Add tests for SecureAppContext

src/libraries/System.Runtime/tests/System.Runtime.Tests.csproj

@@ -149,6 +149,7 @@
     <Compile Include="System\WeakReferenceTests.cs" />
     <Compile Include="System\AppContext\AppContext.Switch.cs" />
     <Compile Include="System\AppContext\AppContext.Switch.Validation.cs" />
+    <Compile Include="System\AppContext\SecureAppContext.cs" />
     <Compile Include="System\Collections\Generic\KeyNotFoundExceptionTests.cs" />
     <Compile Include="System\Collections\Generic\KeyValuePairTests.cs" />
     <Compile Include="System\Collections\ObjectModel\CollectionTests.cs" />

src/libraries/System.Runtime/tests/System/AppContext/SecureAppContext.cs

@@ -0,0 +1,82 @@
+// 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.Reflection;
+using Microsoft.DotNet.RemoteExecutor;
+using Xunit;
+using Xunit.Sdk;
+
+namespace System.Tests
+{
+    public partial class SecureAppContextTests
+    {
+        // these tests only make sense on platforms where reflection is expected to forbid overwriting initonly fields
+        public static bool RunForbidSettingInitonlyTests => PlatformDetection.IsNetCore && RemoteExecutor.IsSupported;
+
+        [ConditionalFact(nameof(RunForbidSettingInitonlyTests))]
+        public void CannotUseReflectionToChangeValues()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                Type secureAppContextType = typeof(AppContext).Assembly.GetType("System.SecureAppContext", throwOnError: true);
+
+                foreach (FieldInfo field in secureAppContextType.GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static))
+                {
+                    try
+                    {
+                        Assert.True(field.FieldType.IsValueType, "Field is a reference type; instance members may be subject to mutation.");
+                        Assert.True(field.IsInitOnly, "Field is mutable.");
+
+                        object originalValue = field.GetValue(null);
+                        Assert.Throws<FieldAccessException>(() => field.SetValue(null, originalValue));
+                    }
+                    catch (Exception ex)
+                    {
+                        throw new Exception($"Failure when testing field {field.Name}.", ex);
+                    }
+                }
+            }).Dispose();
+        }
+
+        [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [InlineData("Switch.System.Runtime.Serialization.SerializationGuard", "SerializationGuardEnabled", true)]
+        [InlineData("System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization", "BinaryFormatterEnabled", true)]
+        public void PopulatedAtAppDomainStart(string switchName, string propertyName, bool defaultValue)
+        {
+            RunTest(switchName, propertyName, null, defaultValue);
+            RunTest(switchName, propertyName, true, true);
+            RunTest(switchName, propertyName, false, false);
+
+            static void RunTest(string switchName, string propertyName, bool? configuredValue, bool expectedValue)
+            {
+                try
+                {
+                    RemoteInvokeOptions options = null;
+                    if (configuredValue != null)
+                    {
+                        options = new RemoteInvokeOptions();
+                        options.RuntimeConfigurationOptions[switchName] = configuredValue.ToString();
+                    }
+
+                    RemoteExecutor.Invoke((switchName, propertyName, expectedValue) =>
+                    {
+                        Type secureAppContextType = typeof(AppContext).Assembly.GetType("System.SecureAppContext", throwOnError: true);
+                        PropertyInfo pi = secureAppContextType.GetProperty(propertyName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static);
+                        if (pi == null)
+                        {
+                            throw new XunitException($"Property {propertyName} not found.");
+                        }
+
+                        Assert.Equal(bool.Parse(expectedValue), pi.GetValue(null));
+                    },
+                    switchName, propertyName, expectedValue.ToString(), options).Dispose();
+                }
+                catch (Exception ex)
+                {
+                    throw new Exception($"Exception occurred for scenario configuredValue={configuredValue?.ToString() ?? "<null>"}, expectedValue={expectedValue}.", ex);
+                }
+            }
+        }
+    }
+}