CreateDelegate with unknown types

Question

I am trying to create Delegate for reading/writing properties of unknown type of class at runtime.

I have a generic class Main<T> and a method which looks like this:

Delegate.CreateDelegate(typeof(Func<T, object>), get)

where get is a MethodInfo of the property that should be read. The problem is that when the property returns int (I guess this happens for value types) the above code throws ArgumentException because the method cannot be bound. In case of string it works well.

To solve the problem I changed the code so that corresponding Delegate type is generated by using MakeGenericType. So now the code is:

Type func = typeof(Func<,>);
Type generic = func.MakeGenericType(typeof(T), get.ReturnType);
var result = Delegate.CreateDelegate(generic, get)

The problem now is that the created delegate instance of generic so I have to use DynamicInvoke which would be as slow as using pure reflection to read the field.

So my question is why is that the first snippet of code fails with value types. According to MSDN it should work as it says that

The return type of a delegate is compatible with the return type of a method if the return type of the method is more restrictive than the return type of the delegate

and how to execute the delegate in the second snippet so that it is faster than reflection.

Thanks.

Answer 1

Here's one way to solve your problem. Create a generic method:

public static Func<T, object> MakeDelegate<U>(MethodInfo @get)
{
    var f = (Func<T, U>)Delegate.CreateDelegate(typeof(Func<T, U>), @get);
    return t => f(t);
}

This way, C#'s compiler takes care of inserting the necessary boxing (if any) to convert f(t) (of type U) to object. Now you can use reflection to call this MakeDelegate method with U set to @get.ReturnType, and what you get back will be a Func<T, object> which can be called without needing to resort to using DynamicInvoke.

Answer 2

Your original code can only work for reference types. That's why string wasn't a problem, it directly derives from System.Object. That a value type derives from ValueType and Object is a nice illusion on paper but actually requires code. The C# compiler automatically emits that code, it requires a boxing conversion. That's the part that is missing here, there is no runtime conversion from int to object without the BOX opcode.

You can get that opcode in your code, but you'll have to use System.Reflection.Emit.

Before you go there, first check if what you've got now is actually too slow. The expense of reflection is digging the metadata out of the assembly. That was done when you created the delegate, the type info is cached after that.