mcs/mcs/nullable.cs

   1 //
   2 // nullable.cs: Nullable types support
   3 //
   4 // Authors: Martin Baulig (martin@ximian.com)
   5 //          Miguel de Icaza (miguel@ximian.com)
   6 //          Marek Safar (marek.safar@gmail.com)
   7 //
   8 // Dual licensed under the terms of the MIT X11 or GNU GPL
   9 //
  10 // Copyright 2001, 2002, 2003 Ximian, Inc (http://www.ximian.com)
  11 // Copyright 2004-2008 Novell, Inc
  12 //
  13
  14 using System;
  15 using System.Reflection;
  16 using System.Reflection.Emit;
  17 using System.Collections;
  18
  19 namespace Mono.CSharp.Nullable
  20 {
  21         public class NullableType : TypeExpr
  22         {
  23                 Expression underlying;
  24
  25                 public NullableType (Expression underlying, Location l)
  26                 {
  27                         this.underlying = underlying;
  28                         loc = l;
  29
  30                         eclass = ExprClass.Type;
  31                 }
  32
  33                 public NullableType (Type type, Location loc)
  34                         : this (new TypeExpression (type, loc), loc)
  35                 { }
  36
  37                 protected override TypeExpr DoResolveAsTypeStep (IResolveContext ec)
  38                 {
  39                         TypeArguments args = new TypeArguments (loc);
  40                         args.Add (underlying);
  41
  42                         if (TypeManager.generic_nullable_type == null) {
  43                                 TypeManager.generic_nullable_type = TypeManager.CoreLookupType (
  44                                         "System", "Nullable`1", Kind.Struct, true);
  45                         }
  46
  47                         ConstructedType ctype = new ConstructedType (TypeManager.generic_nullable_type, args, loc);
  48                         return ctype.ResolveAsTypeTerminal (ec, false);
  49                 }
  50
  51                 public override TypeExpr ResolveAsTypeTerminal (IResolveContext ec, bool silent)
  52                 {
  53                         return ResolveAsBaseTerminal (ec, silent);
  54                 }
  55         }
  56
  57         public sealed class NullableInfo
  58         {
  59                 public readonly Type Type;
  60                 public readonly Type UnderlyingType;
  61                 public MethodInfo HasValue;
  62                 public MethodInfo Value;
  63                 public MethodInfo GetValueOrDefault;
  64                 public ConstructorInfo Constructor;
  65
  66                 public NullableInfo (Type type)
  67                 {
  68                         Type = type;
  69                         UnderlyingType = TypeManager.GetTypeArguments (type) [0];
  70
  71                         PropertyInfo has_value_pi = TypeManager.GetPredefinedProperty (type, "HasValue", Location.Null, Type.EmptyTypes);
  72                         PropertyInfo value_pi = TypeManager.GetPredefinedProperty (type, "Value", Location.Null, Type.EmptyTypes);
  73                         GetValueOrDefault = TypeManager.GetPredefinedMethod (type, "GetValueOrDefault", Location.Null, Type.EmptyTypes);
  74
  75                         HasValue = has_value_pi.GetGetMethod (false);
  76                         Value = value_pi.GetGetMethod (false);
  77 #if MS_COMPATIBLE
  78                         if (UnderlyingType.Module == CodeGen.Module.Builder) {
  79                                 Type o_type = TypeManager.DropGenericTypeArguments (type);
  80                                 Constructor = TypeBuilder.GetConstructor (type,
  81                                         TypeManager.GetPredefinedConstructor (o_type, Location.Null, o_type.GetGenericArguments ()));
  82                                 return;
  83                         }
  84 #endif
  85                         Constructor = type.GetConstructor (new Type[] { UnderlyingType });
  86                 }
  87         }
  88
  89         public class Unwrap : Expression, IMemoryLocation, IAssignMethod
  90         {
  91                 Expression expr;
  92                 NullableInfo info;
  93
  94                 LocalTemporary temp;
  95
  96                 protected Unwrap (Expression expr)
  97                 {
  98                         this.expr = expr;
  99                         this.loc = expr.Location;
 100                 }
 101
 102                 public static Unwrap Create (Expression expr, EmitContext ec)
 103                 {
 104                         return new Unwrap (expr).Resolve (ec) as Unwrap;
 105                 }
 106
 107                 public override Expression CreateExpressionTree (EmitContext ec)
 108                 {
 109                         return expr.CreateExpressionTree (ec);
 110                 }
 111
 112                 public override Expression DoResolve (EmitContext ec)
 113                 {
 114                         if (expr == null)
 115                                 return null;
 116
 117                         info = new NullableInfo (expr.Type);
 118                         type = info.UnderlyingType;
 119                         eclass = expr.eclass;
 120                         return this;
 121                 }
 122
 123                 public override Expression DoResolveLValue (EmitContext ec, Expression right_side)
 124                 {
 125                         return DoResolve (ec);
 126                 }
 127
 128                 public override void Emit (EmitContext ec)
 129                 {
 130                         Store (ec);
 131                         AddressOf (ec, AddressOp.LoadStore);
 132                         ec.ig.EmitCall (OpCodes.Call, info.Value, null);
 133                 }
 134
 135                 public void EmitCheck (EmitContext ec)
 136                 {
 137                         Store (ec);
 138                         AddressOf (ec, AddressOp.LoadStore);
 139                         ec.ig.EmitCall (OpCodes.Call, info.HasValue, null);
 140                 }
 141
 142                 public void EmitGetValueOrDefault (EmitContext ec)
 143                 {
 144                         Store (ec);
 145                         AddressOf (ec, AddressOp.LoadStore);
 146                         ec.ig.EmitCall (OpCodes.Call, info.GetValueOrDefault, null);
 147                 }
 148
 149                 public override bool Equals (object obj)
 150                 {
 151                         Unwrap uw = obj as Unwrap;
