What is the fastest way in .Net 3.5 to call a method by string name?

Question

So the obvious way to do this is..

var handler = GetType().GetMethod(methodName, BindingFlags.NonPublic |
                                              BindingFlags.Instance);

handler.Invoke(this, new object[] {e});

And I can add caching to keep the methods around but I'm wondering if there is a completely different and faster way?

Answer 1

The fastest way would be to cache a typed delegate; if you know the signature is always:

void PersonInstance.MethodName(string s);

Then you could create an Action<Person,string> via Delegate.CreateDelegate:

var action = (Action<Person,string>)Delegate.CreateDelegate(
    typeof(Action<Person,string>), method);

This can then be cached against the name, and invoked as:

action(personInstance, value);

Note that the cache here is critical; the reflection to locate the method and prepare a typed-delegate is non-trivial.

It gets harder if the signature is unpredictable, as DynamicInvoke is relatively slow. The fastest way would be to use DynamicMethod and ILGenerator to write a shim method (on the fly) that takes an object[] for the parameters and unpacks and casts it to match the signature - then you can store an Action<object, object[]> or Func<object,object[],object>. That is, however, an advanced topic. I could provide an example if really needed. Essentially writing (at runtime):

void DummyMethod(object target, object[] args) {
    ((Person)target).MethodName((int)args[0],(string)args[1]);
}

Here's an example of doing that (note: it doesn't handle ref/out args at the moment, and possibly a few other scenarios - and I've left the "cache" side of things as an exercise for the reader):

using System;
using System.Reflection;
using System.Reflection.Emit;

class Program
{
    static void Main()
    {
        var method = typeof(Foo).GetMethod("Bar");
        var func = Wrap(method);
        object[] args = { 123, "abc"};
        var foo = new Foo();
        object result = func(foo, args);
    }

    static Func<object, object[], object> Wrap(MethodInfo method)
    {
        var dm = new DynamicMethod(method.Name, typeof(object), new Type[] {
            typeof(object), typeof(object[])
        }, method.DeclaringType, true);
        var il = dm.GetILGenerator();

        if (!method.IsStatic)
        {
            il.Emit(OpCodes.Ldarg_0);
            il.Emit(OpCodes.Unbox_Any, method.DeclaringType);
        }
        var parameters = method.GetParameters();
        for (int i = 0; i < parameters.Length; i++)
        {
            il.Emit(OpCodes.Ldarg_1);
            il.Emit(OpCodes.Ldc_I4, i);
            il.Emit(OpCodes.Ldelem_Ref);
            il.Emit(OpCodes.Unbox_Any, parameters[i].ParameterType);
        }
        il.EmitCall(method.IsStatic || method.DeclaringType.IsValueType ?
            OpCodes.Call : OpCodes.Callvirt, method, null);
        if (method.ReturnType == null || method.ReturnType == typeof(void))
        {
            il.Emit(OpCodes.Ldnull);
        }
        else if (method.ReturnType.IsValueType)
        {
            il.Emit(OpCodes.Box, method.ReturnType);
        }
        il.Emit(OpCodes.Ret);
        return (Func<object, object[], object>)dm.CreateDelegate(typeof(Func<object, object[], object>));
    }
}

public class Foo
{
    public string Bar(int x, string y)
    {
        return x + y;
    }
}