Question

I’m having trouble emitting a call to a delegate whose type is unfinished at the time of the emit. I’ll elaborate: I’ve declared the following delegate type:

// Delegate type. The 'firstArgument' will be 'this', i.e., this is an open
// instance method: the implicit argument is here given explicitly, in
// 'firstArgument'. (See link below for explanation on open instance delegates).
public delegate Object DirectReadAccessor<T>(T firstArgument);

And now I’m trying to dynamically (i.e., with a TypeBuilder) create the following class:

public MyClass {

    // Array of delegates. T has been replaced with MyClass because the
    // argument will be 'this', which is of type MyClass.
    private static DirectReadAccessor<MyClass>[] directReadAccessors;

    // Method that looks up a delegate in the array of delegates and calls it
    // with 'this'.
    public Object DirectRead(int i) {
        directReadAccessors[i](this);
    }

    // Method that is called by the declaring type to pass an array with the
    // MethodInfo of some methods. MyClass then creates delegates for these
    // methods and stores them in the directReadAccessors array.
    public static void InitializeClass(MethodInfo[] directReadAccessorsMInfo) {
        int length = directReadAccessorsMInfo.Length;
        Type[] typeArguments = new Type[] { typeof(MyClass) };
        directReadAccessors = new DirectReadAccessor<MyClass>[length];
        // For each method in directReadAccessorsMInfo...
        for (int i = 0; i < length; i++) {
            // Create a delegate and store it in directReadAccessors.
            directReadAccessors[i] = (DirectReadAccessor<MyClass>)
                   Delegate.CreateDelegate(
                          DirectReadAccessor<MyClass>, // Type of the delegate.
                          null, // Specify null first argument so that it's
                                // *open* instance.
                          directReadAccessorsMInfo[i].MakeGenericMethod(typeArguments) // The method.
                    );
        }
    }

}

*on open instance delegates.

This has been tricky because MyClass doesn’t exist when I’m trying to declare the field directReadAccessors, which is of type DirectReadAccessor[], or when I emit the method InitalizeClass, which again uses MyClass, that doesn’t exist yet (that’s what I’m creating). However, I’ve managed to do all this, but now I’m having trouble with method DirectRead, since I don’t know how to call the delegate once I have it on the stack. Apparently what I need is the following emit:

ilGenerator.Emit(OpCodes.Callvirt, invokeMInfo);

where invokeMInfo is the method Invoke on DirectReadAccessor, and which I should obtain like so:

MethodInfo invokeMInfo = typeof(DirectReadAccessor<MyClass>).GetMethod(
        "Invoke",                        // Name of the method.
        BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, // Binding attributes.
        null,                            // Binder.
        new Type[] { typeof(MyClass) },  // Types of the arguments.
        null                             // Modifiers for the arguments.
);

Again, the problem is that neither MyClass nor DirectReadAccessor exist yet. I have the TypeBuilder for MyClass and the unfinished DirectReadAccessor type, which I’ve created like this:

directReadAccessorType = typeof(DirectReadAccessor<>).MakeGenericType(typeBuilder);

But if I try to call GetMethod(“Invoke”, ….) on directReadAccessorType as shown above I get a NotSupportedException, because I cannot obtain the method Invoke for an unfinished type. I’ve tested this assumption by making the same call after finalizing the type with:

typeBuilder.CreateType();

And indeed I do not get the exception in that case. However, I need to be able to get the Invoke method’s MethodInfo before finalizing the type, while I’m emitting the code for InitializeClass.

It’s a strange situation: I’ll have the delegate when I need it, but I cannot produce the code to invoke it. Can anyone offer any help?

Thanks so much, and sorry for the lengthy post.

Answer 1

That’s a tricky problem. I think that you should be able to use TypeBuilder.GetMethod to get the MethodInfo you need, using a call along the lines of

TypeBuilder.GetMethod(directReadAccessorType, 
    typeof(DirectReadAccessor<>).GetMethod("Invoke"));