 152                         return uw != null && expr.Equals (uw.expr);
 153                 }
 154
 155                 public Expression Original {
 156                         get {
 157                                 return expr;
 158                         }
 159                 }
 160
 161                 public override int GetHashCode ()
 162                 {
 163                         return expr.GetHashCode ();
 164                 }
 165
 166                 public override bool IsNull {
 167                         get {
 168                                 return expr.IsNull;
 169                         }
 170                 }
 171
 172                 void Store (EmitContext ec)
 173                 {
 174                         if (expr is VariableReference)
 175                                 return;
 176
 177                         if (temp != null)
 178                                 return;
 179
 180                         expr.Emit (ec);
 181                         LocalVariable.Store (ec);
 182                 }
 183
 184                 public void Load (EmitContext ec)
 185                 {
 186                         if (expr is VariableReference)
 187                                 expr.Emit (ec);
 188                         else
 189                                 LocalVariable.Emit (ec);
 190                 }
 191
 192                 public override void MutateHoistedGenericType (AnonymousMethodStorey storey)
 193                 {
 194                         type = storey.MutateType (type);
 195                         info.Constructor = storey.MutateConstructor (info.Constructor);
 196                         info.HasValue = storey.MutateGenericMethod (info.HasValue);
 197                         info.GetValueOrDefault = storey.MutateGenericMethod (info.GetValueOrDefault);
 198                         info.Value = storey.MutateGenericMethod (info.Value);
 199                 }
 200
 201                 public void AddressOf (EmitContext ec, AddressOp mode)
 202                 {
 203                         IMemoryLocation ml = expr as VariableReference;
 204                         if (ml != null)
 205                                 ml.AddressOf (ec, mode);
 206                         else
 207                                 LocalVariable.AddressOf (ec, mode);
 208                 }
 209
 210                 //
 211                 // Keeps result of non-variable expression
 212                 //
 213                 LocalTemporary LocalVariable {
 214                         get {
 215                                 if (temp == null)
 216                                         temp = new LocalTemporary (info.Type);
 217                                 return temp;
 218                         }
 219                 }
 220
 221                 public void Emit (EmitContext ec, bool leave_copy)
 222                 {
 223                         if (leave_copy)
 224                                 Load (ec);
 225
 226                         Emit (ec);
 227                 }
 228
 229                 public void EmitAssign (EmitContext ec, Expression source,
 230                                         bool leave_copy, bool prepare_for_load)
 231                 {
 232                         InternalWrap wrap = new InternalWrap (source, info, loc);
 233                         ((IAssignMethod) expr).EmitAssign (ec, wrap, leave_copy, false);
 234                 }
 235
 236                 protected class InternalWrap : Expression
 237                 {
 238                         public Expression expr;
 239                         public NullableInfo info;
 240
 241                         public InternalWrap (Expression expr, NullableInfo info, Location loc)
 242                         {
 243                                 this.expr = expr;
 244                                 this.info = info;
 245                                 this.loc = loc;
 246
 247                                 type = info.Type;
 248                                 eclass = ExprClass.Value;
 249                         }
 250
 251                         public override Expression CreateExpressionTree (EmitContext ec)
 252                         {
 253                                 throw new NotSupportedException ("ET");
 254                         }
 255
 256                         public override Expression DoResolve (EmitContext ec)
 257                         {
 258                                 return this;
 259                         }
 260
 261                         public override void Emit (EmitContext ec)
 262                         {
 263                                 expr.Emit (ec);
 264                                 ec.ig.Emit (OpCodes.Newobj, info.Constructor);
 265                         }
 266                 }
 267         }
 268
 269         public class Wrap : TypeCast
 270         {
 271                 readonly NullableInfo info;
 272
 273                 protected Wrap (Expression expr, Type type)
 274                         : base (expr, type)
 275                 {
 276                         info = new NullableInfo (type);
 277                         eclass = ExprClass.Value;
 278                 }
 279
 280                 public static Expression Create (Expression expr, Type type)
 281                 {
 282                         //
 283                         // Avoid unwraping and wraping of the same type
 284                         //
 285                         Unwrap unwrap = expr as Unwrap;
 286                         if (unwrap != null && TypeManager.IsEqual (expr.Type, TypeManager.GetTypeArguments (type) [0]))
 287                                 return unwrap.Original;
 288
 289                         return new Wrap (expr, type);
 290                 }
 291
 292                 public override void Emit (EmitContext ec)
 293                 {
 294                         child.Emit (ec);
 295                         ec.ig.Emit (OpCodes.Newobj, info.Constructor);
 296                 }
 297         }
 298
 299         //
 300         // Represents null literal lifted to nullable type
 301         //
 302         public class LiftedNull : EmptyConstantCast, IMemoryLocation
 303         {
 304                 private LiftedNull (Type nullable_type, Location loc)
 305                         : base (new NullLiteral (loc), nullable_type)
 306                 {
 307                         eclass = ExprClass.Value;
 308                 }
 309
