Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.Web/Util/ReflectionUtil.cs

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+//------------------------------------------------------------------------------
+// <copyright file="ReflectionUtil.cs" company="Microsoft">
+//     Copyright (c) Microsoft Corporation.  All rights reserved.
+// </copyright>                                                                
+//------------------------------------------------------------------------------
+
+namespace System.Web.Util {
+    using System;
+    using System.Reflection;
+    using System.Reflection.Emit;
+    using System.Security.Permissions;
+
+    // Provides helper methods for performing reflection over managed objects.
+
+    [SecurityPermission(SecurityAction.LinkDemand, UnmanagedCode = true)]
+    internal static class ReflectionUtil {
+
+        // Resets an object to its "default" state, e.g. where each instance field is given the value default(TField).
+        public static void Reset<T>(T obj) where T : class {
+            ResetUtil<T>.ResetFn(obj);
+        }
+
+        private static class ResetUtil<T> where T : class {
+            internal readonly static Action<T> ResetFn = CreateResetFn();
+
+            private static Action<T> CreateResetFn() {
+                Type targetType = typeof(T);
+                DynamicMethod dynamicMethod = CreateDynamicMethodWithAssert();
+                ILGenerator ilGen = dynamicMethod.GetILGenerator();
+
+                // for each field in the target type, reset to default(TField)
+                FieldInfo[] allFields = targetType.GetFields(BindingFlags.Instance | BindingFlags.DeclaredOnly | BindingFlags.NonPublic | BindingFlags.Public);
+                foreach (FieldInfo fieldInfo in allFields) {
+                    if (fieldInfo.IsInitOnly || fieldInfo.IsDefined(typeof(DoNotResetAttribute))) {
+                        // This field is not eligible to be reset because it is marked readonly or [DoNotReset].
+                        continue;
+                    }
+
+                    // obj.field = default(TField);
+                    // Opcodes.Initobj can be used for both value and reference types; see ECMA 335, Partition III, Sec. 4.5 "initobj"
+                    // (ref: http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-335.pdf)
+                    ilGen.Emit(OpCodes.Ldarg_0);
+                    ilGen.Emit(OpCodes.Ldflda, fieldInfo);
+                    ilGen.Emit(OpCodes.Initobj, fieldInfo.FieldType);
+                }
+
+                ilGen.Emit(OpCodes.Ret);
+                // dynamicMethod = obj => {
+                //   obj.field1 = default(TField1);
+                //   obj.field2 = default(TField2);
+                //   ...
+                // };
+                return (Action<T>)dynamicMethod.CreateDelegate(typeof(Action<T>));
+            }
+
+            [ReflectionPermission(SecurityAction.Assert, MemberAccess = true)] // needed to create a DynamicMethod inside the target type
+            private static DynamicMethod CreateDynamicMethodWithAssert() {
+                Type targetType = typeof(T);
+                return new DynamicMethod(
+                    name: "Reset-" + targetType.Name,
+                    returnType: typeof(void),
+                    parameterTypes: new Type[] { targetType },
+                    owner: targetType,
+                    skipVisibility: true);
+            }
+        }
+
+    }
+}