Keep tracking the file offset in memory, don't perform expensive Seek calls on every Read|WriteAsync

src/coreclr/System.Private.CoreLib/src/System/Threading/ClrThreadPoolPreAllocatedOverlapped.cs

@@ -95,5 +95,7 @@ unsafe void IDeferredDisposable.OnFinalRelease(bool disposed)
                 }
             }
         }
+
+        internal bool IsUserObject(byte[]? buffer) => _overlapped.IsUserObject(buffer);
     }
 }

src/coreclr/System.Private.CoreLib/src/System/Threading/Overlapped.cs

@@ -132,6 +132,8 @@ internal sealed unsafe class OverlappedData
             return AllocateNativeOverlapped();
         }
 
+        internal bool IsUserObject(byte[]? buffer) => ReferenceEquals(_userObject, buffer);
+
         [MethodImpl(MethodImplOptions.InternalCall)]
         private extern NativeOverlapped* AllocateNativeOverlapped();
 
@@ -258,6 +260,8 @@ public static unsafe void Free(NativeOverlapped* nativeOverlappedPtr)
             OverlappedData.GetOverlappedFromNative(nativeOverlappedPtr)._overlapped._overlappedData = null;
             OverlappedData.FreeNativeOverlapped(nativeOverlappedPtr);
         }
+
+        internal bool IsUserObject(byte[]? buffer) => _overlappedData!.IsUserObject(buffer);
     }
 
     #endregion class Overlapped

src/libraries/Common/src/Interop/Windows/Kernel32/Interop.ReadFile_SafeHandle_NativeOverlapped.cs

@@ -16,5 +16,13 @@ internal static extern unsafe int ReadFile(
             int numBytesToRead,
             IntPtr numBytesRead_mustBeZero,
             NativeOverlapped* overlapped);
+
+        [DllImport(Libraries.Kernel32, SetLastError = true)]
+        internal static extern unsafe int ReadFile(
+            SafeHandle handle,
+            byte* bytes,
+            int numBytesToRead,
+            out int numBytesRead,
+            NativeOverlapped* overlapped);
     }
 }

src/libraries/Common/src/Interop/Windows/Kernel32/Interop.WriteFile_SafeHandle_NativeOverlapped.cs

@@ -17,5 +17,8 @@ internal static partial class Kernel32
         // and pass in an address for the numBytesRead parameter.
         [DllImport(Libraries.Kernel32, SetLastError = true)]
         internal static extern unsafe int WriteFile(SafeHandle handle, byte* bytes, int numBytesToWrite, IntPtr numBytesWritten_mustBeZero, NativeOverlapped* lpOverlapped);
+
+        [DllImport(Libraries.Kernel32, SetLastError = true)]
+        internal static extern unsafe int WriteFile(SafeHandle handle, byte* bytes, int numBytesToWrite, out int numBytesWritten, NativeOverlapped* lpOverlapped);
     }
 }

src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs

@@ -182,14 +182,14 @@ public static string GetDistroVersionString()
             }
         }
 
-        private static readonly Lazy<bool> m_isInvariant = new Lazy<bool>(GetIsInvariantGlobalization);
+        private static readonly Lazy<bool> m_isInvariant = new Lazy<bool>(() => GetStaticNonPublicBooleanPropertyValue("System.Globalization.GlobalizationMode", "Invariant"));
 
-        private static bool GetIsInvariantGlobalization()
+        private static bool GetStaticNonPublicBooleanPropertyValue(string typeName, string propertyName)
         {
-            Type globalizationMode = Type.GetType("System.Globalization.GlobalizationMode");
+            Type globalizationMode = Type.GetType(typeName);
             if (globalizationMode != null)
             {
-                MethodInfo methodInfo = globalizationMode.GetProperty("Invariant", BindingFlags.NonPublic | BindingFlags.Static)?.GetMethod;
+                MethodInfo methodInfo = globalizationMode.GetProperty(propertyName, BindingFlags.NonPublic | BindingFlags.Static)?.GetMethod;
                 if (methodInfo != null)
                 {
                     return (bool)methodInfo.Invoke(null, null);
@@ -230,6 +230,10 @@ private static Version GetICUVersion()
                               version & 0xFF);
         }
 
