How can I emit a dynamic method returning ref?

Question

I look through all the inputs and outputs of ref, and I am unable to fix the dynamic method that ref returns.

Handmade lambds and existing methods work as expected:

class Widget
{
    public int Length;
}
delegate ref int WidgetMeasurer(Widget w);

WidgetMeasurer GetMeasurerA()
{
    return w => ref w.Length;
}

static ref int MeasureWidget(Widget w) => ref w.Length;
WidgetMeasurer GetMeasurerB()
{
    return MeasureWidget;
}

But emitting a dynamic method fails. Note . I use Sigil here. Sorry, I'm less familiar with System.Reflection.Emit.

WidgetMeasurer GetMeasurerC()
{
    FieldInfo lengthField = typeof(Widget).GetField(nameof(Widget.Length));
    var emitter = Emit<WidgetMeasurer>.NewDynamicMethod()
        .LoadArgument(0)
        .LoadFieldAddress(lengthField)
        .Return();
    return emitter.CreateDelegate();
}

This is not true when NewDynamicMethodthrowing 'The return Type contains some invalid type (i.e. null, ByRef)'. This makes sense, as I understand that it is WidgetMeasurercoming back under the hood Int32&.

The question is, is there any first or third party method that I can use to emit code that mimics the first two examples (which I empirically know is working correctly)? If not, is this a logical restriction?

EDIT: System.Reflection.Emit ( ):

WidgetMeasurer GetMeasurerD()
{
    FieldInfo lengthField = typeof(Widget).GetField(nameof(Widget.Length));

    Type returnType = typeof(int).MakeByRefType();
    Type[] paramTypes = { typeof(Widget) };
    DynamicMethod method = new DynamicMethod("", returnType, paramTypes);

    ILGenerator il = method.GetILGenerator();
    il.Emit(OpCodes.Ldarg_0);
    il.Emit(OpCodes.Ldflda, lengthField);
    il.Emit(OpCodes.Ret);

    return (WidgetMeasurer)method.CreateDelegate(typeof(WidgetMeasurer));
}

Answer 1

, DynamicMethod, . , . , , , Widget ( , InternalsVisibleTo ).

static void Main(string[] args)
{
    var widget = new Widget();
    GetLengthMeasurer()(widget) = 7;
    Console.WriteLine(widget.Length);
}

private static WidgetMeasurer GetLengthMeasurer()
{
    var fieldInfo = typeof(Widget).GetField("Length");
    var asmName = new AssemblyName("WidgetDynamicAssembly." + Guid.NewGuid().ToString());
    var asmBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(asmName, AssemblyBuilderAccess.RunAndCollect);
    var moduleBuilder = asmBuilder.DefineDynamicModule("<Module>");
    var typeBuilder = moduleBuilder.DefineType("WidgetHelper");
    var methodBuilder = typeBuilder.DefineMethod("GetLength", MethodAttributes.Static | MethodAttributes.Public, typeof(int).MakeByRefType(), new[] { typeof(Widget) });
    var ilGen = methodBuilder.GetILGenerator();
    ilGen.Emit(OpCodes.Ldarg_0);
    ilGen.Emit(OpCodes.Ldflda, fieldInfo);
    ilGen.Emit(OpCodes.Ret);
    var type = typeBuilder.CreateType();
    var mi = type.GetMethod(methodBuilder.Name);
    var del = (WidgetMeasurer)mi.CreateDelegate(typeof(WidgetMeasurer));
    return del;
}

:

7