File: System\Text\Json\Serialization\Metadata\ReflectionEmitMemberAccessor.cs
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Project: src\runtime\src\libraries\System.Text.Json\src\System.Text.Json.csproj (System.Text.Json)
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

#if NETFRAMEWORK || NET
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Reflection;
using System.Reflection.Emit;

namespace System.Text.Json.Serialization.Metadata
{
    [RequiresDynamicCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]
    [RequiresUnreferencedCode(JsonSerializer.SerializationRequiresDynamicCodeMessage)]
    internal sealed class ReflectionEmitMemberAccessor : MemberAccessor
    {
        public ReflectionEmitMemberAccessor()
        {
        }

        public override Func<object>? CreateParameterlessConstructor(Type type, ConstructorInfo? constructorInfo)
        {
            Debug.Assert(type != null);
            Debug.Assert(constructorInfo is null || constructorInfo.GetParameters().Length == 0);

            if (type.IsAbstract)
            {
                return null;
            }

            if (constructorInfo is null && !type.IsValueType)
            {
                return null;
            }

            var dynamicMethod = new DynamicMethod(
                ConstructorInfo.ConstructorName,
                JsonTypeInfo.ObjectType,
                Type.EmptyTypes,
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            if (constructorInfo is null)
            {
                Debug.Assert(type.IsValueType);

                LocalBuilder local = generator.DeclareLocal(type);

                generator.Emit(OpCodes.Ldloca_S, local);
                generator.Emit(OpCodes.Initobj, type);
                generator.Emit(OpCodes.Ldloc, local);
                generator.Emit(OpCodes.Box, type);
            }
            else
            {
                generator.Emit(OpCodes.Newobj, constructorInfo);
                if (type.IsValueType)
                {
                    // Since C# 10 it's now possible to have parameterless constructors in structs
                    generator.Emit(OpCodes.Box, type);
                }
            }

            generator.Emit(OpCodes.Ret);

            return CreateDelegate<Func<object>>(dynamicMethod);
        }

        public override Func<object[], T> CreateParameterizedConstructor<T>(ConstructorInfo constructor) =>
            CreateDelegate<Func<object[], T>>(CreateParameterizedConstructor(constructor));

        private static DynamicMethod CreateParameterizedConstructor(ConstructorInfo constructor)
        {
            Type? type = constructor.DeclaringType;

            Debug.Assert(type != null);
            Debug.Assert(!type.IsAbstract);
            Debug.Assert(constructor.IsPublic && !constructor.IsStatic);

            ParameterInfo[] parameters = constructor.GetParameters();
            int parameterCount = parameters.Length;

            var dynamicMethod = new DynamicMethod(
                ConstructorInfo.ConstructorName,
                type,
                new[] { typeof(object[]) },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            // For byref parameters, we need to store values in local variables and pass addresses.
            // For out parameters, we just need a default-initialized local to pass by address.
            LocalBuilder?[] locals = new LocalBuilder?[parameterCount];

            // Track the mapping from constructor parameter index to args[] index.
            // out parameters don't have entries in args[].
            int argsIndex = 0;
            int[] argsIndices = new int[parameterCount];
            for (int i = 0; i < parameterCount; i++)
            {
                if (parameters[i].IsOut)
                {
                    argsIndices[i] = -1; // out parameters don't have an args entry
                }
                else
                {
                    argsIndices[i] = argsIndex++;
                }
            }

            for (int i = 0; i < parameterCount; i++)
            {
                Type paramType = parameters[i].ParameterType;
                if (paramType.IsByRef)
                {
                    // Declare a local for the underlying type.
                    Type elementType = paramType.GetElementType()!;
                    locals[i] = generator.DeclareLocal(elementType);

                    if (parameters[i].IsOut)
                    {
                        // For out parameters, just initialize the local to default.
                        // We don't load from args[] since out params aren't in the metadata.
                        if (elementType.IsValueType)
                        {
                            generator.Emit(OpCodes.Ldloca, locals[i]!);
                            generator.Emit(OpCodes.Initobj, elementType);
                        }
                        else
                        {
                            generator.Emit(OpCodes.Ldnull);
                            generator.Emit(OpCodes.Stloc, locals[i]!);
                        }
                    }
                    else
                    {
                        // Load value from object array, unbox it, and store in the local.
                        generator.Emit(OpCodes.Ldarg_0);
                        generator.Emit(OpCodes.Ldc_I4, argsIndices[i]);
                        generator.Emit(OpCodes.Ldelem_Ref);
                        generator.Emit(OpCodes.Unbox_Any, elementType);
                        generator.Emit(OpCodes.Stloc, locals[i]!);
                    }
                }
            }