 310                 public static Constant Create (Type nullable, Location loc)
 311                 {
 312                         return new LiftedNull (nullable, loc);
 313                 }
 314
 315                 public static Expression CreateFromExpression (Expression e)
 316                 {
 317                         Report.Warning (458, 2, e.Location, "The result of the expression is always `null' of type `{0}'",
 318                                 TypeManager.CSharpName (e.Type));
 319
 320                         return ReducedExpression.Create (Create (e.Type, e.Location), e);
 321                 }
 322
 323                 public override Expression CreateExpressionTree (EmitContext ec)
 324                 {
 325                         ArrayList args = new ArrayList (2);
 326                         args.Add (new Argument (this));
 327                         args.Add (new Argument (new TypeOf (new TypeExpression (type, loc), loc)));
 328
 329                         return CreateExpressionFactoryCall ("Constant", args);
 330                 }
 331
 332                 public override void Emit (EmitContext ec)
 333                 {
 334                         // TODO: generate less temporary variables
 335                         LocalTemporary value_target = new LocalTemporary (type);
 336
 337                         value_target.AddressOf (ec, AddressOp.Store);
 338                         ec.ig.Emit (OpCodes.Initobj, type);
 339                         value_target.Emit (ec);
 340                 }
 341
 342                 public void AddressOf (EmitContext ec, AddressOp Mode)
 343                 {
 344                         LocalTemporary value_target = new LocalTemporary (type);
 345
 346                         value_target.AddressOf (ec, AddressOp.Store);
 347                         ec.ig.Emit (OpCodes.Initobj, type);
 348                         ((IMemoryLocation) value_target).AddressOf (ec, Mode);
 349                 }
 350         }
 351
 352         public abstract class Lifted : Expression, IMemoryLocation
 353         {
 354                 Expression expr, underlying, wrap, null_value;
 355                 Unwrap unwrap;
 356
 357                 protected Lifted (Expression expr, Location loc)
 358                 {
 359                         this.expr = expr;
 360                         this.loc = loc;
 361                 }
 362
 363                 public override Expression CreateExpressionTree (EmitContext ec)
 364                 {
 365                         ArrayList args = new ArrayList (2);
 366                         args.Add (new Argument (expr.CreateExpressionTree (ec)));
 367                         args.Add (new Argument (new TypeOf (new TypeExpression (type, loc), loc)));
 368                         return CreateExpressionFactoryCall ("Convert", args);
 369                 }
 370
 371                 public override Expression DoResolve (EmitContext ec)
 372                 {
 373                         expr = expr.Resolve (ec);
 374                         if (expr == null)
 375                                 return null;
 376
 377                         unwrap = Unwrap.Create (expr, ec);
 378                         if (unwrap == null)
 379                                 return null;
 380
 381                         underlying = ResolveUnderlying (unwrap, ec);
 382                         if (underlying == null)
 383                                 return null;
 384
 385                         TypeExpr target_type = new NullableType (underlying.Type, loc);
 386                         target_type = target_type.ResolveAsTypeTerminal (ec, false);
 387                         if (target_type == null)
 388                                 return null;
 389
 390                         wrap = Wrap.Create (underlying, target_type.Type);
 391                         if (wrap == null)
 392                                 return null;
 393
 394                         null_value = LiftedNull.Create (wrap.Type, loc);
 395
 396                         type = wrap.Type;
 397                         eclass = ExprClass.Value;
 398                         return this;
 399                 }
 400
 401                 protected abstract Expression ResolveUnderlying (Expression unwrap, EmitContext ec);
 402
 403                 public override void Emit (EmitContext ec)
 404                 {
 405                         ILGenerator ig = ec.ig;
 406                         Label is_null_label = ig.DefineLabel ();
 407                         Label end_label = ig.DefineLabel ();
 408
 409                         unwrap.EmitCheck (ec);
 410                         ig.Emit (OpCodes.Brfalse, is_null_label);
 411
 412                         wrap.Emit (ec);
 413                         ig.Emit (OpCodes.Br, end_label);
 414
 415                         ig.MarkLabel (is_null_label);
 416                         null_value.Emit (ec);
 417
 418                         ig.MarkLabel (end_label);
 419                 }
 420
 421                 public void AddressOf (EmitContext ec, AddressOp mode)
 422                 {
 423                         unwrap.AddressOf (ec, mode);
 424                 }
 425         }
 426
 427         public class LiftedConversion : Lifted
 428         {
 429                 public readonly bool IsUser;
 430                 public readonly bool IsExplicit;
 431                 public readonly Type TargetType;
 432
 433                 public LiftedConversion (Expression expr, Type target_type, bool is_user,
 434                                          bool is_explicit, Location loc)
 435                         : base (expr, loc)
 436                 {
 437                         this.IsUser = is_user;
 438                         this.IsExplicit = is_explicit;
 439                         this.TargetType = target_type;
 440                 }
 441
 442                 protected override Expression ResolveUnderlying (Expression unwrap, EmitContext ec)
 443                 {
 444                         Type type = TypeManager.GetTypeArguments (TargetType) [0];
 445
 446                         if (IsUser) {
 447                                 if (IsExplicit)
 448                                         return Convert.ExplicitUserConversion (ec, unwrap, type, loc);
 449                                 else
 450                                         return Convert.ImplicitUserConversion (ec, unwrap, type, loc);
 451                         } else {