+        private static readonly Lazy<bool> _legacyFileStream = new Lazy<bool>(() => GetStaticNonPublicBooleanPropertyValue("System.IO.FileStreamHelpers", "UseLegacyStrategy"));
+
+        public static bool IsLegacyFileStreamEnabled => _legacyFileStream.Value;
+
         private static bool GetIsInContainer()
         {
             if (IsWindows)

src/libraries/System.IO.FileSystem/System.IO.FileSystem.sln

@@ -25,6 +25,8 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "ref", "ref", "{32A31E04-255
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{D9FB1730-B750-4C0D-8D24-8C992DEB6034}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.IO.FileSystem.Legacy.Tests", "tests\LegacyTests\System.IO.FileSystem.Legacy.Tests.csproj", "{48E07F12-8597-40DE-8A37-CCBEB9D54012}"
+EndProject
 Global
 	GlobalSection(NestedProjects) = preSolution
 		{D350D6E7-52F1-40A4-B646-C178F6BBB689} = {1A727AF9-4F39-4109-BB8F-813286031DC9}
@@ -37,6 +39,7 @@ Global
 		{06DD5AA8-FDA6-495B-A8D1-8CE83C78DE6C} = {32A31E04-2554-4223-BED8-45757408B4F6}
 		{877E39A8-51CB-463A-AF4C-6FAE4F438075} = {D9FB1730-B750-4C0D-8D24-8C992DEB6034}
 		{D7DF8034-3AE5-4DEF-BCC4-6353239391BF} = {D9FB1730-B750-4C0D-8D24-8C992DEB6034}
+		{48E07F12-8597-40DE-8A37-CCBEB9D54012} = {1A727AF9-4F39-4109-BB8F-813286031DC9}
 	EndGlobalSection
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -83,6 +86,10 @@ Global
 		{06DD5AA8-FDA6-495B-A8D1-8CE83C78DE6C}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{06DD5AA8-FDA6-495B-A8D1-8CE83C78DE6C}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{06DD5AA8-FDA6-495B-A8D1-8CE83C78DE6C}.Release|Any CPU.Build.0 = Release|Any CPU
+		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{48E07F12-8597-40DE-8A37-CCBEB9D54012}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

src/libraries/System.IO.FileSystem/tests/FileStream/SafeFileHandle.cs

@@ -58,13 +58,13 @@ public void AccessFlushesFileClosesHandle()
             }
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsLegacyFileStreamEnabled))]
         public async Task ThrowWhenHandlePositionIsChanged_sync()
         {
             await ThrowWhenHandlePositionIsChanged(useAsync: false);
         }
 
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported), nameof(PlatformDetection.IsLegacyFileStreamEnabled))]
         public async Task ThrowWhenHandlePositionIsChanged_async()
         {
             await ThrowWhenHandlePositionIsChanged(useAsync: true);
@@ -104,14 +104,7 @@ private async Task ThrowWhenHandlePositionIsChanged(bool useAsync)
 
                     fs.WriteByte(0);
                     fsr.Position++;
-                    if (useAsync && OperatingSystem.IsWindows()) // Async I/O behaviors differ due to kernel-based implementation on Windows
-                    {
-                        Assert.Throws<IOException>(() => FSAssert.CompletesSynchronously(fs.ReadAsync(new byte[1], 0, 1)));
-                    }
-                    else
-                    {
-                        await Assert.ThrowsAsync<IOException>(() => fs.ReadAsync(new byte[1], 0, 1));
-                    }
+                    await Assert.ThrowsAsync<IOException>(() => fs.ReadAsync(new byte[1], 0, 1));
 
                     fs.WriteByte(0);
                     fsr.Position++;

