Question

I’m trying to use Reflection.Emit to emit a class that will inherit from multiple interfaces at run-time, and I cannot know which interfaces ahead of time.

As per MSDN/TypeBuilder.DefineMethodOverride:

To override a method of a base class or to implement a method of an
interface, simply emit a method with the same name and signature as
the method to be overridden or implemented

Here’s my code that overrides an interface method:

private void OverrideMethod(TypeBuilder typeBuilder, 
                            Type interfaceToOverride,
                            MethodInfo methodToOverride)
{
    // Create the method stub
    MethodBuilder methodBuilder = typeBuilder.DefineMethod(
        methodToOverride.Name,
        MethodAttributes.Public
        | MethodAttributes.HideBySig
        | MethodAttributes.NewSlot
        | MethodAttributes.Virtual
        | MethodAttributes.Final,
        CallingConventions.HasThis,
        methodToOverride.ReturnType,
        methodToOverride.GetParameters().Select(p => p.ParameterType).ToArray()
    );

    // Implement the overriding method
    ILGenerator il = methodBuilder.GetILGenerator();

    // ... a bunch of calls to il.Emit ...

    // Return 
    il.Emit(OpCodes.Ret);
}

This works, except when I inherit from two interfaces who both have a method with the same name. Obviously this is because I’m not giving a fully-qualified name for the method. I’m not sure how to do this properly.

Changing methodToOverride.Name to interfaceToOverride.FullName + "." + methodToOverride.Name did not work: I get an error when emitting, “TypeLoadException: class does not have implementation.

Using DefineMethodOverride partially worked, but for some reason did not when I tested it against nested interfaces. Also, the documentation linked above explicitly says not to do this.

What’s the right approach to get around this issue?

Answer 1

The proper way to do this is to define a method named <InterfaceName>.<MethodName> and then call TypeBuilder.DefineMethodOverride.

The DefineMethodOverride method is used when a method body and a
method declaration have different names.

In your case, method body name is <InterfaceName>.<MethodName>, while method declaration name is <MethodName>. So it’s fine to use TypeBuilder.DefineMethodOverride.

Example usage:

private void OverrideMethod(TypeBuilder typeBuilder, 
                            Type interfaceToOverride,
                            MethodInfo methodToOverride)
{
    // Create the method stub
    MethodBuilder methodBuilder = typeBuilder.DefineMethod(
        /* Change method name here */
        string.Format("{0}.{1}", interfaceToOverride.FullName,
            methodToOverride.Name),
        MethodAttributes.Public
        | MethodAttributes.HideBySig
        | MethodAttributes.NewSlot
        | MethodAttributes.Virtual
        | MethodAttributes.Final,
        CallingConventions.HasThis,
        methodToOverride.ReturnType,
        methodToOverride.GetParameters().Select(p => p.ParameterType).ToArray()
    );

    // Implement the overriding method
    ILGenerator il = methodBuilder.GetILGenerator();

    // ... a bunch of calls to il.Emit ...

    // Return 
    il.Emit(OpCodes.Ret);

    // And define a methodimpl, which consists of a pair of metadata tokens.
    // One token points to an implementation, and the other token points
    // to a declaration that the body implements
    typeBuilder.DefineMethodOverride(methodBuilder, methodToOverride);
}

Note

Actually, you can define your method (using TypeBuilder.DefineMethod) with any name. But it should differ from <MethodName> and you must call TypeBuilder.DefineMethodOverride to “link” method body with method declaration.