Question

I have some code that uses a MethodInfo of a generic method found on a generated type. To avoid some reflection, I have the code use the

ldtoken Method
ldtoken Type
call GetMethodFromHandle(RuntimeMethodHandle,RunTimeTypeHandle)

Pattern to generate the MethodInfos at compile time.

However, if the methodInfo belongs to a generic type and itself is a generic method things get screwy.
Here is some code that simply generates a GM that emits an open version of its methodInfo.
If I call it to retrieve the method than try to close it over a specific type I get a perplexing exception::

System.Reflection.MethodInfo GM[M]() is not a GenericMethodDefinition. MakeGenericMethod may only be called on a method for which MethodBase.IsGenericMethodDefinition is true.

Here is the relevant code::

var aBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("Test"), AssemblyBuilderAccess.RunAndSave);
var mBuilder = aBuilder.DefineDynamicModule(aBuilder.GetName().Name, true);
var typeBuilder = mBuilder.DefineType("NameSpace.Generic`1",TypeAttributes.AutoClass | TypeAttributes.Sealed | TypeAttributes.Public,typeof(object));
var TypeGenerics = typeBuilder.DefineGenericParameters(new[] { "T" });
var methodBuilder = typeBuilder.DefineMethod("GM", MethodAttributes.Public | MethodAttributes.Static | MethodAttributes.HideBySig);
var methodGenerics = methodBuilder.DefineGenericParameters(new[] { "M" });
methodBuilder.SetSignature(typeof(MethodInfo), null, null, Type.EmptyTypes, null, null);
var ilgenerator = methodBuilder.GetILGenerator();
var typeBuilderClosedOverT = typeBuilder.MakeGenericType(TypeGenerics);
ilgenerator.Emit(OpCodes.Ldtoken, methodBuilder);
ilgenerator.Emit(OpCodes.Ldtoken, typeBuilderClosedOverT);
ilgenerator.Emit(OpCodes.Call, 
    typeof(MethodBase).GetMethod(
        "GetMethodFromHandle", 
        BindingFlags.Public | BindingFlags.Static,
        null,
        new[] { typeof(RuntimeMethodHandle), typeof(RuntimeTypeHandle) },
        null
    )
);
ilgenerator.Emit(OpCodes.Castclass,typeof(MethodInfo));
ilgenerator.Emit(OpCodes.Ret);
var bakedType = typeBuilder.CreateType();
var methodInfo = bakedType.MakeGenericType(typeof(int)).GetMethod("GM").MakeGenericMethod(typeof(bool)).Invoke(null, null) as MethodInfo;
var methodInfoClosedOverBool = methodInfo.MakeGenericMethod(typeof(bool));

It seems the only time my code screws up is if it’s a genericmethod on a non-generic type. If the code is rewritten so that its about a normal method on a normal type, or a generic method on a normal type, or a normal method on a generic type it all works. It’s only the combination of both that causes errors. Am I doing something wrong?

I submitted a bug about this issue:
https://connect.microsoft.com/VisualStudio/feedback/details/775989/clr-cannot-emit-a-token-for-an-open-generic-method-on-a-generic-type

Answer 1

Looks like a CLR issue to me, because the same thing happens if you write the IL by hand and use ilasm. That is, given a generic class G and a nongeneric class N, each with a generic method M, then trying to get the generic method definition from the non-generic class works:

ldtoken    method void class N::M<[1]>()
ldtoken    class N<!T>
call       class [mscorlib]System.Reflection.MethodBase [mscorlib]
             System.Reflection.MethodBase::GetMethodFromHandle(
                valuetype [mscorlib]System.RuntimeMethodHandle,
                valuetype [mscorlib]System.RuntimeTypeHandle)
castclass  [mscorlib]System.Reflection.MethodInfo
ret

but the MethodInfo returned from the generic class is not a generic method definition (but it almost is; it’s D.MakeGenericMethod(D.GetGenericArguments()) where D is the method definition you want):

ldtoken    method void class G`1<!T>::M<[1]>()
ldtoken    class G`1<!T>
call       class [mscorlib]System.Reflection.MethodBase [mscorlib]
             System.Reflection.MethodBase::GetMethodFromHandle(
                valuetype [mscorlib]System.RuntimeMethodHandle,
                valuetype [mscorlib]System.RuntimeTypeHandle)
castclass  [mscorlib]System.Reflection.MethodInfo
ret