 452                                 if (IsExplicit)
 453                                         return Convert.ExplicitConversion (ec, unwrap, type, loc);
 454                                 else
 455                                         return Convert.ImplicitConversion (ec, unwrap, type, loc);
 456                         }
 457                 }
 458         }
 459
 460         public class LiftedUnaryOperator : Unary, IMemoryLocation
 461         {
 462                 Unwrap unwrap;
 463                 Expression user_operator;
 464
 465                 public LiftedUnaryOperator (Unary.Operator op, Expression expr)
 466                         : base (op, expr)
 467                 {
 468                 }
 469
 470                 public void AddressOf (EmitContext ec, AddressOp mode)
 471                 {
 472                         unwrap.AddressOf (ec, mode);
 473                 }
 474
 475                 public override Expression CreateExpressionTree (EmitContext ec)
 476                 {
 477                         if (user_operator != null)
 478                                 return user_operator.CreateExpressionTree (ec);
 479
 480                         if (Oper == Operator.UnaryPlus)
 481                                 return Expr.CreateExpressionTree (ec);
 482
 483                         return base.CreateExpressionTree (ec);
 484                 }
 485
 486                 public override Expression DoResolve (EmitContext ec)
 487                 {
 488                         if (eclass != ExprClass.Invalid)
 489                                 return this;
 490
 491                         unwrap = Unwrap.Create (Expr, ec);
 492                         if (unwrap == null)
 493                                 return null;
 494
 495                         Expression res = base.ResolveOperator (ec, unwrap);
 496                         if (res != this) {
 497                                 if (user_operator == null)
 498                                         return res;
 499                         } else {
 500                                 res = Expr = LiftExpression (ec, Expr);
 501                         }
 502
 503                         if (res == null)
 504                                 return null;
 505
 506                         eclass = ExprClass.Value;
 507                         type = res.Type;
 508                         return this;
 509                 }
 510
 511                 public override void Emit (EmitContext ec)
 512                 {
 513                         ILGenerator ig = ec.ig;
 514                         Label is_null_label = ig.DefineLabel ();
 515                         Label end_label = ig.DefineLabel ();
 516
 517                         unwrap.EmitCheck (ec);
 518                         ig.Emit (OpCodes.Brfalse, is_null_label);
 519
 520                         NullableInfo ni = new NullableInfo (type);
 521
 522                         if (user_operator != null) {
 523                                 user_operator.Emit (ec);
 524                         } else {
 525                                 EmitOperator (ec, ni.UnderlyingType);
 526                         }
 527
 528                         ig.Emit (OpCodes.Newobj, ni.Constructor);
 529                         ig.Emit (OpCodes.Br_S, end_label);
 530
 531                         ig.MarkLabel (is_null_label);
 532                         LiftedNull.Create (type, loc).Emit (ec);
 533
 534                         ig.MarkLabel (end_label);
 535                 }
 536
 537                 Expression LiftExpression (EmitContext ec, Expression expr)
 538                 {
 539                         TypeExpr lifted_type = new NullableType (expr.Type, expr.Location);
 540                         lifted_type = lifted_type.ResolveAsTypeTerminal (ec, false);
 541                         if (lifted_type == null)
 542                                 return null;
 543
 544                         expr.Type = lifted_type.Type;
 545                         return expr;
 546                 }
 547
 548                 protected override Expression ResolveEnumOperator (EmitContext ec, Expression expr)
 549                 {
 550                         expr = base.ResolveEnumOperator (ec, expr);
 551                         if (expr == null)
 552                                 return null;
 553
 554                         Expr = LiftExpression (ec, Expr);
 555                         return LiftExpression (ec, expr);
 556                 }
 557
 558                 protected override Expression ResolveUserOperator (EmitContext ec, Expression expr)
 559                 {
 560                         expr = base.ResolveUserOperator (ec, expr);
 561                         if (expr == null)
 562                                 return null;
 563
 564                         //
 565                         // When a user operator is of non-nullable type
 566                         //
 567                         if (Expr is Unwrap) {
 568                                 user_operator = LiftExpression (ec, expr);
 569                                 return user_operator;
 570                         }
 571
 572                         return expr;
 573                 }
 574         }
 575
 576         public class LiftedBinaryOperator : Binary
 577         {
 578                 Unwrap left_unwrap, right_unwrap;
 579                 bool left_null_lifted, right_null_lifted;
 580                 Expression left_orig, right_orig;
 581                 Expression user_operator;
 582                 ConstructorInfo wrap_ctor;
 583
 584                 public LiftedBinaryOperator (Binary.Operator op, Expression left, Expression right,
 585                                              Location loc)
 586                         : base (op, left, right)
 587                 {
 588                         this.loc = loc;
 589                 }
 590
 591                 public override Expression CreateExpressionTree (EmitContext ec)
 592                 {
 593                         if (user_operator != null)
 594                                 return user_operator.CreateExpressionTree (ec);
 595
 596                         return base.CreateExpressionTree (ec);
 597                 }
 598
 599                 //
 600                 // CSC 2 has this behavior, it allows structs to be compared
 601                 // with the null literal *outside* of a generics context and
 602                 // inlines that as true or false.
 603                 //
 604                 Expression CreateNullConstant (Expression expr)
 605                 {
 606                         // FIXME: Handle side effect constants
 607                         Constant c = new BoolConstant (Oper == Operator.Inequality, loc);
 608
 609                         if ((Oper & Operator.EqualityMask) != 0) {
 610                                 Report.Warning (472, 2, loc, "The result of comparing value type `{0}' with null is `{1}'",
 611                                                 expr.GetSignatureForError (), c.AsString ());
 612                         } else {
 613                                 Report.Warning (464, 2, loc, "The result of comparing type `{0}' with null is always `{1}'",
 614                                                 expr.GetSignatureForError (), c.AsString ());
 615                         }
 616
 617                         return ReducedExpression.Create (c, this);
 618                 }
 619
 620                 public override Expression DoResolve (EmitContext ec)
 621                 {
 622                         if (eclass != ExprClass.Invalid)
 623                                 return this;
 624
 625                         if ((Oper & Operator.LogicalMask) != 0) {
 626                                 Error_OperatorCannotBeApplied (left, right);
 627                                 return null;
 628                         }
 629
 630                         left_orig = left;
 631                         if (TypeManager.IsNullableType (left.Type)) {
 632                                 left = left_unwrap = Unwrap.Create (left, ec);
 633                                 if (left == null)
 634                                         return null;
 635                         }
 636
 637                         right_orig = right;
 638                         if (TypeManager.IsNullableType (right.Type)) {
 639                                 right = right_unwrap = Unwrap.Create (right, ec);
 640                                 if (right == null)
 641                                         return null;
 642                         }
 643
 644                         //
 645                         // Some details are in 6.4.2, 7.2.7
 646                         // Arguments can be lifted for equal operators when the return type is bool and both
 647                         // arguments are of same type
 648                         //
 649                         if (left is NullLiteral) {
 650                                 left = right;
 651                                 left_null_lifted = true;
 652                                 type = TypeManager.bool_type;
 653                         }
 654
 655                         if (right is NullLiteral) {
 656                                 right = left;
 657                                 right_null_lifted = true;
 658                                 type = TypeManager.bool_type;
 659                         }
 660
 661                         eclass = ExprClass.Value;
 662                         return DoResolveCore (ec, left_orig, right_orig);
 663                 }
 664
 665                 void EmitBitwiseBoolean (EmitContext ec)
 666                 {
 667                         ILGenerator ig = ec.ig;
 668
 669                         Label load_left = ig.DefineLabel ();
 670                         Label load_right = ig.DefineLabel ();
 671                         Label end_label = ig.DefineLabel ();
 672
 673                         left_unwrap.EmitGetValueOrDefault (ec);
 674                         ig.Emit (OpCodes.Brtrue_S, load_right);
 675
 676                         right_unwrap.EmitGetValueOrDefault (ec);
 677                         ig.Emit (OpCodes.Brtrue_S, load_left);
 678
 679                         left_unwrap.EmitCheck (ec);
 680                         ig.Emit (OpCodes.Brfalse_S, load_right);
 681
 682                         // load left
 683                         ig.MarkLabel (load_left);
 684
 685                         if (Oper == Operator.BitwiseAnd) {
 686                                 left_unwrap.Load (ec);
 687                         } else {
 688                                 right_unwrap.Load (ec);
 689                                 right_unwrap = left_unwrap;
 690                         }
 691                         ig.Emit (OpCodes.Br_S, end_label);
 692
 693                         // load right
 694                         ig.MarkLabel (load_right);
 695                         right_unwrap.Load (ec);
 696
 697                         ig.MarkLabel (end_label);
 698                 }
 699
 700                 //
 701                 // Emits optimized equality or inequality operator when possible
 702                 //
 703                 bool EmitEquality (EmitContext ec)
 704                 {
 705                         ILGenerator ig = ec.ig;
 706
 707                         //
 708                         // Either left or right is null
 709                         //
 710                         if (left_unwrap != null && (right_null_lifted || right.IsNull)) {
 711                                 left_unwrap.EmitCheck (ec);
 712                                 if (Oper == Binary.Operator.Equality) {
 713                                         ig.Emit (OpCodes.Ldc_I4_0);
 714                                         ig.Emit (OpCodes.Ceq);
 715                                 }
 716                                 return true;
 717                         }
 718
 719                         if (right_unwrap != null && (left_null_lifted || left.IsNull)) {
 720                                 right_unwrap.EmitCheck (ec);
 721                                 if (Oper == Binary.Operator.Equality) {
 722                                         ig.Emit (OpCodes.Ldc_I4_0);
 723                                         ig.Emit (OpCodes.Ceq);
 724                                 }
 725                                 return true;
 726                         }
 727
 728                         if (user_operator != null)
 729                                 return false;
 730
 731                         Label dissimilar_label = ig.DefineLabel ();
 732                         Label end_label = ig.DefineLabel ();
 733
 734                         if (left_unwrap != null)
 735                                 left_unwrap.EmitGetValueOrDefault (ec);
 736                         else
 737                                 left.Emit (ec);
 738
 739                         if (right_unwrap != null)
 740                                 right_unwrap.EmitGetValueOrDefault (ec);
 741                         else
 742                                 right.Emit (ec);
 743
 744                         ig.Emit (OpCodes.Bne_Un_S, dissimilar_label);
 745
 746                         if (left_unwrap != null)
 747                                 left_unwrap.EmitCheck (ec);
 748                         if (right_unwrap != null)
 749                                 right_unwrap.EmitCheck (ec);
 750
 751                         if (left_unwrap != null && right_unwrap != null) {
 752                                 if (Oper == Operator.Inequality)
 753                                         ig.Emit (OpCodes.Xor);
 754                                 else
 755                                         ig.Emit (OpCodes.Ceq);
 756                         } else {
 757                                 if (Oper == Operator.Inequality) {
 758                                         ig.Emit (OpCodes.Ldc_I4_0);
 759                                         ig.Emit (OpCodes.Ceq);
 760                                 }
 761                         }
 762
 763                         ig.Emit (OpCodes.Br_S, end_label);
 764
 765                         ig.MarkLabel (dissimilar_label);
 766                         if (Oper == Operator.Inequality)
 767                                 ig.Emit (OpCodes.Ldc_I4_1);
 768                         else
 769                                 ig.Emit (OpCodes.Ldc_I4_0);
 770
 771                         ig.MarkLabel (end_label);
 772                         return true;
 773                 }
 774
 775                 public override void EmitBranchable (EmitContext ec, Label target, bool onTrue)
 776                 {
 777                         Emit (ec);
 778                         ec.ig.Emit (onTrue ? OpCodes.Brtrue : OpCodes.Brfalse, target);
 779                 }
 780
 781                 public override void Emit (EmitContext ec)
 782                 {
 783                         //
 784                         // Optimize same expression operation