            // Now push all arguments onto the stack.
            for (int i = 0; i < parameterCount; i++)
            {
                Type paramType = parameters[i].ParameterType;

                if (paramType.IsByRef)
                {
                    // Load address of the local variable.
                    generator.Emit(OpCodes.Ldloca, locals[i]!);
                }
                else
                {
                    // Load value from object array and unbox.
                    generator.Emit(OpCodes.Ldarg_0);
                    generator.Emit(OpCodes.Ldc_I4, argsIndices[i]);
                    generator.Emit(OpCodes.Ldelem_Ref);
                    generator.Emit(OpCodes.Unbox_Any, paramType);
                }
            }

            generator.Emit(OpCodes.Newobj, constructor);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override JsonTypeInfo.ParameterizedConstructorDelegate<T, TArg0, TArg1, TArg2, TArg3>?
            CreateParameterizedConstructor<T, TArg0, TArg1, TArg2, TArg3>(ConstructorInfo constructor) =>
            CreateDelegate<JsonTypeInfo.ParameterizedConstructorDelegate<T, TArg0, TArg1, TArg2, TArg3>>(
                CreateParameterizedConstructor(constructor, typeof(TArg0), typeof(TArg1), typeof(TArg2), typeof(TArg3)));

        public override Func<object?, T> CreateSingleParameterConstructor<T>(ConstructorInfo constructor) =>
            CreateDelegate<Func<object?, T>>(CreateSingleParameterConstructor(constructor));

        private static DynamicMethod CreateSingleParameterConstructor(ConstructorInfo constructor)
        {
            Type? type = constructor.DeclaringType;

            Debug.Assert(type != null);
            Debug.Assert(!type.IsAbstract);
            Debug.Assert(constructor.IsPublic && !constructor.IsStatic);

            ParameterInfo[] parameters = constructor.GetParameters();
            Debug.Assert(parameters.Length == 1);

            Type parameterType = parameters[0].ParameterType;
            Debug.Assert(!parameterType.IsByRef);

            var dynamicMethod = new DynamicMethod(
                ConstructorInfo.ConstructorName,
                type,
                new[] { JsonTypeInfo.ObjectType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();
            EmitObjectArgument(generator, parameterType);
            generator.Emit(OpCodes.Newobj, constructor);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        private static void EmitObjectArgument(ILGenerator generator, Type parameterType)
        {
            Type? nullableUnderlyingType = Nullable.GetUnderlyingType(parameterType);
            if (nullableUnderlyingType is not null)
            {
                Label notNull = generator.DefineLabel();
                Label done = generator.DefineLabel();
                LocalBuilder nullableLocal = generator.DeclareLocal(parameterType);

                generator.Emit(OpCodes.Ldarg_0);
                generator.Emit(OpCodes.Brtrue_S, notNull);

                generator.Emit(OpCodes.Ldloca_S, nullableLocal);
                generator.Emit(OpCodes.Initobj, parameterType);
                generator.Emit(OpCodes.Ldloc, nullableLocal);
                generator.Emit(OpCodes.Br_S, done);

                generator.MarkLabel(notNull);
                generator.Emit(OpCodes.Ldarg_0);
                generator.Emit(OpCodes.Unbox_Any, nullableUnderlyingType);
                generator.Emit(OpCodes.Newobj, parameterType.GetConstructor(new[] { nullableUnderlyingType })!);

                generator.MarkLabel(done);
            }
            else
            {
                generator.Emit(OpCodes.Ldarg_0);
                generator.Emit(parameterType.IsValueType ? OpCodes.Unbox_Any : OpCodes.Castclass, parameterType);
            }
        }

        private static DynamicMethod? CreateParameterizedConstructor(ConstructorInfo constructor, Type parameterType1, Type parameterType2, Type parameterType3, Type parameterType4)
        {
            Type? type = constructor.DeclaringType;

            Debug.Assert(type != null);
            Debug.Assert(!type.IsAbstract);
            Debug.Assert(!constructor.IsStatic);

