Disable Net5CompatFileStream by default

src/libraries/System.IO.FileSystem/tests/Net5CompatTests/Net5CompatSwitchTests.cs

@@ -20,7 +20,7 @@ public static void LegacySwitchIsHonored()
                     .GetField("_strategy", BindingFlags.NonPublic | BindingFlags.Instance)
                     .GetValue(fileStream);
 
-                Assert.DoesNotContain("Net5Compat", strategy.GetType().FullName);
+                Assert.Contains("Net5Compat", strategy.GetType().FullName);
             }
 
             File.Delete(filePath);

src/libraries/System.IO.FileSystem/tests/Net5CompatTests/runtimeconfig.template.json

@@ -1,5 +1,5 @@
 {
     "configProperties": {
-        "System.IO.UseNet5CompatFileStream": false
+        "System.IO.UseNet5CompatFileStream": true
     }
 }

src/libraries/System.IO/tests/Net5CompatTests/runtimeconfig.template.json

@@ -1,5 +1,5 @@
 {
     "configProperties": {
-        "System.IO.UseNet5CompatFileStream": false
+        "System.IO.UseNet5CompatFileStream": true
     }
 }

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/FileStreamHelpers.cs

@@ -7,21 +7,7 @@ namespace System.IO.Strategies
 {
     internal static partial class FileStreamHelpers
     {
-        // It's enabled by default. We are going to change that once we fix #16354, #25905 and #24847.
-        internal static bool UseNet5CompatStrategy { get; } = GetNet5CompatFileStreamSetting();
-
-        private static bool GetNet5CompatFileStreamSetting()
-        {
-            if (AppContext.TryGetSwitch("System.IO.UseNet5CompatFileStream", out bool fileConfig))
-            {
-                return fileConfig;
-            }
-
-            string? envVar = Environment.GetEnvironmentVariable("DOTNET_SYSTEM_IO_USENET5COMPATFILESTREAM");
-            return envVar is null
-                ? true // Net5Compat is currently enabled by default;
-                : bool.IsTrueStringIgnoreCase(envVar) || envVar.Equals("1");
-        }
+        internal static bool UseNet5CompatStrategy { get; } = AppContextConfigHelper.GetBooleanConfig("System.IO.UseNet5CompatFileStream", "DOTNET_SYSTEM_IO_USENET5COMPATFILESTREAM");
 
         internal static FileStreamStrategy ChooseStrategy(FileStream fileStream, SafeFileHandle handle, FileAccess access, FileShare share, int bufferSize, bool isAsync)
             => WrapIfDerivedType(fileStream, ChooseStrategyCore(handle, access, share, bufferSize, isAsync));