src/libraries/System.IO.FileSystem/tests/FileStream/WriteAsync.cs

@@ -220,7 +220,7 @@ public async Task ManyConcurrentWriteAsyncs_OuterLoop(
                 {
                     writes[i] = WriteAsync(fs, expectedData, i * writeSize, writeSize, cancellationToken);
                     Assert.Null(writes[i].Exception);
-                    if (useAsync)
+                    if (useAsync && PlatformDetection.IsLegacyFileStreamEnabled)
                     {
                         Assert.Equal((i + 1) * writeSize, fs.Position);
                     }

src/libraries/System.IO.FileSystem/tests/LegacyTests/LegacySwitchTests.cs

@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Reflection;
+using Xunit;
+
+namespace System.IO.Tests
+{
+    public class LegacySwitchTests
+    {
+        [Fact]
+        public static void LegacySwitchIsHonored()
+        {
+            string filePath = Path.Combine(Path.GetTempPath(), Path.GetTempFileName());
+
+            using (FileStream fileStream = File.Create(filePath))
+            {
+                Stream strategy = fileStream
+                    .GetType()
+                    .GetField("_strategy", BindingFlags.NonPublic | BindingFlags.Instance)
+                    .GetValue(fileStream) as Stream;
+
+                Assert.DoesNotContain(strategy.GetType().FullName, "Legacy");
+            }
+
+            File.Delete(filePath);
+        }
+    }
+}

src/libraries/System.IO.FileSystem/tests/LegacyTests/System.IO.FileSystem.Legacy.Tests.csproj

@@ -0,0 +1,42 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
+    <!-- currently we have non Legacy strategy only for Windows, so we run the tests only on Windows -->
+    <TargetFrameworks>$(NetCoreAppCurrent)-windows</TargetFrameworks>
+
+    <RunTestsJSArguments>--working-dir=/test-dir</RunTestsJSArguments>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="..\**\*.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetsUnix)' == 'true'">
+    <Compile Remove="..\**\*.Windows.cs" />
+    <Compile Remove="..\**\*.Browser.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetsWindows)' == 'true'">
+    <Compile Remove="..\**\*.Unix.cs" />
+    <Compile Remove="..\**\*.Browser.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(TargetsBrowser)' == 'true'">
+    <Compile Remove="..\**\*.Unix.cs" />
+    <Compile Remove="..\**\*.Windows.cs" />
+  </ItemGroup>  
+  <ItemGroup>
+    <!-- Helpers -->
+    <Compile Include="$(CommonPath)Interop\Windows\Kernel32\Interop.GenericOperations.cs" Link="Interop\Windows\Interop.GenericOperations.cs" />
+    <Compile Include="$(CommonTestPath)System\Buffers\NativeMemoryManager.cs" Link="Common\System\Buffers\NativeMemoryManager.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\TempFile.cs" Link="Common\System\IO\TempFile.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\PathFeatures.cs" Link="Common\System\IO\PathFeatures.cs" />
+
+    <Content Include="..\DirectoryInfo\test-dir\dummy.txt" Link="test-dir\dummy.txt" />
+
+    <Compile Include="$(CommonTestPath)Tests\System\IO\StreamConformanceTests.cs" Link="Common\System\IO\StreamConformanceTests.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\ConnectedStreams.cs" Link="Common\System\IO\ConnectedStreams.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\CallTrackingStream.cs" Link="Common\System\IO\CallTrackingStream.cs" />
+    <Compile Include="$(CommonPath)System\Net\ArrayBuffer.cs" Link="ProductionCode\Common\System\Net\ArrayBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Net\MultiArrayBuffer.cs" Link="ProductionCode\Common\System\Net\MultiArrayBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Net\StreamBuffer.cs" Link="ProductionCode\Common\System\Net\StreamBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs" Link="Common\System\Threading\Tasks\TaskToApm.cs" />
+  </ItemGroup>
+</Project>