            ParameterInfo[] parameters = constructor.GetParameters();
            int parameterCount = parameters.Length;

            var dynamicMethod = new DynamicMethod(
                ConstructorInfo.ConstructorName,
                type,
                new[] { parameterType1, parameterType2, parameterType3, parameterType4 },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            // For byref parameters, we need locals because we receive values but need to pass addresses.
            LocalBuilder?[] locals = new LocalBuilder?[parameterCount];
            for (int index = 0; index < parameterCount; index++)
            {
                if (parameters[index].ParameterType.IsByRef)
                {
                    Type elementType = parameters[index].ParameterType.GetElementType()!;
                    locals[index] = generator.DeclareLocal(elementType);

                    if (parameters[index].IsOut)
                    {
                        // For out parameters, initialize to default (the caller passes placeholder values).
                        if (elementType.IsValueType)
                        {
                            generator.Emit(OpCodes.Ldloca, locals[index]!);
                            generator.Emit(OpCodes.Initobj, elementType);
                        }
                        else
                        {
                            generator.Emit(OpCodes.Ldnull);
                            generator.Emit(OpCodes.Stloc, locals[index]!);
                        }
                    }
                    else
                    {
                        // For in/ref parameters, copy the argument value to the local.
                        generator.Emit(
                            index switch
                            {
                                0 => OpCodes.Ldarg_0,
                                1 => OpCodes.Ldarg_1,
                                2 => OpCodes.Ldarg_2,
                                3 => OpCodes.Ldarg_3,
                                _ => throw new InvalidOperationException()
                            });
                        generator.Emit(OpCodes.Stloc, locals[index]!);
                    }
                }
            }

            // Now push all arguments onto the stack.
            for (int index = 0; index < parameterCount; index++)
            {
                Debug.Assert(index <= JsonConstants.UnboxedParameterCountThreshold);

                if (parameters[index].ParameterType.IsByRef)
                {
                    // Load address of the local variable.
                    generator.Emit(OpCodes.Ldloca, locals[index]!);
                }
                else
                {
                    // Load the argument value directly.
                    generator.Emit(
                        index switch
                        {
                            0 => OpCodes.Ldarg_0,
                            1 => OpCodes.Ldarg_1,
                            2 => OpCodes.Ldarg_2,
                            3 => OpCodes.Ldarg_3,
                            _ => throw new InvalidOperationException()
                        });
                }
            }

            generator.Emit(OpCodes.Newobj, constructor);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Action<TCollection, object?> CreateAddMethodDelegate<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] TCollection>() =>
            CreateDelegate<Action<TCollection, object?>>(CreateAddMethodDelegate(typeof(TCollection)));

        private static DynamicMethod CreateAddMethodDelegate(
            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] Type collectionType)
        {
            // We verified this won't be null when we created the converter that calls this method.
            MethodInfo realMethod = (collectionType.GetMethod("Push") ?? collectionType.GetMethod("Enqueue"))!;

            var dynamicMethod = new DynamicMethod(
                realMethod.Name,
                typeof(void),
                new[] { collectionType, JsonTypeInfo.ObjectType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(OpCodes.Ldarg_1);
            generator.Emit(OpCodes.Callvirt, realMethod);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Func<IEnumerable<TElement>, TCollection> CreateImmutableEnumerableCreateRangeDelegate<TCollection, TElement>() =>
            CreateDelegate<Func<IEnumerable<TElement>, TCollection>>(
                CreateImmutableEnumerableCreateRangeDelegate(typeof(TCollection), typeof(TElement), typeof(IEnumerable<TElement>)));

        private static DynamicMethod CreateImmutableEnumerableCreateRangeDelegate(Type collectionType, Type elementType, Type enumerableType)
        {
            MethodInfo realMethod = collectionType.GetImmutableEnumerableCreateRangeMethod(elementType);

            var dynamicMethod = new DynamicMethod(
                realMethod.Name,
                collectionType,
                new[] { enumerableType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(OpCodes.Call, realMethod);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Func<IEnumerable<KeyValuePair<TKey, TValue>>, TCollection> CreateImmutableDictionaryCreateRangeDelegate<TCollection, TKey, TValue>() =>
            CreateDelegate<Func<IEnumerable<KeyValuePair<TKey, TValue>>, TCollection>>(
                CreateImmutableDictionaryCreateRangeDelegate(typeof(TCollection), typeof(TKey), typeof(TValue), typeof(IEnumerable<KeyValuePair<TKey, TValue>>)));

