Move AppDomain local data store to AppContext and cleanup AppContext (#21180)

Contributes to #21028

1 files changed, 39 insertions, 0 deletions

diff --git a/src/System.Private.CoreLib/shared/System/LocalAppContextSwitches.Common.cs b/src/System.Private.CoreLib/shared/System/LocalAppContextSwitches.Common.cs
new file mode 100644
index 0000000000..521848f2ef
--- /dev/null
+++ b/src/System.Private.CoreLib/shared/System/LocalAppContextSwitches.Common.cs
@@ -0,0 +1,39 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Runtime.CompilerServices;
+
+namespace System
+{
+    // Helper method for local caching of compatibility quirks. Keep this lean and simple - this file is included into
+    // every framework assembly that implements any compatibility quirks.
+    internal static partial class LocalAppContextSwitches
+    {
+        // Returns value of given switch using provided cache.
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool GetCachedSwitchValue(string switchName, ref int cachedSwitchValue)
+        {
+            // The cached switch value has 3 states: 0 - unknown, 1 - true, -1 - false
+            if (cachedSwitchValue < 0) return false;
+            if (cachedSwitchValue > 0) return true;
+
+            return GetCachedSwitchValueInternal(switchName, ref cachedSwitchValue);
+        }
+
+        private static bool GetCachedSwitchValueInternal(string switchName, ref int cachedSwitchValue)
+        {
+            bool isSwitchEnabled;
+            AppContext.TryGetSwitch(switchName, out isSwitchEnabled);
+
+            AppContext.TryGetSwitch(@"TestSwitch.LocalAppContext.DisableCaching", out bool disableCaching);
+            if (!disableCaching)
+            {
+                cachedSwitchValue = isSwitchEnabled ? 1 /*true*/ : -1 /*false*/;
+            }
+
+            return isSwitchEnabled;
+        }
+    }
+}