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 Dictionary<string, SwitchValueState> s_switchMap = new Dictionary<string, SwitchValueState>();
22 private static readonly object s_syncLock = new object();
24 public static string BaseDirectory
27 [System.Security.SecuritySafeCritical]
31 // The value of APP_CONTEXT_BASE_DIRECTORY key has to be a string and it is not allowed to be any other type.
32 // Otherwise the caller will get invalid cast exception
33 return (string) AppDomain.CurrentDomain.GetData("APP_CONTEXT_BASE_DIRECTORY") ?? AppDomain.CurrentDomain.BaseDirectory;
41 // populate the AppContext with the default set of values
42 AppContextDefaultValues.PopulateDefaultValues();
46 /// Try to get the value of the switch.
48 /// <param name="switchName">The name of the switch</param>
49 /// <param name="isEnabled">A variable where to place the value of the switch</param>
50 /// <returns>A return value of true represents that the switch was set and <paramref name="isEnabled"/> contains the value of the switch</returns>
51 public static bool TryGetSwitch(string switchName, out bool isEnabled)
53 if (switchName == null)
54 throw new ArgumentNullException("switchName");
55 if (switchName.Length == 0)
56 throw new ArgumentException(Environment.GetResourceString("Argument_EmptyName"), "switchName");
58 // By default, the switch is not enabled.
61 SwitchValueState switchValue;
64 if (s_switchMap.TryGetValue(switchName, out switchValue))
66 // The value is in the dictionary.
67 // There are 3 cases here:
68 // 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).
69 // 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
70 // Will be unknown to mscorlib.dll and we want to prevent checking the overrides every time we check this switch
71 // 2. The switch has a valid value AND we have read the overrides for it
72 // Example: TryGetSwitch is called for a switch set via SetSwitch
73 // 3. The switch has the default value and we need to check for overrides
74 // Example: TryGetSwitch is called for the first time for a switch that has a default value
76 // 1. The value is unknown
77 if (switchValue == SwitchValueState.UnknownValue)
83 // We get the value of isEnabled from the value that we stored in the dictionary
84 isEnabled = (switchValue & SwitchValueState.HasTrueValue) == SwitchValueState.HasTrueValue;
86 // 2. The switch has a valid value AND we have checked for overrides
87 if ((switchValue & SwitchValueState.HasLookedForOverride) == SwitchValueState.HasLookedForOverride)
92 // 3. The switch has a valid value, but we need to check for overrides.
93 // Regardless of whether or not the switch has an override, we need to update the value to reflect
94 // the fact that we checked for overrides.
96 if (AppContextDefaultValues.TryGetSwitchOverride(switchName, out overrideValue))
98 // we found an override!
99 isEnabled = overrideValue;
102 // Update the switch in the dictionary to mark it as 'checked for override'
103 s_switchMap[switchName] = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
104 | SwitchValueState.HasLookedForOverride;
110 // The value is NOT in the dictionary
111 // In this case we need to see if we have an override defined for the value.
112 // There are 2 cases:
113 // 1. The value has an override specified. In this case we need to add the value to the dictionary
114 // and mark it as checked for overrides
115 // Example: In a servicing event, System.Xml introduces a switch and an override is specified.
116 // The value is not found in mscorlib (as System.Xml ships independent of mscorlib)
117 // 2. The value does not have an override specified
118 // In this case, we want to capture the fact that we looked for a value and found nothing by adding
119 // an entry in the dictionary with the 'sentinel' value of 'SwitchValueState.UnknownValue'.
120 // Example: This will prevent us from trying to find overrides for values that we don't have in the dictionary
122 // 1. The value has an override specified.
124 if (AppContextDefaultValues.TryGetSwitchOverride(switchName, out overrideValue))
126 isEnabled = overrideValue;
128 // Update the switch in the dictionary to mark it as 'checked for override'
129 s_switchMap[switchName] = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
130 | SwitchValueState.HasLookedForOverride;
135 // 2. The value does not have an override.
136 s_switchMap[switchName] = SwitchValueState.UnknownValue;
139 return false; // we did not find a value for the switch
143 /// Assign a switch a value
145 /// <param name="switchName">The name of the switch</param>
146 /// <param name="isEnabled">The value to assign</param>
147 public static void SetSwitch(string switchName, bool isEnabled)
149 if (switchName == null)
150 throw new ArgumentNullException("switchName");
151 if (switchName.Length == 0)
152 throw new ArgumentException(Environment.GetResourceString("Argument_EmptyName"), "switchName");
156 // Store the new value and the fact that we checked in the dictionary
157 s_switchMap[switchName] = (isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue)
158 | SwitchValueState.HasLookedForOverride;
163 /// This method is going to be called from the AppContextDefaultValues class when setting up the
164 /// default values for the switches. !!!! This method is called during the static constructor so it does not
165 /// take a lock !!!! If you are planning to use this outside of that, please ensure proper locking.
167 internal static void DefineSwitchDefault(string switchName, bool isEnabled)
169 s_switchMap[switchName] = isEnabled ? SwitchValueState.HasTrueValue : SwitchValueState.HasFalseValue;