3 // Copyright (c) Microsoft Corporation. All rights reserved.
7 using System.Collections.Generic;
11 public static class AppContext
14 private enum SwitchValueState
18 HasLookedForOverride = 0x4,
19 UnknownValue = 0x8 // Has no default and could not find an override
21 private static readonly Dictionary<string, SwitchValueState> s_switchMap = new Dictionary<string, SwitchValueState>();
23 public static string BaseDirectory
26 [System.Security.SecuritySafeCritical]
30 // The value of APP_CONTEXT_BASE_DIRECTORY key has to be a string and it is not allowed to be any other type.
31 // Otherwise the caller will get invalid cast exception
32 return (string) AppDomain.CurrentDomain.GetData("APP_CONTEXT_BASE_DIRECTORY") ?? AppDomain.CurrentDomain.BaseDirectory;
37 public static string TargetFrameworkName
41 throw new NotImplementedException();
45 public static object GetData (string name)
47 throw new NotImplementedException();
55 // populate the AppContext with the default set of values
56 AppContextDefaultValues.PopulateDefaultValues();
60 /// Try to get the value of the switch.
62 /// <param name="switchName">The name of the switch</param>
63 /// <param name="isEnabled">A variable where to place the value of the switch</param>
64 /// <returns>A return value of true represents that the switch was set and <paramref name="isEnabled"/> contains the value of the switch</returns>
65 public static bool TryGetSwitch(string switchName, out bool isEnabled)
67 if (switchName == null)
68 throw new ArgumentNullException("switchName");
69 if (switchName.Length == 0)
70 throw new ArgumentException(Environment.GetResourceString("Argument_EmptyName"), "switchName");
72 // By default, the switch is not enabled.
75 SwitchValueState switchValue;
78 if (s_switchMap.TryGetValue(switchName, out switchValue))
80 // The value is in the dictionary.
81 // There are 3 cases here:
82 // 1. The value of the switch is 'unknown'. This means that the switch name is not known to the system (either via defaults or checking overrides).
83 // Example: This is the case when, during a servicing event, a switch is added to System.Xml which ships before mscorlib. The value of the switch
84 // Will be unknown to mscorlib.dll and we want to prevent checking the overrides every time we check this switch
85 // 2. The switch has a valid value AND we have read the overrides for it
86 // Example: TryGetSwitch is called for a switch set via SetSwitch
87 // 3. The switch has the default value and we need to check for overrides
88 // Example: TryGetSwitch is called for the first time for a switch that has a default value
90 // 1. The value is unknown
91 if (switchValue == SwitchValueState.UnknownValue)
97 // We get the value of isEnabled from the value that we stored in the dictionary
98 isEnabled = (switchValue & SwitchValueState.HasTrueValue) == SwitchValueState.HasTrueValue;
100 // 2. The switch has a valid value AND we have checked for overrides
101 if ((switchValue & SwitchValueState.HasLookedForOverride) == SwitchValueState.HasLookedForOverride)
106 // 3. The switch has a valid value, but we need to check for overrides.
107 // Regardless of whether or not the switch has an override, we need to update the value to reflect
108 // the fact that we checked for overrides.
110 if (AppContextDefaultValues.TryGetSwitchOverride(switchName, out overrideValue))
112 // we found an override!
113 isEnabled = overrideValue;
116 // Update the switch in the dictionary to mark it as 'checked for override'
117 s_switchMap[switchName] = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
118 | SwitchValueState.HasLookedForOverride;
124 // The value is NOT in the dictionary
125 // In this case we need to see if we have an override defined for the value.
126 // There are 2 cases:
127 // 1. The value has an override specified. In this case we need to add the value to the dictionary
128 // and mark it as checked for overrides
129 // Example: In a servicing event, System.Xml introduces a switch and an override is specified.
130 // The value is not found in mscorlib (as System.Xml ships independent of mscorlib)
131 // 2. The value does not have an override specified
132 // In this case, we want to capture the fact that we looked for a value and found nothing by adding
133 // an entry in the dictionary with the 'sentinel' value of 'SwitchValueState.UnknownValue'.
134 // Example: This will prevent us from trying to find overrides for values that we don't have in the dictionary
136 // 1. The value has an override specified.
138 if (AppContextDefaultValues.TryGetSwitchOverride(switchName, out overrideValue))
140 isEnabled = overrideValue;
142 // Update the switch in the dictionary to mark it as 'checked for override'
143 s_switchMap[switchName] = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
144 | SwitchValueState.HasLookedForOverride;
149 // 2. The value does not have an override.
150 s_switchMap[switchName] = SwitchValueState.UnknownValue;
153 return false; // we did not find a value for the switch
157 /// Assign a switch a value
159 /// <param name="switchName">The name of the switch</param>
160 /// <param name="isEnabled">The value to assign</param>
161 public static void SetSwitch(string switchName, bool isEnabled)
163 if (switchName == null)
164 throw new ArgumentNullException("switchName");
165 if (switchName.Length == 0)
166 throw new ArgumentException(Environment.GetResourceString("Argument_EmptyName"), "switchName");
168 SwitchValueState switchValue = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
169 | SwitchValueState.HasLookedForOverride;
172 // Store the new value and the fact that we checked in the dictionary
173 s_switchMap[switchName] = switchValue;
178 /// This method is going to be called from the AppContextDefaultValues class when setting up the
179 /// default values for the switches. !!!! This method is called during the static constructor so it does not
180 /// take a lock !!!! If you are planning to use this outside of that, please ensure proper locking.
182 internal static void DefineSwitchDefault(string switchName, bool isEnabled)
184 s_switchMap[switchName] = isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue;