eventHandler += value;
                if (eventHandler == null) {
                    eventHandler = internalEventHandler;
                }
            }
            remove {
                eventHandler -= value;
                if (eventHandler == null) {
                    eventHandler = internalEventHandler;
                }
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.IsCompiled"]/*' />
        /// <devdoc>
        ///    <para>IsCompiled is true when the schema set is in compiled state</para>
        /// </devdoc>
        public bool IsCompiled {
            get { 
                return isCompiled; 
            }
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.XmlResolver"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        public XmlResolver XmlResolver {
            set {
                readerSettings.XmlResolver = value;
            }
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.CompilationSettings"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        public XmlSchemaCompilationSettings CompilationSettings {
            get {
                return compilationSettings;
            }
            set {
               compilationSettings = value;
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Count"]/*' />
        /// <devdoc>
        ///    <para>Returns the count of schemas in the set</para>
        /// </devdoc>
        public int Count {
            get {
                return schemas.Count;
            }
        }
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.GlobalElements"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        public XmlSchemaObjectTable GlobalElements {
            get {
                if (elements == null) {
                    elements = new XmlSchemaObjectTable();
                }
                return elements;
            }
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.GlobalAttributes"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        public XmlSchemaObjectTable GlobalAttributes {
            get {
                if (attributes == null) {
                    attributes = new XmlSchemaObjectTable();
                }
                return attributes;
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.GlobalTypes"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        public XmlSchemaObjectTable GlobalTypes {
            get {
                if (schemaTypes == null) {
                    schemaTypes = new XmlSchemaObjectTable();
                }
                return schemaTypes;
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.SubstitutionGroups"]/*' />
        /// <devdoc>
        ///    <para></para>
        /// </devdoc>
        /// 
        internal XmlSchemaObjectTable SubstitutionGroups {
            get {
                if (substitutionGroups == null) {
                    substitutionGroups = new XmlSchemaObjectTable();
                }
                return substitutionGroups;
            }
        }

        /// <summary>
        /// Table of all types extensions
        /// </summary>
        internal Hashtable SchemaLocations {
            get {
                return schemaLocations;
            }
        }

        /// <summary>
        /// Table of all types extensions
        /// </summary>
        internal XmlSchemaObjectTable TypeExtensions {
            get {
                if (typeExtensions == null) {
                    typeExtensions = new XmlSchemaObjectTable();
                }
                return typeExtensions;
            }
        }
//Public Methods

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Add1"]/*' />
        /// <devdoc>
        ///    <para>Add the schema located by the given URL into the schema schemas.
        ///       If the given schema references other namespaces, the schemas for those other
        ///       namespaces are NOT automatically loaded.</para>
        /// </devdoc>
        [ResourceConsumption(ResourceScope.Machine)]
        [ResourceExposure(ResourceScope.Machine)]
        public XmlSchema Add(String targetNamespace, String schemaUri) {
            if (schemaUri == null || schemaUri.Length == 0) {
                throw new ArgumentNullException("schemaUri");
            }
            if (targetNamespace != null) {
                targetNamespace = XmlComplianceUtil.CDataNormalize(targetNamespace);
            }
            XmlSchema schema = null;
            lock (InternalSyncObject) {
                //Check if schema from url has already been added
                XmlResolver tempResolver = readerSettings.GetXmlResolver();
                if ( tempResolver == null ) {
                    tempResolver = new XmlUrlResolver();
                }
                Uri tempSchemaUri = tempResolver.ResolveUri(null, schemaUri);
                if (IsSchemaLoaded(tempSchemaUri, targetNamespace, out schema)) {
                    return schema;
                }
                else {
                    //Url already not processed; Load SOM from url
                    XmlReader reader = XmlReader.Create(schemaUri, readerSettings);
                    try {
                        schema = Add(targetNamespace, ParseSchema(targetNamespace, reader)); //
                        while(reader.Read());// wellformness check; 
                    }
                    finally {
                        reader.Close();
                    }
                }
            }
            return schema;
        }
        

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Add4"]/*' />
        /// <devdoc>
        ///    <para>Add the given schema into the schema schemas.
        ///       If the given schema references other namespaces, the schemas for those
        ///       other namespaces are NOT automatically loaded.</para>
        /// </devdoc>
        public XmlSchema Add(String targetNamespace, XmlReader schemaDocument) {
            if (schemaDocument == null) {
                throw new ArgumentNullException("schemaDocument");
            }
            if (targetNamespace != null) {
                targetNamespace = XmlComplianceUtil.CDataNormalize(targetNamespace);
            }
            lock (InternalSyncObject) {
                XmlSchema schema = null;
                Uri schemaUri = new Uri(schemaDocument.BaseURI, UriKind.RelativeOrAbsolute);
                if (IsSchemaLoaded(schemaUri, targetNamespace, out schema)) {
                    return schema;
                }
                else {
                    DtdProcessing dtdProcessing = this.readerSettings.DtdProcessing;
                    SetDtdProcessing(schemaDocument);
                    schema = Add(targetNamespace, ParseSchema(targetNamespace, schemaDocument));
                    this.readerSettings.DtdProcessing = dtdProcessing; //reset dtdProcessing setting
                    return schema;
                }
            }
        }
        