 785                         //
 786                         if (right_unwrap != null && right.Equals (left))
 787                                 right_unwrap = left_unwrap;
 788
 789                         if (user_operator == null && IsBitwiseBoolean) {
 790                                 EmitBitwiseBoolean (ec);
 791                                 return;
 792                         }
 793
 794                         if ((Oper & Operator.EqualityMask) != 0) {
 795                                 if (EmitEquality (ec))
 796                                         return;
 797                         }
 798
 799                         ILGenerator ig = ec.ig;
 800
 801                         Label is_null_label = ig.DefineLabel ();
 802                         Label end_label = ig.DefineLabel ();
 803
 804                         if (left_unwrap != null) {
 805                                 left_unwrap.EmitCheck (ec);
 806                                 ig.Emit (OpCodes.Brfalse, is_null_label);
 807                         }
 808
 809                         //
 810                         // Don't emit HasValue check when left and right expressions are same
 811                         //
 812                         if (right_unwrap != null && !left.Equals (right)) {
 813                                 right_unwrap.EmitCheck (ec);
 814                                 ig.Emit (OpCodes.Brfalse, is_null_label);
 815                         }
 816
 817                         EmitOperator (ec, left.Type);
 818
 819                         if (wrap_ctor != null)
 820                                 ig.Emit (OpCodes.Newobj, wrap_ctor);
 821
 822                         ig.Emit (OpCodes.Br_S, end_label);
 823                         ig.MarkLabel (is_null_label);
 824
 825                         if ((Oper & Operator.ComparisonMask) != 0) {
 826                                 //
 827                                 // Emit true when equality operator both operands are same
 828                                 // or inequality operator operands are not
 829                                 //
 830                                 if ((Oper == Operator.Equality && left_unwrap == right_unwrap) ||
 831                                         (Oper == Operator.Inequality && left_unwrap != right_unwrap))
 832                                         ig.Emit (OpCodes.Ldc_I4_1);
 833                                 else
 834                                         ig.Emit (OpCodes.Ldc_I4_0);
 835                         } else {
 836                                 LiftedNull.Create (type, loc).Emit (ec);
 837                         }
 838
 839                         ig.MarkLabel (end_label);
 840                 }
 841
 842                 protected override void EmitOperator (EmitContext ec, Type l)
 843                 {
 844                         if (user_operator != null) {
 845                                 user_operator.Emit (ec);
 846                                 return;
 847                         }
 848
 849                         if (TypeManager.IsNullableType (l))
 850                                 l = TypeManager.GetTypeArguments (l) [0];
 851
 852                         base.EmitOperator (ec, l);
 853                 }
 854
 855                 bool IsBitwiseBoolean {
 856                         get {
 857                                 return (Oper & Operator.BitwiseMask) != 0 && left_unwrap != null && right_unwrap != null &&
 858                                 left_unwrap.Type == TypeManager.bool_type && right_unwrap.Type == TypeManager.bool_type;
 859                         }
 860                 }
 861
 862                 Expression LiftResult (EmitContext ec, Expression res_expr)
 863                 {
 864                         TypeExpr lifted_type;
 865
 866                         //
 867                         // Avoid double conversion
 868                         //
 869                         if (left_unwrap == null || left_null_lifted || !TypeManager.IsEqual (left_unwrap.Type, left.Type) || (left_unwrap != null && right_null_lifted)) {
 870                                 lifted_type = new NullableType (left.Type, loc);
 871                                 lifted_type = lifted_type.ResolveAsTypeTerminal (ec, false);
 872                                 if (lifted_type == null)
 873                                         return null;
 874
 875                                 if (left is UserCast || left is TypeCast)
 876                                         left.Type = lifted_type.Type;
 877                                 else
 878                                         left = EmptyCast.Create (left, lifted_type.Type);
 879                         }
 880
 881                         if (right_unwrap == null || right_null_lifted || !TypeManager.IsEqual (right_unwrap.Type, right.Type) || (right_unwrap != null && left_null_lifted)) {
 882                                 lifted_type = new NullableType (right.Type, loc);
 883                                 lifted_type = lifted_type.ResolveAsTypeTerminal (ec, false);
 884                                 if (lifted_type == null)
 885                                         return null;
 886
 887                                 if (right is UserCast || right is TypeCast)
 888                                         right.Type = lifted_type.Type;
 889                                 else
 890                                         right = EmptyCast.Create (right, lifted_type.Type);
 891                         }
 892
 893                         if ((Oper & Operator.ComparisonMask) == 0) {
 894                                 lifted_type = new NullableType (res_expr.Type, loc);
 895                                 lifted_type = lifted_type.ResolveAsTypeTerminal (ec, false);
 896                                 if (lifted_type == null)
 897                                         return null;
 898
 899                                 wrap_ctor = new NullableInfo (lifted_type.Type).Constructor;
 900                                 type = res_expr.Type = lifted_type.Type;
 901                         }
 902
 903                         if (left_null_lifted) {
 904                                 left = LiftedNull.Create (right.Type, left.Location);
 905
 906                                 if ((Oper & (Operator.ArithmeticMask | Operator.ShiftMask)) != 0)
 907                                         return LiftedNull.CreateFromExpression (res_expr);
 908
 909                                 //
 910                                 // Value types and null comparison
 911                                 //
 912                                 if (right_unwrap == null || (Oper & Operator.RelationalMask) != 0)
 913                                         return CreateNullConstant (right_orig).Resolve (ec);
 914                         }
 915
 916                         if (right_null_lifted) {
 917                                 right = LiftedNull.Create (left.Type, right.Location);
 918
 919                                 if ((Oper & (Operator.ArithmeticMask | Operator.ShiftMask)) != 0)
 920                                         return LiftedNull.CreateFromExpression (res_expr);
 921
 922                                 //
 923                                 // Value types and null comparison
 924                                 //
 925                                 if (left_unwrap == null || (Oper & Operator.RelationalMask) != 0)
 926                                         return CreateNullConstant (left_orig).Resolve (ec);
 927                         }
 928
 929                         return res_expr;
 930                 }
 931
 932                 protected override Expression ResolveOperatorPredefined (EmitContext ec, Binary.PredefinedOperator [] operators, bool primitives_only, Type enum_type)
 933                 {
 934                         Expression e = base.ResolveOperatorPredefined (ec, operators, primitives_only, enum_type);
 935
 936                         if (e == this || enum_type != null)
 937                                 return LiftResult (ec, e);
 938
 939                         //
 940                         // 7.9.9 Equality operators and null
 941                         //
 942                         // The == and != operators permit one operand to be a value of a nullable type and
 943                         // the other to be the null literal, even if no predefined or user-defined operator
 944                         // (in unlifted or lifted form) exists for the operation.