        private static DynamicMethod CreateImmutableDictionaryCreateRangeDelegate(Type collectionType, Type keyType, Type valueType, Type enumerableType)
        {
            MethodInfo realMethod = collectionType.GetImmutableDictionaryCreateRangeMethod(keyType, valueType);

            var dynamicMethod = new DynamicMethod(
                realMethod.Name,
                collectionType,
                new[] { enumerableType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(OpCodes.Call, realMethod);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Func<object, TProperty> CreatePropertyGetter<TProperty>(PropertyInfo propertyInfo) =>
            CreateDelegate<Func<object, TProperty>>(CreatePropertyGetter(propertyInfo, typeof(TProperty)));

        public override Func<TDeclaringType, TProperty> CreatePropertyGetter<TDeclaringType, TProperty>(PropertyInfo propertyInfo) =>
            CreateDelegate<Func<TDeclaringType, TProperty>>(CreatePropertyGetter(propertyInfo, typeof(TDeclaringType), typeof(TProperty)));

        private static DynamicMethod CreatePropertyGetter(PropertyInfo propertyInfo, Type runtimePropertyType)
        {
            MethodInfo? realMethod = propertyInfo.GetMethod;
            Debug.Assert(realMethod != null);

            Type? declaringType = propertyInfo.DeclaringType;
            Debug.Assert(declaringType != null);

            Type declaredPropertyType = propertyInfo.PropertyType;

            DynamicMethod dynamicMethod = CreateGetterMethod(propertyInfo.Name, runtimePropertyType);
            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);

            if (declaringType.IsValueType)
            {
                generator.Emit(OpCodes.Unbox, declaringType);
                generator.Emit(OpCodes.Call, realMethod);
            }
            else
            {
                generator.Emit(OpCodes.Castclass, declaringType);
                generator.Emit(OpCodes.Callvirt, realMethod);
            }

            // declaredPropertyType: Type of the property
            // runtimePropertyType:  <T> of JsonConverter / JsonPropertyInfo

            if (declaredPropertyType != runtimePropertyType && declaredPropertyType.IsValueType)
            {
                // Not supported scenario: possible if declaredPropertyType == int? and runtimePropertyType == int
                // We should catch that particular case earlier in converter generation.
                Debug.Assert(!runtimePropertyType.IsValueType);

                generator.Emit(OpCodes.Box, declaredPropertyType);
            }

            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        private static DynamicMethod CreatePropertyGetter(PropertyInfo propertyInfo, Type declaringType, Type runtimePropertyType)
        {
            MethodInfo? realMethod = propertyInfo.GetMethod;
            Debug.Assert(realMethod != null);

            Type? propertyDeclaringType = propertyInfo.DeclaringType;
            Debug.Assert(propertyDeclaringType != null);
            Debug.Assert(propertyDeclaringType.IsAssignableFrom(declaringType));

            Type declaredPropertyType = propertyInfo.PropertyType;

            DynamicMethod dynamicMethod = CreateGetterMethod(propertyInfo.Name, declaringType, runtimePropertyType);
            ILGenerator generator = dynamicMethod.GetILGenerator();

            if (declaringType.IsValueType)
            {
                generator.Emit(OpCodes.Ldarga_S, 0);
                if (propertyDeclaringType.IsInterface)
                {
                    generator.Emit(OpCodes.Constrained, declaringType);
                    generator.Emit(OpCodes.Callvirt, realMethod);
                }
                else
                {
                    generator.Emit(OpCodes.Call, realMethod);
                }
            }
            else
            {
                generator.Emit(OpCodes.Ldarg_0);

                if (declaringType != propertyDeclaringType)
                {
                    generator.Emit(OpCodes.Castclass, propertyDeclaringType);
                }

                generator.Emit(OpCodes.Callvirt, realMethod);
            }

            if (declaredPropertyType != runtimePropertyType && declaredPropertyType.IsValueType)
            {
                Debug.Assert(!runtimePropertyType.IsValueType);

                generator.Emit(OpCodes.Box, declaredPropertyType);
            }

            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Action<object, TProperty> CreatePropertySetter<TProperty>(PropertyInfo propertyInfo) =>
            CreateDelegate<Action<object, TProperty>>(CreatePropertySetter(propertyInfo, typeof(TProperty)));

        private static DynamicMethod CreatePropertySetter(PropertyInfo propertyInfo, Type runtimePropertyType)
        {
            MethodInfo? realMethod = propertyInfo.SetMethod;
            Debug.Assert(realMethod != null);