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Add5"]/*' />
        /// <devdoc>
        ///    <para>Adds all the namespaces defined in the given schemas
        ///       (including their associated schemas) to this schemas.</para>
        /// </devdoc>
        public void Add(XmlSchemaSet schemas) {
            if (schemas == null) {
                throw new ArgumentNullException("schemas");
            }
            if (this == schemas) {
                return;
            }
            bool thisLockObtained = false;
            bool schemasLockObtained = false;
            try {
                while(true) {
                    Monitor.TryEnter(InternalSyncObject, ref thisLockObtained);
                    if (thisLockObtained) {
                        Monitor.TryEnter(schemas.InternalSyncObject, ref schemasLockObtained);
                        if (schemasLockObtained) {
                            break;
                        }
                        else {
                            Monitor.Exit(InternalSyncObject); //Give up this lock and try both again
                            thisLockObtained = false;
                            Thread.Yield(); //Let the thread that holds the lock run
                            continue;
                        }
                    }
                }
                
                XmlSchema currentSchema;
                //
                if (schemas.IsCompiled) { 
                    CopyFromCompiledSet(schemas);
                }
                else { 
                    bool remove = false;
                    string tns = null;
                    foreach(XmlSchema schema in schemas.SortedSchemas.Values) {
                        tns = schema.TargetNamespace;
                        if (tns == null) {
                            tns = string.Empty;
                        }
                        if (this.schemas.ContainsKey(schema.SchemaId) || FindSchemaByNSAndUrl(schema.BaseUri, tns, null) != null) { //Do not already existing url
                            continue;
                        }
                        currentSchema = Add(schema.TargetNamespace, schema);
                        if(currentSchema == null) {
                            remove = true;
                            break;
                        }
                    }
                    //Remove all from the set if even one schema in the passed in set is not preprocessed.
                    if (remove) {
                        foreach(XmlSchema schema in schemas.SortedSchemas.Values) { //Remove all previously added schemas from the set
                            this.schemas.Remove(schema.SchemaId); //Might remove schema that was already there and was not added thru this operation
                            schemaLocations.Remove(schema.BaseUri);
                        }
                    }
                }
            }
            finally { //release locks on sets
                if (thisLockObtained) {
                    Monitor.Exit(InternalSyncObject);
                }
                if (schemasLockObtained) {
                    Monitor.Exit(schemas.InternalSyncObject);
                }
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Add6"]/*' />
        public XmlSchema Add(XmlSchema schema) {
            if (schema == null) {
                throw new ArgumentNullException("schema");
            }
            lock (InternalSyncObject) {
                if (schemas.ContainsKey(schema.SchemaId)) {
                    return schema;
                }
                return Add(schema.TargetNamespace, schema);
            }
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Remove"]/*' />
        public XmlSchema Remove(XmlSchema schema) {
            return Remove(schema, true);
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.RemoveRecursive"]/*' />
        public bool RemoveRecursive(XmlSchema schemaToRemove) {

            if (schemaToRemove == null) {
                throw new ArgumentNullException("schemaToRemove");
            }
            if (!schemas.ContainsKey(schemaToRemove.SchemaId)) {
                return false;
            }

            lock (InternalSyncObject) { //Need to lock here so that remove cannot be called while the set is being compiled
                if (schemas.ContainsKey(schemaToRemove.SchemaId)) { //Need to check again
                    
                    //Build disallowedNamespaces list
                    Hashtable disallowedNamespaces = new Hashtable();
                    disallowedNamespaces.Add(GetTargetNamespace(schemaToRemove), schemaToRemove);
                    string importedNS;
                    for (int i = 0; i < schemaToRemove.ImportedNamespaces.Count; i++) {
                        importedNS = (string)schemaToRemove.ImportedNamespaces[i];
                        if (disallowedNamespaces[importedNS] == null) {
                            disallowedNamespaces.Add(importedNS, importedNS);
                        }
                    }

                    //Removal list is all schemas imported by this schema directly or indirectly
                    //Need to check if other schemas in the set import schemaToRemove / any of its imports
                    ArrayList needToCheckSchemaList = new ArrayList();
                    XmlSchema mainSchema;
                    for (int i =0; i < schemas.Count; i++) {
                        mainSchema = (XmlSchema)schemas.GetByIndex(i);
                        if (mainSchema == schemaToRemove || 
                            schemaToRemove.ImportedSchemas.Contains(mainSchema)) {
                            continue;
                        }
                        needToCheckSchemaList.Add(mainSchema);
                    }

                    mainSchema = null;
                    for (int i = 0; i < needToCheckSchemaList.Count; i++) { //Perf: Not using nested foreach here
                        mainSchema = (XmlSchema)needToCheckSchemaList[i];
                        
                        if (mainSchema.ImportedNamespaces.Count > 0) {
                            foreach(string tns in disallowedNamespaces.Keys) {
                                if (mainSchema.ImportedNamespaces.Contains(tns)) {
                                    SendValidationEvent(new XmlSchemaException(Res.Sch_SchemaNotRemoved, string.Empty), XmlSeverityType.Warning);
                                    return false;
                                }
                            }
                        }
                    }

                    Remove(schemaToRemove, true);
                    for (int i = 0; i < schemaToRemove.ImportedSchemas.Count; ++i) {
                        XmlSchema impSchema = (XmlSchema)schemaToRemove.ImportedSchemas[i];
                        Remove(impSchema, true);
                    }
                    return true;
                }
            }
            return false;
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Contains1"]/*' />
        public bool Contains(String targetNamespace) {
           if (targetNamespace == null) {
               targetNamespace = string.Empty;
           }
           return targetNamespaces[targetNamespace] != null;
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Contains2"]/*' />
        public bool Contains(XmlSchema schema) {
            if (schema == null) {
                throw new ArgumentNullException("schema");
            }
            return schemas.ContainsValue(schema);
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Compile"]/*' />
        /// <devdoc>
        ///    <para>[To be supplied.]</para>
        /// </devdoc>
        public void Compile() {
            if (isCompiled) {
                return;
            }
            if (schemas.Count == 0) {
                ClearTables(); //Clear any previously present compiled state left by calling just Remove() on the set
                cachedCompiledInfo = new SchemaInfo();
                isCompiled = true;
                compileAll = false;
                return;
            }
            lock (InternalSyncObject) {
                