 945                         //
 946                         if (e == null && (Oper & Operator.EqualityMask) != 0) {
 947                                 if ((left_null_lifted && right_unwrap != null) || (right_null_lifted && left_unwrap != null))
 948                                         return LiftResult (ec, this);
 949                         }
 950
 951                         return e;
 952                 }
 953
 954                 protected override Expression ResolveUserOperator (EmitContext ec, Type l, Type r)
 955                 {
 956                         Expression expr = base.ResolveUserOperator (ec, l, r);
 957                         if (expr == null)
 958                                 return null;
 959
 960                         expr = LiftResult (ec, expr);
 961                         if (expr is Constant)
 962                                 return expr;
 963
 964                         type = expr.Type;
 965                         user_operator = expr;
 966                         return this;
 967                 }
 968         }
 969
 970         public class NullCoalescingOperator : Expression
 971         {
 972                 Expression left, right;
 973                 Unwrap unwrap;
 974
 975                 public NullCoalescingOperator (Expression left, Expression right, Location loc)
 976                 {
 977                         this.left = left;
 978                         this.right = right;
 979                         this.loc = loc;
 980                 }
 981
 982                 public override Expression CreateExpressionTree (EmitContext ec)
 983                 {
 984                         if (left is NullLiteral)
 985                                 Report.Error (845, loc, "An expression tree cannot contain a coalescing operator with null left side");
 986
 987                         UserCast uc = left as UserCast;
 988                         Expression conversion = null;
 989                         if (uc != null) {
 990                                 left = uc.Source;
 991
 992                                 ArrayList c_args = new ArrayList (2);
 993                                 c_args.Add (new Argument (uc.CreateExpressionTree (ec)));
 994                                 c_args.Add (new Argument (left.CreateExpressionTree (ec)));
 995                                 conversion = CreateExpressionFactoryCall ("Lambda", c_args);
 996                         }
 997
 998                         ArrayList args = new ArrayList (3);
 999                         args.Add (new Argument (left.CreateExpressionTree (ec)));
1000                         args.Add (new Argument (right.CreateExpressionTree (ec)));
1001                         if (conversion != null)
1002                                 args.Add (new Argument (conversion));
1003
1004                         return CreateExpressionFactoryCall ("Coalesce", args);
1005                 }
1006
1007                 Expression ConvertExpression (EmitContext ec)
1008                 {
1009                         // TODO: ImplicitConversionExists should take care of this
1010                         if (left.eclass == ExprClass.MethodGroup)
1011                                 return null;
1012
1013                         Type ltype = left.Type;
1014
1015                         //
1016                         // If left is a nullable type and an implicit conversion exists from right to underlying type of left,
1017                         // the result is underlying type of left
1018                         //
1019                         if (TypeManager.IsNullableType (ltype)) {
1020                                 unwrap = Unwrap.Create (left, ec);
1021                                 if (unwrap == null)
1022                                         return null;
1023
1024                                 if (Convert.ImplicitConversionExists (ec, right, unwrap.Type)) {
1025                                         left = unwrap;
1026                                         type = left.Type;
1027                                         right = Convert.ImplicitConversion (ec, right, type, loc);
1028                                         return this;
1029                                 }
1030                         } else if (TypeManager.IsReferenceType (ltype)) {
1031                                 if (Convert.ImplicitConversionExists (ec, right, ltype)) {
1032                                         //
1033                                         // Reduce (constant ?? expr) to constant
1034                                         //
1035                                         Constant lc = left as Constant;
1036                                         if (lc != null && !lc.IsDefaultValue)
1037                                                 return new SideEffectConstant (lc, right, loc).Resolve (ec);
1038
1039                                         //
1040                                         // Reduce (left ?? null) to left OR (null-constant ?? right) to right
1041                                         //
1042                                         if (right.IsNull || lc != null)
1043                                                 return ReducedExpression.Create (lc != null ? right : left, this).Resolve (ec);
1044
1045                                         right = Convert.ImplicitConversion (ec, right, ltype, loc);
1046                                         type = left.Type;
1047                                         return this;
1048                                 }
1049                         } else {
1050                                 return null;
1051                         }
1052
1053                         Type rtype = right.Type;
1054                         if (!Convert.ImplicitConversionExists (ec, unwrap != null ? unwrap : left, rtype) || right.eclass == ExprClass.MethodGroup)
1055                                 return null;
1056
1057                         //
1058                         // Reduce (null ?? right) to right
1059                         //
1060                         if (left.IsNull)
