Builds a Delegate from MethodInfo ?

Question

After googling and landing on SO and having read this other question

Is it possible to build a correct Delegate from a MethodInfo if you didn't know the number or types of parameters at compile time?

More on this: can this be done elegantly without the use of Reflection.Emit or type builders?

This is sorta a bummer for me because Delegate.CreateDelegate requires me to specify the correct Delegate type as the first parameter or else it would throw exceptions or invoke an incorrect method.

I'm building some ninja gears and this would helps a lot... Thanks!

---

Here's a generic solution:

/// <summary>
/// Builds a Delegate instance from the supplied MethodInfo object and a target to invoke against.
/// </summary>
public static Delegate ToDelegate(MethodInfo mi, object target)
{
    if (mi == null) throw new ArgumentNullException("mi");

    Type delegateType;

    var typeArgs = mi.GetParameters()
        .Select(p => p.ParameterType)
        .ToList();

    // builds a delegate type
    if (mi.ReturnType == typeof(void)) {
        delegateType = Expression.GetActionType(typeArgs.ToArray());

    } else {
        typeArgs.Add(mi.ReturnType);
        delegateType = Expression.GetFuncType(typeArgs.ToArray());
    }

    // creates a binded delegate if target is supplied
    var result = (target == null)
        ? Delegate.CreateDelegate(delegateType, mi)
        : Delegate.CreateDelegate(delegateType, target, mi);

    return result;
}

---

Note: I am building a Silverlight application that would replace a built-years-ago javascript application in which I have multiple Javascript interfaces that calls into the same Silverlight [ScriptableMember] method.

All those legacy JS interfaces need to be supported as well as new interface for accessing new features, so something that automatically setups the JS interface and "delegates" the call to the right Silverlight method would helps speed up work a lot.

I can't post code here, so that's the summary.

Answer 1

If you don't know the number or type of parameters in advance, presumably that means you don't know the delegate type you want to create either?

If that's the case, you're stuck in the absolutely general case.

However, for most common cases (no ref/out parameters, few enough parameters to use one of the existing types) you could get away with one of the Func or Action delegates. (.NET 4.0 has Func/Action types for huge numbers of parameters, so really you'd only need to worry about out/ref parameters.) If the method has a non-void return type use Func, otherwise use Action. Work out which type to use based on the number of parameters, e.g.

static readonly Type[] FuncTypes = { typeof(Func), 
    typeof(Func<>), typeof(Func<,>), typeof(Func<,,>), /* etc */ };

Use Type.MakeGenericType using the parameter types and return type to get the right delegate type, then Delegate.CreateDelegate should work.

I don't have time to work up a sample right now, but let me know if you want me to later on.

One question: how are you intending to use this delegate? Something else is going to need to know how to execute it, surely...

Answer 2

To be honest, if you don't know the type at compile time, there isn't a huge amount of benefit in creating a Delegate. You don't want to use DynamicInvoke; it will be about as slow as reflection. The main exception to this is when there is a delegate-type lurking in the shadows, for example when subscribing to an event - in which case EventInfo makes this available.

For info, in .NET 3.5 on Expression, there is:

Expression.GetActionType(params Type[] typeArgs);
Expression.GetFuncType(params Type[] typeArgs)

That might help to an extent:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
static class Program {
    static void Main() {
        DoStuff("Test1");
        DoStuff("Test2");
    }
    static void DoStuff(string methodName) {
        MethodInfo method = typeof(Program).GetMethod(methodName);
        List<Type> args = new List<Type>(
            method.GetParameters().Select(p => p.ParameterType));
        Type delegateType;
        if (method.ReturnType == typeof(void)) {
            delegateType = Expression.GetActionType(args.ToArray());
        } else {
            args.Add(method.ReturnType);
            delegateType = Expression.GetFuncType(args.ToArray());
        }
        Delegate d = Delegate.CreateDelegate(delegateType, null, method);
        Console.WriteLine(d);
    }
    public static void Test1(int i, DateTime when) { }
    public static float Test2(string x) { return 0; }
}