                if (!isCompiled) { //Locking before checking isCompiled to avoid problems with double locking
                    Compiler compiler = new Compiler(nameTable, eventHandler, schemaForSchema, compilationSettings);
                    SchemaInfo newCompiledInfo = new SchemaInfo();
                    int schemaIndex = 0;
                    if (!compileAll) { //if we are not compiling everything again, Move the pre-compiled schemas to the compiler's tables
                        compiler.ImportAllCompiledSchemas(this); 
                    }
                    try { //First thing to do in the try block is to acquire locks since finally will try to release them. 
                        //If we dont accuire the locks first, and an exception occurs in the code before the locking code, then Threading.SynchronizationLockException will be thrown
                        //when attempting to release it in the finally block
                        XmlSchema currentSchema;
                        XmlSchema xmlNSSchema = Preprocessor.GetBuildInSchema();
                        for (schemaIndex = 0; schemaIndex < schemas.Count; schemaIndex++) {
                            currentSchema = (XmlSchema)schemas.GetByIndex(schemaIndex);
                            
                            //Lock schema to be compiled
#pragma warning disable 0618 
                            //@
                            Monitor.Enter(currentSchema);
#pragma warning restore 0618
                            if (!currentSchema.IsPreprocessed)
                            {
                                SendValidationEvent(new XmlSchemaException(Res.Sch_SchemaNotPreprocessed, string.Empty), XmlSeverityType.Error);
                                isCompiled = false;
                                return;
                            }
                            if (currentSchema.IsCompiledBySet) {
                                if (!compileAll) {
                                    continue;
                                }
                                else if ((object)currentSchema == (object)xmlNSSchema) { // prepare for xml namespace schema without cleanup
                                    compiler.Prepare(currentSchema, false);
                                    continue;
                                }
                            }
                            compiler.Prepare(currentSchema, true);
                        }

                        isCompiled = compiler.Execute(this, newCompiledInfo);
                        if (isCompiled) {
                            if (!compileAll) {
                                newCompiledInfo.Add(cachedCompiledInfo, eventHandler); //Add all the items from the old to the new compiled object
                            }
                            compileAll = false;
                            cachedCompiledInfo = newCompiledInfo; //Replace the compiled info in the set after successful compilation
                        }
                    }
                    finally {
                        //Release locks on all schemas
                        XmlSchema currentSchema;
                        if (schemaIndex == schemas.Count) {
                            schemaIndex--;
                        }
                        for (int i = schemaIndex; i >= 0; i--) {
                            currentSchema = (XmlSchema)schemas.GetByIndex(i);
                            if (currentSchema == Preprocessor.GetBuildInSchema()) { //dont re-set compiled flags for xml namespace schema
                                Monitor.Exit(currentSchema);
                                continue;
                            }
                            currentSchema.IsCompiledBySet = isCompiled;
                            Monitor.Exit(currentSchema);
                        }
                    }
                }
            }
            return;
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Reprocess"]/*' />
        /// <devdoc>
        ///    <para>[To be supplied.]</para>
        /// </devdoc>
        public XmlSchema Reprocess(XmlSchema schema) {
            // Due to 




            if (schema == null) {
                throw new ArgumentNullException("schema");
            }
            if (!schemas.ContainsKey(schema.SchemaId)) {
                throw new ArgumentException(Res.GetString(Res.Sch_SchemaDoesNotExist), "schema");
            }
            XmlSchema originalSchema = schema;
            lock (InternalSyncObject) { //Lock set so that set cannot be compiled in another thread

                // This code is copied from method:
                // Remove(XmlSchema schema, bool forceCompile) 
                // If you changed anything here go and change the same in Remove(XmlSchema schema, bool forceCompile) method
                #region Copied from Remove(XmlSchema schema, bool forceCompile)

                RemoveSchemaFromGlobalTables(schema);
                RemoveSchemaFromCaches(schema);
                if (schema.BaseUri != null) {
                    schemaLocations.Remove(schema.BaseUri);
                }
                string tns = GetTargetNamespace(schema);
                if (Schemas(tns).Count == 0) { //This is the only schema for that namespace
                    targetNamespaces.Remove(tns);
                }
                isCompiled = false;
                compileAll = true; //Force compilation of the whole set; This is when the set is not completely thread-safe

                #endregion //Copied from Remove(XmlSchema schema, bool forceCompile)


                // This code is copied from method:
                // Add(string targetNamespace, XmlSchema schema)
                // If you changed anything here go and change the same in Add(string targetNamespace, XmlSchema schema) method
                #region Copied from Add(string targetNamespace, XmlSchema schema)

                if (schema.ErrorCount != 0) { //Schema with parsing errors cannot be loaded
                    return originalSchema;
                }
                if (PreprocessSchema(ref schema, schema.TargetNamespace)) { //No perf opt for already compiled schemas

                    // This code is copied from method:
                    // AddSchemaToSet(XmlSchema schema)
                    // If you changed anything here go and change the same in AddSchemaToSet(XmlSchema schema) method
                    #region Copied from AddSchemaToSet(XmlSchema schema)

