DynamicMethod Byref Return Type

[mikernet]

Would like to be able to set a ref return type like this:

var method = new DynamicMethod("Method", typeof(object).MakeByRefType(), new[] { typeof(object) }, ownerType, true);

Currently have to use this workaround/hack to avoid exception being thrown:

var method = new DynamicMethod("Method", typeof(object), new[] { typeof(object) }, ownerType, true);
method.GetType().GetField("m_returnType", BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly).SetValue(method, typeof(object).MakeByRefType());

[mgravell]

Interestingly, this limitation only applies to DynamicMethod. If you use TypeBuilder, you can create a by-ref return method:

delegate ref object SomeDelegateType(object[] arr);
...
var asm = AppDomain.CurrentDomain.DefineDynamicAssembly(
    new AssemblyName("foo"), AssemblyBuilderAccess.Run);
var module = asm.DefineDynamicModule("foo");
var tb = module.DefineType("bar", TypeAttributes.Class, typeof(object));
var mb = tb.DefineMethod("method",
      MethodAttributes.Public | MethodAttributes.Static,
      typeof(object).MakeByRefType(), new Type[] { typeof(object[]) });
var il = mb.GetILGenerator();
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldc_I4_2);
il.Emit(OpCodes.Ldelema, typeof(object));
il.Emit(OpCodes.Ret);

var type = tb.CreateType();
var method = type.GetMethod("method");
var del = (SomeDelegateType)Delegate.CreateDelegate(typeof(SomeDelegateType), null, method);

object[] arr = new object[4] { "abc", "def", "ghi", "jkl" };

ref object r = ref del(arr);
Console.WriteLine(r); // ghi

The custom delegate here is required because you can't invoke this type of method via reflection, since a managed pointer (ref object in this case) can not be boxed.

[mikernet]

Yeah, the Init() method in DynamicMethod explicitly checks if the return type is byref and throws if it is.

[jkotas]

This was fixed by dotnet/coreclr#15106 . This fix is included in .NET Core 2.1

[mikernet]

Oh my mistake...I looked though "DynamicMethod" issues but it didn't come up because of the space between the words in the other issue :/