Fix OSConditionAttribute to detect OS via reflection for .NET Framework on Mono

src/TestFramework/TestFramework/Attributes/TestMethod/OSConditionAttribute.cs

@@ -21,6 +21,11 @@
 #endif
 #endif
 
+#if NETFRAMEWORK
+    // Cache the detected OS to avoid repeated reflection calls
+    private static readonly OperatingSystems? s_detectedOS = DetectCurrentOS();
+#endif
+
     private readonly OperatingSystems _operatingSystems;
 
     /// <summary>
@@ -50,9 +55,15 @@
     /// Gets a value indicating whether the test method or test class should be ignored.
     /// </summary>
     public override bool IsConditionMet
-#if NET462
-        // On .NET Framework, we are sure we are running on Windows.
-        => (_operatingSystems & OperatingSystems.Windows) != 0;
+#if NETFRAMEWORK
+    {
+        get
+        {
+            // If we couldn't detect the OS via reflection, assume Windows
+            OperatingSystems currentOS = s_detectedOS ?? OperatingSystems.Windows;
+            return (_operatingSystems & currentOS) != 0;
+        }
+    }
 #else
     {
         get
@@ -79,6 +90,82 @@
     }
 #endif
 
+#if NETFRAMEWORK
+    private static OperatingSystems? DetectCurrentOS()
+    {
+        // RuntimeInformation.IsOSPlatform is available in .NET Framework 4.7.1+.
+        // For older .NET Framework versions or environments where the API is not available,
+        // we return null to fall back to assuming Windows.
+        // This also handles Mono which supports RuntimeInformation API and can run on non-Windows platforms.
+        Type? runtimeInformationType = Type.GetType("System.Runtime.InteropServices.RuntimeInformation, System.Runtime.InteropServices.RuntimeInformation")
+            ?? Type.GetType("System.Runtime.InteropServices.RuntimeInformation, mscorlib");
+
+        if (runtimeInformationType is null)
+        {
+            return null;
+        }
+
+        MethodInfo? isOSPlatformMethod = runtimeInformationType.GetMethod("IsOSPlatform", BindingFlags.Public | BindingFlags.Static);
+        if (isOSPlatformMethod is null)
+        {
+            return null;
+        }
+
+        Type? osPlatformType = Type.GetType("System.Runtime.InteropServices.OSPlatform, System.Runtime.InteropServices.RuntimeInformation")
+            ?? Type.GetType("System.Runtime.InteropServices.OSPlatform, mscorlib");
+
+        if (osPlatformType is null)
+        {
+            return null;
+        }
+
+        MethodInfo? createMethod = osPlatformType.GetMethod("Create", BindingFlags.Public | BindingFlags.Static);
+        if (createMethod is null)
+        {
+            return null;
+        }
+
+        if (IsOSPlatformViaReflection(isOSPlatformMethod, createMethod, "Windows"))
+        {
+            return OperatingSystems.Windows;
+        }
+        else if (IsOSPlatformViaReflection(isOSPlatformMethod, createMethod, "Linux"))
+        {
+            return OperatingSystems.Linux;
+        }
+        else if (IsOSPlatformViaReflection(isOSPlatformMethod, createMethod, "OSX"))
+        {
+            return OperatingSystems.OSX;
+        }
+        else if (IsOSPlatformViaReflection(isOSPlatformMethod, createMethod, "FreeBSD"))
+        {
+            return OperatingSystems.FreeBSD;
+        }
+
+        return null;
+    }
+
+    private static bool IsOSPlatformViaReflection(MethodInfo isOSPlatformMethod, MethodInfo createMethod, string osName)
+    {
+        try
+        {
+            object? osPlatform = createMethod.Invoke(null, new object[] { osName });
+            if (osPlatform is null)
+            {
+                return false;
+            }
+
+            object? result = isOSPlatformMethod.Invoke(null, new object[] { osPlatform });
+            return result is true;
+        }
+        catch
+        {
+            // Reflection invocation failed, treat as OS not matching
+            return false;
+        }
+    }
+#endif
+
     /// <summary>
     /// Gets the group name for this attribute.
     /// </summary>