                    //Add to targetNamespaces table
                    if (targetNamespaces[tns] == null) {
                        targetNamespaces.Add(tns, tns);
                    }
                    if (schemaForSchema == null && tns == XmlReservedNs.NsXs && schema.SchemaTypes[DatatypeImplementation.QnAnyType] != null) { //it has xs:anyType
                        schemaForSchema = schema;
                    }
                    for (int i = 0; i < schema.ImportedSchemas.Count; ++i) {    //Once preprocessed external schemas property is set
                        XmlSchema s = (XmlSchema)schema.ImportedSchemas[i];
                        if (!schemas.ContainsKey(s.SchemaId)) {
                            schemas.Add(s.SchemaId, s);
                        }
                        tns = GetTargetNamespace(s);
                        if (targetNamespaces[tns] == null) {
                            targetNamespaces.Add(tns, tns);
                        }
                        if (schemaForSchema == null && tns == XmlReservedNs.NsXs && schema.SchemaTypes[DatatypeImplementation.QnAnyType] != null) { //it has xs:anyType
                            schemaForSchema = schema;
                        }
                    }
                    #endregion //Copied from AddSchemaToSet(XmlSchema schema)
                    return schema;
                }
                #endregion // Copied from Add(string targetNamespace, XmlSchema schema)

                return originalSchema;
            }
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.CopyTo"]/*' />
        /// <devdoc>
        ///    <para>[To be supplied.]</para>
        /// </devdoc>
        public void CopyTo(XmlSchema[] schemas, int index) {
            if (schemas == null)
                throw new ArgumentNullException("schemas");
            if (index < 0 || index > schemas.Length -1 )
                throw new ArgumentOutOfRangeException("index");
            this.schemas.Values.CopyTo(schemas, index);
        }

        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Schemas1"]/*' />
        /// <devdoc>
        ///    <para>[To be supplied.]</para>
        /// </devdoc>
        public ICollection Schemas() {       
            return schemas.Values; 
        }
        
        /// <include file='doc\XmlSchemaSet.uex' path='docs/doc[@for="XmlSchemaSet.Schemas1"]/*' />
        /// <devdoc>
        ///    <para>[To be supplied.]</para>
        /// </devdoc>
        public ICollection Schemas(String targetNamespace) {       
            ArrayList tnsSchemas = new ArrayList();
            XmlSchema currentSchema;
            if (targetNamespace == null) {
                targetNamespace = string.Empty;
            }
            for (int i=0; i < schemas.Count; i++) {
                currentSchema = (XmlSchema)schemas.GetByIndex(i);
                if (GetTargetNamespace(currentSchema) == targetNamespace) {
                    tnsSchemas.Add(currentSchema);
                }
            }
            return tnsSchemas;
        }
        
//Internal Methods

        private XmlSchema Add(string targetNamespace, XmlSchema schema) {
            // Due to 


            if (schema == null || schema.ErrorCount != 0) { //Schema with parsing errors cannot be loaded
                return null;    
            }

            // This code is copied to method:
            // Reprocess(XmlSchema schema) 
            // If you changed anything here go and change the same in Reprocess(XmlSchema schema) method
            if (PreprocessSchema(ref schema, targetNamespace)) { //No perf opt for already compiled schemas
                AddSchemaToSet(schema);
                isCompiled = false;
                return schema;
            }
            return null;
        }

#if TRUST_COMPILE_STATE
        private void AddCompiledSchema(XmlSchema schema) {
            if (schema.IsCompiledBySet ) { //trust compiled state always if it is not a chameleon schema
                VerifyTables();
                SchemaInfo newCompiledInfo = new SchemaInfo();
                XmlSchemaObjectTable substitutionGroupsTable = null;
                if (!AddToCompiledInfo(schema, newCompiledInfo, ref substitutionGroupsTable)) { //Error while adding main schema
                    return null;
                }
                foreach (XmlSchema impSchema in schema.ImportedSchemas) {
                    if (!AddToCompiledInfo(impSchema, newCompiledInfo, ref substitutionGroupsTable)) { //Error while adding imports
                        return null;
                    }
                }
                newCompiledInfo.Add(cachedCompiledInfo, eventHandler); //Add existing compiled info
                cachedCompiledInfo = newCompiledInfo;
                if (substitutionGroupsTable != null) {
                    ProcessNewSubstitutionGroups(substitutionGroupsTable, true);
                }
                if (schemas.Count == 0) { //If its the first compiled schema being added, then set doesnt need to be compiled
                    isCompiled = true;
                    compileAll = false;
                }
                AddSchemaToSet(schema);
                return schema;
            }
        }

        private bool AddToCompiledInfo(XmlSchema schema, SchemaInfo newCompiledInfo, ref XmlSchemaObjectTable substTable) {
            //Add schema's compiled tables to the set
            if (schema.BaseUri != null && schemaLocations[schema.BaseUri] == null) { //Update schemaLocations table
                schemaLocations.Add(schema.BaseUri, schema);
            }

            foreach (XmlSchemaElement element in schema.Elements.Values) {
                if(!AddToTable(elements, element.QualifiedName, element)) {
                    RemoveSchemaFromGlobalTables(schema);
                    return false;
                }
                XmlQualifiedName head = element.SubstitutionGroup;
                if (!head.IsEmpty) {
                    if (substTable == null) {
                        substTable = new XmlSchemaObjectTable();
                    }
                    XmlSchemaSubstitutionGroup substitutionGroup = (XmlSchemaSubstitutionGroup)substTable[head];
                    if (substitutionGroup == null) {
                        substitutionGroup = new XmlSchemaSubstitutionGroup();
                        substitutionGroup.Examplar = head;
                        substTable.Add(head, substitutionGroup);
                    }
                    ArrayList members = substitutionGroup.Members;
                    if (!members.Contains(element)) { //Members might contain element if the same schema is included and imported through different paths. Imp, hence will be added to set directly
                        members.Add(element);
                    }
                }
            }
            foreach (XmlSchemaAttribute attribute in schema.Attributes.Values) {
                if (!AddToTable(attributes, attribute.QualifiedName, attribute)) {
                    RemoveSchemaFromGlobalTables(schema);
                    return false;
                }
            }
            foreach (XmlSchemaType schemaType in schema.SchemaTypes.Values) {
                if (!AddToTable(schemaTypes, schemaType.QualifiedName, schemaType)) {
                    RemoveSchemaFromGlobalTables(schema);
                    return false;
                }
            }
            schema.AddCompiledInfo(newCompiledInfo);
            
