Imported Upstream version 4.3.2.467

Former-commit-id: 9c2cb47f45fa221e661ab616387c9cda183f283d

external/referencesource/System.ServiceModel/InternalApis/Clr/inc/LocalAppContext.cs

@@ -0,0 +1,128 @@
+// ==++==
+// 
+//   Copyright (c) Microsoft Corporation.  All rights reserved.
+// 
+// ==--==
+
+// NOTE: This file should not be included in mscorlib. This should only be included in FX libraries that need to provide switches
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Threading;
+
+namespace System
+{
+    internal static partial class LocalAppContext
+    {
+        private delegate bool TryGetSwitchDelegate(string switchName, out bool value);
+
+        private static TryGetSwitchDelegate TryGetSwitchFromCentralAppContext;
+        private static bool s_canForwardCalls;
+
+        private static Dictionary<string, bool> s_switchMap = new Dictionary<string, bool>();
+        private static readonly object s_syncLock = new object();
+
+        private static bool DisableCaching { get; set; }
+
+        static LocalAppContext()
+        {
+            // Try to setup the callback into the central AppContext
+            s_canForwardCalls = SetupDelegate();
+
+            // Populate the default values of the local app context 
+            AppContextDefaultValues.PopulateDefaultValues();
+
+            // Cache the value of the switch that help with testing
+            DisableCaching = IsSwitchEnabled(@"TestSwitch.LocalAppContext.DisableCaching");
+        }
+
+        public static bool IsSwitchEnabled(string switchName)
+        {
+            if (s_canForwardCalls)
+            {
+                bool isEnabledCentrally;
+                if (TryGetSwitchFromCentralAppContext(switchName, out isEnabledCentrally))
+                {
+                    // we found the switch, so return whatever value it has
+                    return isEnabledCentrally;
+                }
+                // if we could not get the value from the central authority, try the local storage.
+            }
+
+            return IsSwitchEnabledLocal(switchName);
+        }
+
+        private static bool IsSwitchEnabledLocal(string switchName)
+        {
+            // read the value from the set of local defaults
+            bool isEnabled, isPresent;
+            lock (s_switchMap)
+            {
+                isPresent = s_switchMap.TryGetValue(switchName, out isEnabled);
+            }
+
+            // If the value is in the set of local switches, reutrn the value
+            if (isPresent)
+            {
+                return isEnabled;
+            }
+
+            // if we could not find the switch name, we should return 'false'
+            // This will preserve the concept of switches been 'off' unless explicitly set to 'on'
+            return false;
+        }
+
+        private static bool SetupDelegate()
+        {
+            Type appContextType = typeof(object).Assembly.GetType("System.AppContext");
+            if (appContextType == null)
+                return false;
+
+            MethodInfo method = appContextType.GetMethod(
+                                            "TryGetSwitch",  // the method name
+                                            BindingFlags.Static | BindingFlags.Public,  // binding flags
+                                            null, // use the default binder
+                                            new Type[] { typeof(string), typeof(bool).MakeByRefType() },
+                                            null); // parameterModifiers - this is ignored by the default binder 
+            if (method == null)
+                return false;
+
+            // Create delegate if we found the method.
+            TryGetSwitchFromCentralAppContext = (TryGetSwitchDelegate)Delegate.CreateDelegate(typeof(TryGetSwitchDelegate), method);
+
+            return true;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool GetCachedSwitchValue(string switchName, ref int switchValue)
+        {
+            if (switchValue < 0) return false;
+            if (switchValue > 0) return true;
+
+            return GetCachedSwitchValueInternal(switchName, ref switchValue);
+        }
+
+        private static bool GetCachedSwitchValueInternal(string switchName, ref int switchValue)
+        {
+            if (LocalAppContext.DisableCaching)
+            {
+                return LocalAppContext.IsSwitchEnabled(switchName);
+            }
+
+            bool isEnabled = LocalAppContext.IsSwitchEnabled(switchName);
+            switchValue = isEnabled ? 1 /*true*/ : -1 /*false*/;
+            return isEnabled;
+        }
+
+        /// <summary>
+        /// This method is going to be called from the AppContextDefaultValues class when setting up the 
+        /// default values for the switches. !!!! This method is called during the static constructor so it does not
+        /// take a lock !!!! If you are planning to use this outside of that, please ensure proper locking.
+        /// </summary>
+        internal static void DefineSwitchDefault(string switchName, bool initialValue)
+        {
+            s_switchMap[switchName] = initialValue;
+        }
+    }
+}