            Type? declaringType = propertyInfo.DeclaringType;
            Debug.Assert(declaringType != null);

            Type declaredPropertyType = propertyInfo.PropertyType;

            DynamicMethod dynamicMethod = CreateSetterMethod(propertyInfo.Name, runtimePropertyType);
            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(declaringType.IsValueType ? OpCodes.Unbox : OpCodes.Castclass, declaringType);
            generator.Emit(OpCodes.Ldarg_1);

            // declaredPropertyType: Type of the property
            // runtimePropertyType:  <T> of JsonConverter / JsonPropertyInfo

            if (declaredPropertyType != runtimePropertyType && declaredPropertyType.IsValueType)
            {
                // Not supported scenario: possible if e.g. declaredPropertyType == int? and runtimePropertyType == int
                // We should catch that particular case earlier in converter generation.
                Debug.Assert(!runtimePropertyType.IsValueType);

                generator.Emit(OpCodes.Unbox_Any, declaredPropertyType);
            }

            generator.Emit(declaringType.IsValueType ? OpCodes.Call : OpCodes.Callvirt, realMethod);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Func<object, TProperty> CreateFieldGetter<TProperty>(FieldInfo fieldInfo) =>
            CreateDelegate<Func<object, TProperty>>(CreateFieldGetter(fieldInfo, typeof(TProperty)));

        private static DynamicMethod CreateFieldGetter(FieldInfo fieldInfo, Type runtimeFieldType)
        {
            Type? declaringType = fieldInfo.DeclaringType;
            Debug.Assert(declaringType != null);

            Type declaredFieldType = fieldInfo.FieldType;

            DynamicMethod dynamicMethod = CreateGetterMethod(fieldInfo.Name, runtimeFieldType);
            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(
                declaringType.IsValueType
                    ? OpCodes.Unbox
                    : OpCodes.Castclass,
                declaringType);
            generator.Emit(OpCodes.Ldfld, fieldInfo);

            // declaredFieldType: Type of the field
            // runtimeFieldType:  <T> of JsonConverter / JsonPropertyInfo

            if (declaredFieldType.IsValueType && declaredFieldType != runtimeFieldType)
            {
                generator.Emit(OpCodes.Box, declaredFieldType);
            }

            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override Action<object, TProperty> CreateFieldSetter<TProperty>(FieldInfo fieldInfo) =>
            CreateDelegate<Action<object, TProperty>>(CreateFieldSetter(fieldInfo, typeof(TProperty)));

        private static DynamicMethod CreateFieldSetter(FieldInfo fieldInfo, Type runtimeFieldType)
        {
            Type? declaringType = fieldInfo.DeclaringType;
            Debug.Assert(declaringType != null);

            Type declaredFieldType = fieldInfo.FieldType;

            DynamicMethod dynamicMethod = CreateSetterMethod(fieldInfo.Name, runtimeFieldType);
            ILGenerator generator = dynamicMethod.GetILGenerator();

            generator.Emit(OpCodes.Ldarg_0);
            generator.Emit(declaringType.IsValueType ? OpCodes.Unbox : OpCodes.Castclass, declaringType);
            generator.Emit(OpCodes.Ldarg_1);

            // declaredFieldType: Type of the field
            // runtimeFieldType:  <T> of JsonConverter / JsonPropertyInfo

            if (declaredFieldType != runtimeFieldType && declaredFieldType.IsValueType)
            {
                generator.Emit(OpCodes.Unbox_Any, declaredFieldType);
            }

            generator.Emit(OpCodes.Stfld, fieldInfo);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        public override UnionTryGetValueAccessor<TUnion> CreateUnionTryGetValueAccessor<TUnion>(IReadOnlyList<KeyValuePair<Type, MethodInfo>> entries) =>
            CreateDelegate<UnionTryGetValueAccessor<TUnion>>(CreateUnionTryGetValueAccessor(typeof(TUnion), entries));

        private static DynamicMethod CreateUnionTryGetValueAccessor(Type unionType, IReadOnlyList<KeyValuePair<Type, MethodInfo>> entries)
        {
            Debug.Assert(entries.Count > 0, "Should not build a chained accessor for an empty entry list.");