            return true;
        }
#endif

        //For use by the validator when loading schemaLocations in the instance
        internal void Add(String targetNamespace, XmlReader reader, Hashtable validatedNamespaces) {
            if (reader == null) {
                throw new ArgumentNullException("reader");
            }
            if (targetNamespace == null) {
                targetNamespace = string.Empty;
            }
            if (validatedNamespaces[targetNamespace] != null) {
                if (FindSchemaByNSAndUrl(new Uri(reader.BaseURI, UriKind.RelativeOrAbsolute), targetNamespace, null) != null) {
                    return;
                }
                else {
                    throw new XmlSchemaException(Res.Sch_ComponentAlreadySeenForNS, targetNamespace);
                }
            }

            //Not locking set as this will not be accessible outside the validator
            XmlSchema schema;
            if (IsSchemaLoaded(new Uri(reader.BaseURI, UriKind.RelativeOrAbsolute), targetNamespace, out schema)) {
                return;
            }
            else { //top-level schema not present for same url
                schema = ParseSchema(targetNamespace, reader);

                //Store the previous locations
                DictionaryEntry[] oldLocations = new DictionaryEntry[schemaLocations.Count];
                schemaLocations.CopyTo(oldLocations, 0);

                //Add to set
                Add(targetNamespace, schema);
                if (schema.ImportedSchemas.Count > 0) { //Check imports
                    string tns;
                    for (int i = 0; i < schema.ImportedSchemas.Count; ++i) {
                        XmlSchema impSchema = (XmlSchema)schema.ImportedSchemas[i];
                        tns = impSchema.TargetNamespace;
                        if (tns == null) {
                            tns = string.Empty;
                        }
                        if (validatedNamespaces[tns] != null && (FindSchemaByNSAndUrl(impSchema.BaseUri, tns, oldLocations) == null) ) {
                            RemoveRecursive(schema);
                            throw new XmlSchemaException(Res.Sch_ComponentAlreadySeenForNS, tns);
                        }
                    }
                 }
            }
        }

        internal XmlSchema FindSchemaByNSAndUrl(Uri schemaUri, string ns, DictionaryEntry[] locationsTable) {
            if (schemaUri == null || schemaUri.OriginalString.Length == 0) {
                return null;
            }
            XmlSchema schema = null;
            if (locationsTable == null) {
                schema = (XmlSchema)schemaLocations[schemaUri];
            }
            else {
                for (int i = 0; i < locationsTable.Length; i++) {
                    if (schemaUri.Equals(locationsTable[i].Key)) {
                        schema = (XmlSchema)locationsTable[i].Value;
                        break;
                    }
                }
            }
            if (schema != null) {
                Debug.Assert(ns != null);
                string tns = schema.TargetNamespace == null ? string.Empty : schema.TargetNamespace;
                if (tns == ns) {
                    return schema;
                }
                else if (tns == string.Empty) { //There could be a chameleon for same ns
                    // It is OK to pass in the schema we have found so far, since it must have the schemaUri we're looking for
                    // (we found it that way above) and it must be the original chameleon schema (the one without target ns)
                    // as we don't add the chameleon copies into the locations tables above.
                    Debug.Assert(schema.BaseUri.Equals(schemaUri));
                    ChameleonKey cKey = new ChameleonKey(ns, schema);
                    schema = (XmlSchema)chameleonSchemas[cKey]; //Need not clone if a schema for that namespace already exists
                }
                else {
                    schema = null;
                }
            }
            return schema;
        }

        private void SetDtdProcessing(XmlReader reader) {
            if (reader.Settings != null) {
                this.readerSettings.DtdProcessing = reader.Settings.DtdProcessing;
            }  
            else {
                XmlTextReader v1Reader = reader as XmlTextReader;
                if (v1Reader != null) {
                   this.readerSettings.DtdProcessing = v1Reader.DtdProcessing;
                }
            }
        }

        private void AddSchemaToSet(XmlSchema schema) {
            // Due to 


            schemas.Add(schema.SchemaId, schema);
            //Add to targetNamespaces table

            // This code is copied to method:
            // Reprocess(XmlSchema schema) 
            // If you changed anything here go and change the same in Reprocess(XmlSchema schema) method
            #region This code is copied to Reprocess(XmlSchema schema) method

            string tns = GetTargetNamespace(schema);
            if (targetNamespaces[tns] == null) {
                targetNamespaces.Add(tns, tns);
            }
            if (schemaForSchema == null && tns == XmlReservedNs.NsXs && schema.SchemaTypes[DatatypeImplementation.QnAnyType] != null) { //it has xs:anyType
                schemaForSchema = schema;
            }
            for (int i = 0; i < schema.ImportedSchemas.Count; ++i) {    //Once preprocessed external schemas property is set
                XmlSchema s = (XmlSchema)schema.ImportedSchemas[i]; 
                if (!schemas.ContainsKey(s.SchemaId)) {
                    schemas.Add(s.SchemaId, s);
                }
                tns = GetTargetNamespace(s);
                if (targetNamespaces[tns] == null) {
                    targetNamespaces.Add(tns, tns);
                }
                if (schemaForSchema == null && tns == XmlReservedNs.NsXs && schema.SchemaTypes[DatatypeImplementation.QnAnyType] != null) { //it has xs:anyType
                    schemaForSchema = schema;
                }
            }

            #endregion // This code is copied to Reprocess(XmlSchema schema) method
        }

        private void ProcessNewSubstitutionGroups(XmlSchemaObjectTable substitutionGroupsTable, bool resolve) {
            foreach(XmlSchemaSubstitutionGroup substGroup in substitutionGroupsTable.Values) {
                if (resolve) { //Resolve substitutionGroups within this schema
                    ResolveSubstitutionGroup(substGroup, substitutionGroupsTable);
                }

                //Add or Merge new substitutionGroups with those that already exist in the set 
                XmlQualifiedName head = substGroup.Examplar;
                XmlSchemaSubstitutionGroup oldSubstGroup = (XmlSchemaSubstitutionGroup)substitutionGroups[head];
                if (oldSubstGroup != null) {
                    for (int i = 0; i < substGroup.Members.Count; ++i) {
                        if (!oldSubstGroup.Members.Contains(substGroup.Members[i])) {
                            oldSubstGroup.Members.Add(substGroup.Members[i]);
                        }
                    }
                }
                else {
                    AddToTable(substitutionGroups, head, substGroup);
                }
            }
        }
   
        private void ResolveSubstitutionGroup(XmlSchemaSubstitutionGroup substitutionGroup, XmlSchemaObjectTable substTable) {
            List<XmlSchemaElement> newMembers = null;
            XmlSchemaElement headElement = (XmlSchemaElement)elements[substitutionGroup.Examplar];
            if (substitutionGroup.Members.Contains(headElement)) {// already checked
                return;
            }
            for (int i = 0; i < substitutionGroup.Members.Count; ++i) {
                XmlSchemaElement element = (XmlSchemaElement)substitutionGroup.Members[i];

                //Chain to other head's that are members of this head's substGroup
                XmlSchemaSubstitutionGroup g = (XmlSchemaSubstitutionGroup)substTable[element.QualifiedName];
                if (g != null) {
                    ResolveSubstitutionGroup(g, substTable);
                    for (int j = 0; j < g.Members.Count; ++j) {
                        XmlSchemaElement element1 = (XmlSchemaElement)g.Members[j];
                        if (element1 != element) { //Exclude the head
                            if (newMembers == null) {
                                newMembers = new List<XmlSchemaElement>();
                            }
                            newMembers.Add(element1);
                        }
                    }
                }
            }
            if (newMembers != null) {
                for (int i = 0; i < newMembers.Count; ++i) {
                    substitutionGroup.Members.Add(newMembers[i]);
                }
            }
            substitutionGroup.Members.Add(headElement);
        }

        internal XmlSchema Remove(XmlSchema schema, bool forceCompile) {
            // Due to 

            if (schema == null) {
                throw new ArgumentNullException("schema");
            }
            lock (InternalSyncObject) { //Need to lock here so that remove cannot be called while the set is being compiled
                if (schemas.ContainsKey(schema.SchemaId)) {

                    // This code is copied to method:
                    // Reprocess(XmlSchema schema) 
                    // If you changed anything here go and change the same in Reprocess(XmlSchema schema) method
                    #region This code is copied to Reprocess(XmlSchema schema) method

                    if (forceCompile) {
                        RemoveSchemaFromGlobalTables(schema);
                        RemoveSchemaFromCaches(schema);
                    }
                    schemas.Remove(schema.SchemaId);
                    if (schema.BaseUri != null) {
                        schemaLocations.Remove(schema.BaseUri);
                    }
                    string tns = GetTargetNamespace(schema);
                    if (Schemas(tns).Count == 0) { //This is the only schema for that namespace
                        targetNamespaces.Remove(tns);
                    }
                    if (forceCompile) {
                        isCompiled = false;
                        compileAll = true; //Force compilation of the whole set; This is when the set is not completely thread-safe
                    }
                    return schema;

                    #endregion // This code is copied to Reprocess(XmlSchema schema) method
                }
            }
            return null;
        }

        private void ClearTables() {
            GlobalElements.Clear();
            GlobalAttributes.Clear();
            GlobalTypes.Clear();
            SubstitutionGroups.Clear();
            TypeExtensions.Clear();
        }

        internal bool PreprocessSchema(ref XmlSchema schema, string targetNamespace) {
            Preprocessor prep = new Preprocessor(nameTable, GetSchemaNames(nameTable), eventHandler, compilationSettings);
            prep.XmlResolver = readerSettings.GetXmlResolver_CheckConfig();
            prep.ReaderSettings = readerSettings;
            prep.SchemaLocations = schemaLocations;
            prep.ChameleonSchemas = chameleonSchemas;
            bool hasErrors = prep.Execute(schema, targetNamespace, true);
            schema = prep.RootSchema; //For any root level chameleon cloned
            return hasErrors;
        }
        
        internal XmlSchema ParseSchema(string targetNamespace, XmlReader reader) {
            XmlNameTable readerNameTable = reader.NameTable;
            SchemaNames schemaNames = GetSchemaNames(readerNameTable);
            Parser parser = new Parser(SchemaType.XSD, readerNameTable, schemaNames, eventHandler);
            parser.XmlResolver = readerSettings.GetXmlResolver_CheckConfig();
            SchemaType schemaType;
            try {
                schemaType = parser.Parse(reader, targetNamespace);
            }
            catch(XmlSchemaException e) {
                SendValidationEvent(e, XmlSeverityType.Error);
                return null;
            }
            return parser.XmlSchema;
        }
        
        internal void CopyFromCompiledSet(XmlSchemaSet otherSet) {
            XmlSchema currentSchema;
            SortedList copyFromList = otherSet.SortedSchemas;
            bool setIsCompiled = schemas.Count == 0 ? true : false; 
            ArrayList existingSchemas = new ArrayList();

