From 51378119bd5877cab4c54dc64c853ff3b9dee5fa Mon Sep 17 00:00:00 2001
From: Adam Sitnik <adam.sitnik@gmail.com>
Date: Thu, 1 Apr 2021 16:21:42 +0200
Subject: [PATCH] use a hacky way to run all the tests using new FileStream
 implementation

---
 src/Tests/Common/runtimeconfig.template.json  |  5 ++-
 .../dotnet.Tests/Net5CompatSwitchTests.cs     | 42 +++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 src/Tests/dotnet.Tests/Net5CompatSwitchTests.cs

diff --git a/src/Tests/Common/runtimeconfig.template.json b/src/Tests/Common/runtimeconfig.template.json
index 2c73f3989069..4fe34ff40af3 100644
--- a/src/Tests/Common/runtimeconfig.template.json
+++ b/src/Tests/Common/runtimeconfig.template.json
@@ -1,3 +1,6 @@
 {
-    "rollForwardOnNoCandidateFx": 2
+    "rollForwardOnNoCandidateFx": 2,
+    "configProperties": {
+        "System.IO.UseNet5CompatFileStream": false
+    }
 }
\ No newline at end of file
diff --git a/src/Tests/dotnet.Tests/Net5CompatSwitchTests.cs b/src/Tests/dotnet.Tests/Net5CompatSwitchTests.cs
new file mode 100644
index 000000000000..4bc7296575dc
--- /dev/null
+++ b/src/Tests/dotnet.Tests/Net5CompatSwitchTests.cs
@@ -0,0 +1,42 @@
+// 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.
+
+#if NET6_0
+using System.Reflection;
+using Xunit;
+
+namespace System.IO.Tests
+{
+    public class Net5CompatSwitchTests
+    {
+        [Fact]
+        public static void LegacySwitchIsHonored()
+        {
+            string filePath = Path.Combine(Path.GetTempPath(), Path.GetTempFileName());
+
+            using (FileStream fileStream = File.Create(filePath))
+            {
+                object strategy = fileStream?
+                        .GetType()?
+                        .GetField("_strategy", BindingFlags.NonPublic | BindingFlags.Instance)?
+                        .GetValue(fileStream)
+                    ?? null;
+
+                if (OperatingSystem.IsWindows())
+                {
+                    if (strategy == null)
+                    {
+                        throw new Exception("using an old build");
+                    }
+
+                    Assert.DoesNotContain("Net5Compat", strategy.GetType().FullName);
+                    Assert.DoesNotContain("Legacy", strategy.GetType().FullName);
+                }
+            }
+
+            File.Delete(filePath);
+        }
+    }
+}
+#endif