            // Emit a single DynamicMethod that chains the user-declared
            // 'bool TUnion.TryGetValue(out CaseType)' overloads in caller-supplied order.
            // First successful overload sets the out parameters and returns true; if none
            // match, the trailer writes null/null and returns false. Producing one piece of
            // IL avoids per-entry closure or delegate hop and keeps the dispatch O(chainLength)
            // with no managed allocations on the hot path.
            DynamicMethod dynamicMethod = new DynamicMethod(
                "UnionTryGetValueAccessor",
                typeof(bool),
                new[] { unionType, typeof(Type).MakeByRefType(), typeof(object).MakeByRefType() },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

            ILGenerator generator = dynamicMethod.GetILGenerator();
            bool unionIsValueType = unionType.IsValueType;

            for (int i = 0; i < entries.Count; i++)
            {
                KeyValuePair<Type, MethodInfo> entry = entries[i];
                Type caseType = entry.Key;
                MethodInfo method = entry.Value;
                LocalBuilder extracted = generator.DeclareLocal(caseType);
                Label nextEntry = generator.DefineLabel();

                // if (!union.TryGetValue(out extracted)) goto nextEntry;
                if (unionIsValueType)
                {
                    generator.Emit(OpCodes.Ldarga_S, (byte)0);
                    generator.Emit(OpCodes.Ldloca_S, extracted);
                    generator.Emit(OpCodes.Call, method);
                }
                else
                {
                    generator.Emit(OpCodes.Ldarg_0);
                    generator.Emit(OpCodes.Ldloca_S, extracted);
                    generator.Emit(OpCodes.Callvirt, method);
                }
                generator.Emit(OpCodes.Brfalse, nextEntry);

                // caseType = typeof(CaseType);
                generator.Emit(OpCodes.Ldarg_1);
                generator.Emit(OpCodes.Ldtoken, caseType);
                generator.Emit(OpCodes.Call, s_getTypeFromHandleMethod);
                generator.Emit(OpCodes.Stind_Ref);

                // value = (object?)extracted;
                generator.Emit(OpCodes.Ldarg_2);
                generator.Emit(OpCodes.Ldloc, extracted);
                if (caseType.IsValueType)
                {
                    generator.Emit(OpCodes.Box, caseType);
                }
                generator.Emit(OpCodes.Stind_Ref);

                // return true;
                generator.Emit(OpCodes.Ldc_I4_1);
                generator.Emit(OpCodes.Ret);

                generator.MarkLabel(nextEntry);
            }

            // No overload matched: caseType = null; value = null; return false;
            generator.Emit(OpCodes.Ldarg_1);
            generator.Emit(OpCodes.Ldnull);
            generator.Emit(OpCodes.Stind_Ref);
            generator.Emit(OpCodes.Ldarg_2);
            generator.Emit(OpCodes.Ldnull);
            generator.Emit(OpCodes.Stind_Ref);
            generator.Emit(OpCodes.Ldc_I4_0);
            generator.Emit(OpCodes.Ret);

            return dynamicMethod;
        }

        private static readonly MethodInfo s_getTypeFromHandleMethod =
            typeof(Type).GetMethod(nameof(Type.GetTypeFromHandle), new[] { typeof(RuntimeTypeHandle) })!;

        private static DynamicMethod CreateGetterMethod(string memberName, Type memberType) =>
            new DynamicMethod(
                memberName + "Getter",
                memberType,
                new[] { JsonTypeInfo.ObjectType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

        private static DynamicMethod CreateGetterMethod(string memberName, Type declaringType, Type memberType) =>
            new DynamicMethod(
                memberName + "Getter",
                memberType,
                new[] { declaringType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

        private static DynamicMethod CreateSetterMethod(string memberName, Type memberType) =>
            new DynamicMethod(
                memberName + "Setter",
                typeof(void),
                new[] { JsonTypeInfo.ObjectType, memberType },
                typeof(ReflectionEmitMemberAccessor).Module,
                skipVisibility: true);

        [return: NotNullIfNotNull(nameof(method))]
        private static T? CreateDelegate<T>(DynamicMethod? method) where T : Delegate =>
            (T?)method?.CreateDelegate(typeof(T));
    }
}
#endif