            SchemaInfo newCompiledInfo = new SchemaInfo();
            Uri baseUri;
            for(int i=0; i < copyFromList.Count; i++) {
                currentSchema = (XmlSchema)copyFromList.GetByIndex(i);
                baseUri = currentSchema.BaseUri;
                if (schemas.ContainsKey(currentSchema.SchemaId) || (baseUri != null && baseUri.OriginalString.Length != 0 && schemaLocations[baseUri] != null)) {
                    existingSchemas.Add(currentSchema);
                    continue;
                }
                schemas.Add(currentSchema.SchemaId, currentSchema);
                if (baseUri != null && baseUri.OriginalString.Length != 0) {
                    schemaLocations.Add(baseUri, currentSchema);
                }
                string tns = GetTargetNamespace(currentSchema);
                if (targetNamespaces[tns] == null) {
                    targetNamespaces.Add(tns, tns);
                }
            }

            VerifyTables();
            foreach (XmlSchemaElement element in otherSet.GlobalElements.Values) {
                if(!AddToTable(elements, element.QualifiedName, element)) {
                    goto RemoveAll;
                }
            }
            foreach (XmlSchemaAttribute attribute in otherSet.GlobalAttributes.Values) {
                if (!AddToTable(attributes, attribute.QualifiedName, attribute)) {
                    goto RemoveAll;
                }
            }
            foreach (XmlSchemaType schemaType in otherSet.GlobalTypes.Values) {
                if (!AddToTable(schemaTypes, schemaType.QualifiedName, schemaType)) {
                    goto RemoveAll;
                }
            }
            //
            ProcessNewSubstitutionGroups(otherSet.SubstitutionGroups, false);

            newCompiledInfo.Add(cachedCompiledInfo, eventHandler); //Add all the items from the old to the new compiled object
            newCompiledInfo.Add(otherSet.CompiledInfo,eventHandler); //
            cachedCompiledInfo = newCompiledInfo; //Replace the compiled info in the set after successful compilation
            if (setIsCompiled) {
                isCompiled = true;
                compileAll = false;
            }
            return;
        
            RemoveAll:
                foreach (XmlSchema schemaToRemove in copyFromList.Values) {
                    if (!existingSchemas.Contains(schemaToRemove)) {
                        Remove(schemaToRemove, false);
                    }
                }
                foreach (XmlSchemaElement elementToRemove in otherSet.GlobalElements.Values) {
                    if(!existingSchemas.Contains((XmlSchema)elementToRemove.Parent)) {
                        elements.Remove(elementToRemove.QualifiedName);
                    }
                }
                foreach (XmlSchemaAttribute attributeToRemove in otherSet.GlobalAttributes.Values) {
                    if(!existingSchemas.Contains((XmlSchema)attributeToRemove.Parent)) {
                        attributes.Remove(attributeToRemove.QualifiedName);
                    }
                }
                foreach (XmlSchemaType schemaTypeToRemove in otherSet.GlobalTypes.Values) {
                    if(!existingSchemas.Contains((XmlSchema)schemaTypeToRemove.Parent)) {
                        schemaTypes.Remove(schemaTypeToRemove.QualifiedName);
                    }
                }
        }

        internal SchemaInfo CompiledInfo {
            get {
                return cachedCompiledInfo; 
            }
        }
        
        internal XmlReaderSettings ReaderSettings {
            get {
                return readerSettings;
            }
        }
        
        internal XmlResolver GetResolver() {
            return readerSettings.GetXmlResolver_CheckConfig();
        }
        
        internal ValidationEventHandler GetEventHandler() {
            return eventHandler;
        }

        internal SchemaNames GetSchemaNames(XmlNameTable nt) {
            if (nameTable != nt) {
                return new SchemaNames(nt);
            }
            else {
                if (schemaNames == null) {
                    schemaNames = new SchemaNames( nameTable );
                }
                return schemaNames;
            }
        }
        
        internal bool IsSchemaLoaded(Uri schemaUri, string targetNamespace, out XmlSchema schema) {
            schema = null;
            if (targetNamespace == null) {
                targetNamespace = string.Empty;
            }
            if (GetSchemaByUri(schemaUri, out schema)) {
                if (schemas.ContainsKey(schema.SchemaId) && (targetNamespace.Length == 0 || targetNamespace == schema.TargetNamespace)) { //schema is present in set
                    //Schema found
                }
                else if (schema.TargetNamespace == null) { //If schema not in set or namespace doesnt match, then it might be a chameleon
                    XmlSchema chameleonSchema = FindSchemaByNSAndUrl(schemaUri, targetNamespace, null);
                    if (chameleonSchema != null && schemas.ContainsKey(chameleonSchema.SchemaId)) {
                        schema = chameleonSchema;
                    }
                    else {
                        schema = Add(targetNamespace, schema);
                    }
                }
                else if (targetNamespace.Length != 0 && targetNamespace != schema.TargetNamespace) {
                    SendValidationEvent(new XmlSchemaException(Res.Sch_MismatchTargetNamespaceEx, new string[] { targetNamespace, schema.TargetNamespace }), XmlSeverityType.Error);
                    schema = null;
                }
                else {
                    //If here, schema not present in set but in loc and might be added in loc through an earlier include
                    //S.TNS != null && ( tns == null or tns == s.TNS)
                    AddSchemaToSet(schema);
                }
                return true; //Schema Found
           }
           return false;
        }

        internal bool GetSchemaByUri(Uri schemaUri, out XmlSchema schema) {
            schema = null;
            if (schemaUri == null || schemaUri.OriginalString.Length == 0) {
                return false;                    
            }                    
            schema = (XmlSchema)schemaLocations[schemaUri];
            if (schema != null) {
                return true;
            }
            return false;
        }

        internal string GetTargetNamespace(XmlSchema schema) {
            return schema.TargetNamespace == null ? string.Empty : schema.TargetNamespace;
        }


        internal SortedList SortedSchemas {
            get {
                return schemas;
            }
        }
        
        internal bool CompileAll {
            get {
                return compileAll;
            }
        }