1061                                 return ReducedExpression.Create (right, this).Resolve (ec);
1062
1063                         left = Convert.ImplicitConversion (ec, unwrap != null ? unwrap : left, rtype, loc);
1064                         type = rtype;
1065                         return this;
1066                 }
1067
1068                 public override Expression DoResolve (EmitContext ec)
1069                 {
1070                         if (eclass != ExprClass.Invalid)
1071                                 return this;
1072
1073                         left = left.Resolve (ec);
1074                         right = right.Resolve (ec);
1075
1076                         if (left == null || right == null)
1077                                 return null;
1078
1079                         eclass = ExprClass.Value;
1080
1081                         Expression e = ConvertExpression (ec);
1082                         if (e == null) {
1083                                 Binary.Error_OperatorCannotBeApplied (left, right, "??", loc);
1084                                 return null;
1085                         }
1086
1087                         return e;
1088                 }
1089
1090                 public override void Emit (EmitContext ec)
1091                 {
1092                         ILGenerator ig = ec.ig;
1093
1094                         Label end_label = ig.DefineLabel ();
1095
1096                         if (unwrap != null) {
1097                                 Label is_null_label = ig.DefineLabel ();
1098
1099                                 unwrap.EmitCheck (ec);
1100                                 ig.Emit (OpCodes.Brfalse, is_null_label);
1101
1102                                 left.Emit (ec);
1103                                 ig.Emit (OpCodes.Br, end_label);
1104
1105                                 ig.MarkLabel (is_null_label);
1106                                 right.Emit (ec);
1107
1108                                 ig.MarkLabel (end_label);
1109                                 return;
1110                         }
1111
1112                         left.Emit (ec);
1113
1114                         ig.Emit (OpCodes.Dup);
1115                         ig.Emit (OpCodes.Brtrue, end_label);
1116
1117                         ig.Emit (OpCodes.Pop);
1118                         right.Emit (ec);
1119
1120                         ig.MarkLabel (end_label);
1121                 }
1122
1123                 public override void MutateHoistedGenericType (AnonymousMethodStorey storey)
1124                 {
1125                         left.MutateHoistedGenericType (storey);
1126                         right.MutateHoistedGenericType (storey);
1127                         type = storey.MutateType (type);
1128                 }
1129
1130                 protected override void CloneTo (CloneContext clonectx, Expression t)
1131                 {
1132                         NullCoalescingOperator target = (NullCoalescingOperator) t;
1133
1134                         target.left = left.Clone (clonectx);
1135                         target.right = right.Clone (clonectx);
1136                 }
1137         }
1138
1139         public class LiftedUnaryMutator : ExpressionStatement
1140         {
1141                 public readonly UnaryMutator.Mode Mode;
1142                 Expression expr;
1143                 UnaryMutator underlying;
1144                 Unwrap unwrap;
1145
1146                 public LiftedUnaryMutator (UnaryMutator.Mode mode, Expression expr, Location loc)
1147                 {
1148                         this.expr = expr;
1149                         this.Mode = mode;
1150                         this.loc = loc;
1151
1152                         eclass = ExprClass.Value;
1153                 }
1154
1155                 public override Expression CreateExpressionTree (EmitContext ec)
1156                 {
1157                         return new SimpleAssign (this, this).CreateExpressionTree (ec);
1158                 }
1159
1160                 public override Expression DoResolve (EmitContext ec)
1161                 {
1162                         expr = expr.Resolve (ec);
1163                         if (expr == null)
1164                                 return null;
1165
1166                         unwrap = Unwrap.Create (expr, ec);
1167                         if (unwrap == null)
1168                                 return null;
1169
1170                         underlying = (UnaryMutator) new UnaryMutator (Mode, unwrap).Resolve (ec);
1171                         if (underlying == null)
1172                                 return null;
1173
1174                         type = expr.Type;
1175                         return this;
1176                 }
1177
1178                 void DoEmit (EmitContext ec, bool is_expr)
1179                 {
1180                         ILGenerator ig = ec.ig;
1181                         Label is_null_label = ig.DefineLabel ();
1182                         Label end_label = ig.DefineLabel ();
1183
1184                         unwrap.EmitCheck (ec);
1185                         ig.Emit (OpCodes.Brfalse, is_null_label);
1186
1187                         if (is_expr) {
1188                                 underlying.Emit (ec);
1189                                 ig.Emit (OpCodes.Br_S, end_label);
1190                         } else {
1191                                 underlying.EmitStatement (ec);
1192                         }
1193
1194                         ig.MarkLabel (is_null_label);
1195                         if (is_expr)
1196                                 LiftedNull.Create (type, loc).Emit (ec);
1197
1198                         ig.MarkLabel (end_label);
1199                 }
1200
1201                 public override void Emit (EmitContext ec)
1202                 {
1203                         DoEmit (ec, true);
1204                 }
1205
1206                 public override void EmitStatement (EmitContext ec)
1207                 {
1208                         DoEmit (ec, false);
1209                 }
1210         }
1211 }
1212