System.Reflection.Emit: field or property of ref struct

Question

For the following interface and struct:

internal interface IRecord<T> where T : struct
{
    ref T Values { get; }
}

public struct Entity
{
    public int Field1;
    ...
}

I would like get the following lambda expression via reflection:

Expression<Func<IRecord<Entity>, int>> getter = x => x.Values.Field1;
Expression<Action<IRecord<Entity>, int>> setter = (x, value) => x.Values.Field1 = value;

Unfortunately this doesn't compile: cs8153: an expression tree lambda may not contain a call to a method, property, or indexer that returns by reference. It seems get any member of ref struct is not supported via reflection.

So I have to go for System.Reflection.Emit to generate the following accessor class:

public static class Accessor
{
    public static int GetField1(IRecord<Entity> record) => record.Values.Field1;
    public static void SetField1(IRecord<Entity> record, int value) => record.Values.Field1 = value;
    ...
}

and get the following lambda expression via reflection:

Expression<Func<IRecord<Entity>, int>> getter = x => Accessor.GetField1(x);
Expression<Action<IRecord<Entity>, int>> setter = (x, value) => Accessor.SetField1(x, value);

Here is my code to generate the Accessor class using System.Reflection.Emit:

private static readonly ModuleBuilder ModuleBuilder = GetModuleBuilder();

private static ModuleBuilder GetModuleBuilder()
{
    AssemblyName assemblyName = new AssemblyName("AccessTypeBuilder");
    AssemblyBuilder assemblyBuilder = AssemblyBuilder.DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.Run);
    ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("Module");
    return moduleBuilder;
}

public static Type BuildAccessorType(Type fieldValuesType)
{
    TypeBuilder typeBuilder = ModuleBuilder.DefineType("Accessor", TypeAttributes.Public | TypeAttributes.Abstract | TypeAttributes.Sealed, typeof(object));
    BuildAccessor(typeBuilder, typeof(Entity), "Field1", typeof(int));
    return typeBuilder.CreateType();
}

private static void BuildAccessor(TypeBuilder typeBuilder, Type fieldValuesType, string fieldName, Type dataType)
{
    typeBuilder.DefineGetter(fieldValuesType, $"Get{fieldName}", dataType, fieldName);
    typeBuilder.DefineSetter(fieldValuesType, $"Set{fieldName}", dataType, fieldName);
}

private static void DefineField(this TypeBuilder typeBuilder, Type dataType, string fieldName)
{
    typeBuilder.DefineField(fieldName, dataType, FieldAttributes.Public);
}

private static Type RecordType(this Type fieldValuesType)
{
    return typeof(IRecord<>).MakeGenericType(fieldValuesType);
}

private static MethodInfo FieldValues(this Type fieldValuesType)
{
    var recordType = fieldValuesType.RecordType();
    var property = recordType.GetProperty(nameof(IRecord<int>.Values));
    return property.GetMethod;
}

private static FieldInfo Field(this Type fieldValuesType, string fieldName) => fieldValuesType.GetField(fieldName, BindingFlags.Public | BindingFlags.Instance);

private static void DefineGetter(this TypeBuilder typeBuilder, Type fieldValuesType, string methodName, Type dataType, string fieldName)
{
    var method = typeBuilder.DefineMethod(methodName, MethodAttributes.Public | MethodAttributes.Static, dataType, new Type[] { fieldValuesType.RecordType() });
    var methodBody = method.GetILGenerator();
    methodBody.EmitGetter(fieldValuesType, fieldName);
}

private static void EmitGetter(this ILGenerator methodBody, Type fieldValuesType, string fieldName)
{
    methodBody.Emit(OpCodes.Ldarg_0);
    methodBody.Emit(OpCodes.Callvirt, fieldValuesType.FieldValues());
    methodBody.Emit(OpCodes.Ldfld, fieldValuesType.Field(fieldName));
    methodBody.Emit(OpCodes.Stloc_0);
    methodBody.Emit(OpCodes.Ldloc_0);
    methodBody.Emit(OpCodes.Ret);
}

private static void DefineSetter(this TypeBuilder typeBuilder, Type fieldValuesType, string methodName, Type dataType, string fieldName)
{
    var method = typeBuilder.DefineMethod(methodName, MethodAttributes.Public | MethodAttributes.Static, typeof(void), new Type[] { fieldValuesType.RecordType(), dataType });
    var methodBody = method.GetILGenerator();
    methodBody.EmitSetter(fieldValuesType, fieldName);
}

private static void EmitSetter(this ILGenerator methodBody, Type fieldValuesType, string fieldName)
{
    methodBody.Emit(OpCodes.Nop);
    methodBody.Emit(OpCodes.Ldarg_0);
    methodBody.Emit(OpCodes.Callvirt, fieldValuesType.FieldValues());
    methodBody.Emit(OpCodes.Ldarg_1);
    methodBody.Emit(OpCodes.Stfld, fieldValuesType.Field(fieldName));
    methodBody.Emit(OpCodes.Ret);
}

When consuming the generated Accessor class, I get InvalidProgramException: Common Language Runtime detected an invalid program. when invoking the generated getter; and get System.MethodAccessException: Attempt by method 'Accessor.SetField0(IRecord`1<Entity>, Int32)' to access IRecord`1<Entity>.get_FieldValues()' failed. when invoking the generated setter.

What am I doing wrong? I've spent whole day for this and is pretty frustrated. Any help will be very much appreciated!

Answer 1

Fixed.

1. The IRecord<T> interface must be public;

2. Remove two lines of opcode emitting:

private static void EmitGetter(this ILGenerator methodBody, Type fieldValuesType, string fieldName)
{
    methodBody.Emit(OpCodes.Ldarg_0);
    methodBody.Emit(OpCodes.Callvirt, fieldValuesType.FieldValues());
    methodBody.Emit(OpCodes.Ldfld, fieldValuesType.Field(fieldName));
    //methodBody.Emit(OpCodes.Stloc_0); --removed
    //methodBody.Emit(OpCodes.Ldloc_0); --removed
    methodBody.Emit(OpCodes.Ret);
}