//Private Methods
        private void RemoveSchemaFromCaches(XmlSchema schema) {
            //Remove From ChameleonSchemas and schemaLocations cache
            List<XmlSchema> reprocessList = new List<XmlSchema>();
            schema.GetExternalSchemasList(reprocessList, schema);
            for (int i = 0; i < reprocessList.Count; ++i) { //Remove schema from schemaLocations & chameleonSchemas tables
                if (reprocessList[i].BaseUri != null && reprocessList[i].BaseUri.OriginalString.Length != 0) {
                    schemaLocations.Remove(reprocessList[i].BaseUri);
                }
                //Remove from chameleon table
                ICollection chameleonKeys = chameleonSchemas.Keys;
                ArrayList removalList = new ArrayList();
                foreach(ChameleonKey cKey in chameleonKeys) {
                    if (cKey.chameleonLocation.Equals(reprocessList[i].BaseUri)) {
                        // The key will have the originalSchema set to null if the location was not empty
                        //   otherwise we need to care about it as there may be more chameleon schemas without
                        //   a base URI and we want to remove only those which were created as a clone of the one
                        //   we're removing.
                        if (cKey.originalSchema == null || Ref.ReferenceEquals(cKey.originalSchema, reprocessList[i])) {
                            removalList.Add(cKey);
                        }
                    }
                }
                for (int j = 0; j < removalList.Count; ++j) {
                    chameleonSchemas.Remove(removalList[j]);
                }
            }
        }

        private void RemoveSchemaFromGlobalTables(XmlSchema schema) {
            if (schemas.Count == 0) {
                return;                    
            }
            VerifyTables();                
            foreach (XmlSchemaElement elementToRemove in schema.Elements.Values) {
                XmlSchemaElement elem = (XmlSchemaElement)elements[elementToRemove.QualifiedName];
                if (elem == elementToRemove) {
                    elements.Remove(elementToRemove.QualifiedName);
                }
            }
            foreach (XmlSchemaAttribute attributeToRemove in schema.Attributes.Values) {
                XmlSchemaAttribute attr = (XmlSchemaAttribute)attributes[attributeToRemove.QualifiedName];
                if (attr == attributeToRemove) {
                    attributes.Remove(attributeToRemove.QualifiedName);
                }
            }
            foreach (XmlSchemaType schemaTypeToRemove in schema.SchemaTypes.Values) {
                XmlSchemaType schemaType = (XmlSchemaType)schemaTypes[schemaTypeToRemove.QualifiedName];
                if (schemaType == schemaTypeToRemove) {
                    schemaTypes.Remove(schemaTypeToRemove.QualifiedName);
                }
            }
        }
        private bool AddToTable(XmlSchemaObjectTable table, XmlQualifiedName qname, XmlSchemaObject item) {
            if (qname.Name.Length == 0) {
                return true;
            }
            XmlSchemaObject existingObject = (XmlSchemaObject)table[qname]; 
            if (existingObject != null) {
                if (existingObject == item || existingObject.SourceUri == item.SourceUri) {
                    return true;
                }
                string code = string.Empty;
                if (item is XmlSchemaComplexType) {
                    code = Res.Sch_DupComplexType;
                } 
                else if (item is XmlSchemaSimpleType) {
                    code = Res.Sch_DupSimpleType;
                } 
                else if (item is XmlSchemaElement) {
                    code = Res.Sch_DupGlobalElement;
                } 
                else if (item is XmlSchemaAttribute) {
                    if (qname.Namespace == XmlReservedNs.NsXml) {
                        XmlSchema schemaForXmlNS = Preprocessor.GetBuildInSchema();
                        XmlSchemaObject builtInAttribute = schemaForXmlNS.Attributes[qname];
                        if (existingObject == builtInAttribute) { //replace built-in one
                            table.Insert(qname, item);
                            return true;
                        }
                        else if (item == builtInAttribute) { //trying to overwrite customer's component with built-in, ignore built-in
                            return true;
                        }
                    }
                    code = Res.Sch_DupGlobalAttribute;
                } 
                SendValidationEvent(new XmlSchemaException(code,qname.ToString()), XmlSeverityType.Error);
                return false;
            } 
            else {
                table.Add(qname, item);
                return true;
            }
        }

        private void VerifyTables() {
            if (elements == null) {
                elements = new XmlSchemaObjectTable();
            }
            if (attributes == null) {
                attributes = new XmlSchemaObjectTable();
            }
            if (schemaTypes == null) {
                schemaTypes = new XmlSchemaObjectTable();
            }
            if (substitutionGroups == null) {
                substitutionGroups = new XmlSchemaObjectTable();
            }
        }

        private void InternalValidationCallback(object sender, ValidationEventArgs e ) {
            if (e.Severity == XmlSeverityType.Error) {
                throw e.Exception;
            }
        }
        
        private void SendValidationEvent(XmlSchemaException e, XmlSeverityType severity) {
            if (eventHandler != null) {
                eventHandler(this, new ValidationEventArgs(e, severity));
            } 
            else {
                throw e;
            }
        }
    };
#endif
}