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

@@ -0,0 +1,5 @@
+{
+    "configProperties": {
+        "System.IO.UseLegacyFileStream": true
+    }
+}

src/libraries/System.IO/System.IO.sln

@@ -23,6 +23,8 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "ref", "ref", "{9FDAA57A-696
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{D9FD8082-D04C-4DA8-9F4C-261D1C65A6D3}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "System.IO.Legacy.Tests", "tests\LegacyTests\System.IO.Legacy.Tests.csproj", "{0217540D-FA86-41B3-9754-7BB5096ABA3E}"
+EndProject
 Global
 	GlobalSection(NestedProjects) = preSolution
 		{D11D3624-1322-45D1-A604-7E68CDB85BE8} = {5AD2C433-C661-4AD1-BD9F-D164ADC43512}
@@ -34,6 +36,7 @@ Global
 		{D0D1CDAC-16F8-4382-A219-74A513CC1790} = {9FDAA57A-696B-4CB1-99AE-BCDF91848B75}
 		{0769544B-1A5D-4D74-94FD-899DF6C39D62} = {D9FD8082-D04C-4DA8-9F4C-261D1C65A6D3}
 		{AA5E80B2-A0AA-46F1-B319-5B528BAC382B} = {D9FD8082-D04C-4DA8-9F4C-261D1C65A6D3}
+		{0217540D-FA86-41B3-9754-7BB5096ABA3E} = {5AD2C433-C661-4AD1-BD9F-D164ADC43512}
 	EndGlobalSection
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -76,6 +79,10 @@ Global
 		{D0D1CDAC-16F8-4382-A219-74A513CC1790}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{D0D1CDAC-16F8-4382-A219-74A513CC1790}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{D0D1CDAC-16F8-4382-A219-74A513CC1790}.Release|Any CPU.Build.0 = Release|Any CPU
+		{0217540D-FA86-41B3-9754-7BB5096ABA3E}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{0217540D-FA86-41B3-9754-7BB5096ABA3E}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{0217540D-FA86-41B3-9754-7BB5096ABA3E}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{0217540D-FA86-41B3-9754-7BB5096ABA3E}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

src/libraries/System.IO/tests/LegacyTests/System.IO.Legacy.Tests.csproj

@@ -0,0 +1,24 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <RootNamespace>System.IO</RootNamespace>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <TestRuntime>true</TestRuntime>
+    <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
+    <!-- currently we have non Legacy strategy only for Windows, so we run the tests only on Windows -->
+    <TargetFrameworks>$(NetCoreAppCurrent)-windows</TargetFrameworks>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="..\**\*.cs" />
+    <!-- Helpers -->
+    <Compile Include="$(CommonTestPath)System\Buffers\NativeMemoryManager.cs" Link="Common\System\Buffers\NativeMemoryManager.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\CallTrackingStream.cs" Link="Common\System\IO\CallTrackingStream.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\DelegateStream.cs" Link="Common\System\IO\DelegateStream.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\WrappedMemoryStream.cs" Link="Common\System\IO\WrappedMemoryStream.cs" />
+    <Compile Include="$(CommonTestPath)Tests\System\IO\StreamConformanceTests.cs" Link="Common\System\IO\StreamConformanceTests.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\ConnectedStreams.cs" Link="Common\System\IO\ConnectedStreams.cs" />
+    <Compile Include="$(CommonPath)System\Net\ArrayBuffer.cs" Link="ProductionCode\Common\System\Net\ArrayBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Net\MultiArrayBuffer.cs" Link="ProductionCode\Common\System\Net\MultiArrayBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Net\StreamBuffer.cs" Link="ProductionCode\Common\System\Net\StreamBuffer.cs" />
+    <Compile Include="$(CommonPath)System\Threading\Tasks\TaskToApm.cs" Link="Common\System\Threading\Tasks\TaskToApm.cs" />
+  </ItemGroup>
+</Project>

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

@@ -0,0 +1,5 @@
+{
+    "configProperties": {
+        "System.IO.UseLegacyFileStream": true
+    }
+}

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -393,6 +393,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\BinaryReader.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\BinaryWriter.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\BufferedStream.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\BufferedFileStreamStrategy.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DerivedFileStreamStrategy.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DirectoryNotFoundException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\EncodingCache.cs" />
@@ -1634,14 +1635,17 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\GlobalizationMode.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\HijriCalendar.Win32.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Guid.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\AsyncWindowsFileStreamStrategy.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DisableMediaInsertionPrompt.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DriveInfoInternal.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamHelpers.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamCompletionSource.Win32.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\LegacyFileStreamStrategy.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\Path.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathHelper.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathInternal.Windows.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\LegacyFileStreamStrategy.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\SyncWindowsFileStreamStrategy.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\WindowsFileStreamStrategy.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\PasteArguments.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\Loader\LibraryNameVariation.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Runtime\MemoryFailPoint.Windows.cs" />
@@ -1892,6 +1896,7 @@
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsBrowser)' == 'true'">
     <Compile Include="$(MSBuildThisFileDirectory)System\AppContext.Browser.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\AppContextConfigHelper.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Environment.Browser.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DriveInfoInternal.Browser.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PersistedFiles.Browser.cs" />

src/libraries/System.Private.CoreLib/src/System/AppContextConfigHelper.cs

@@ -10,6 +10,20 @@ internal static class AppContextConfigHelper
         internal static bool GetBooleanConfig(string configName, bool defaultValue) =>
             AppContext.TryGetSwitch(configName, out bool value) ? value : defaultValue;
 
+        internal static bool GetBooleanConfig(string switchName, string envVariable)
+        {
+            if (!AppContext.TryGetSwitch(switchName, out bool ret))
+            {
+                string? switchValue = Environment.GetEnvironmentVariable(envVariable);
+                if (switchValue != null)
+                {
+                    ret = bool.IsTrueStringIgnoreCase(switchValue) || switchValue.Equals("1");
+                }
+            }
+
+            return ret;
+        }
+
         internal static int GetInt32Config(string configName, int defaultValue, bool allowNegative = true)
         {
             try

src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.Windows.cs

@@ -10,7 +10,7 @@ internal static partial class GlobalizationMode
         internal static bool Invariant { get; } = GetInvariantSwitchValue();
 
         internal static bool UseNls { get; } = !Invariant &&
-            (GetSwitchValue("System.Globalization.UseNls", "DOTNET_SYSTEM_GLOBALIZATION_USENLS") ||
+            (AppContextConfigHelper.GetBooleanConfig("System.Globalization.UseNls", "DOTNET_SYSTEM_GLOBALIZATION_USENLS") ||
                 !LoadIcu());
 
         private static bool LoadIcu()

src/libraries/System.Private.CoreLib/src/System/Globalization/GlobalizationMode.cs

@@ -9,25 +9,11 @@ namespace System.Globalization
     internal static partial class GlobalizationMode
     {
         private static bool GetInvariantSwitchValue() =>
-            GetSwitchValue("System.Globalization.Invariant", "DOTNET_SYSTEM_GLOBALIZATION_INVARIANT");
+            AppContextConfigHelper.GetBooleanConfig("System.Globalization.Invariant", "DOTNET_SYSTEM_GLOBALIZATION_INVARIANT");
 
         private static bool TryGetAppLocalIcuSwitchValue([NotNullWhen(true)] out string? value) =>
             TryGetStringValue("System.Globalization.AppLocalIcu", "DOTNET_SYSTEM_GLOBALIZATION_APPLOCALICU", out value);
 
-        private static bool GetSwitchValue(string switchName, string envVariable)
-        {
-            if (!AppContext.TryGetSwitch(switchName, out bool ret))
-            {
-                string? switchValue = Environment.GetEnvironmentVariable(envVariable);
-                if (switchValue != null)
-                {
-                    ret = bool.IsTrueStringIgnoreCase(switchValue) || switchValue.Equals("1");
-                }
-            }
-
-            return ret;
-        }
-
         private static bool TryGetStringValue(string switchName, string envVariable, [NotNullWhen(true)] out string? value)
         {
             value = AppContext.GetData(switchName) as string;