IronPython function with variable number of arguments as a delegate

Question

Hey!

In IronPython I am trying to call a PythonFunction with different numbers of arguments from C#. For instance;

I want to do:

def foo(a, b):
     print a, b

def bar(a, b, c = None):
     print a, b, c

p = App.DynamicEvent()
p.addHandler(foo)
p.addHandler(bar)
p.invoke("Not", "Working")

where addHandler takes a single argument and somehow stores it in a list of methods to be invoked and invoke has a signature like this:

public virtual void invoke(params object[] tArgs)

Because I want to avoid making it specific to the PythonEngine (and thus engine.Operations.Invoke()), I've tried several ways of storing and implementing these things as delegates but I think the crux of my problem is that I don't know how to store some kind of MulticastDelegate base type that is compatible with a PythonFunction?

Perhaps I want to implement my own DynamicInvoke method? Any thoughts and experience would be greatly appreciated!

The reason for wanting to do this is that I want to transparently map calls made from a sealed Javascript engine into IronPython via C#. i.e. in the Javascript call: Client.doThing("something", 4, {"key:"value"}) and handle it in the python with:

def doThing(s, i, d):
    pass

using the following dynamic event binding:

doThingEvent = App.DynamicEvent()
doThingEvent.addHandler(doThing)
WebBrowser.handleMethod("doThing", doThingEvent);

Answer 1

You can pass a PythonFunction as a delegate for example by casting to an Action<...>

i.e. in Python by doing something like:

import sys
import clr
import System
from System import *

def foo(a, b):
     print a, b

def bar(a, b, c = "N.A."):
     print a, b, c

p = App.DynamicEvent()
p.AddHandler( Action[object,object](foo) )
p.AddHandler( Action[object,object,object](bar) )
p.DynamicInvoke("Not", "Working")

In this way your p can be a MulticastDelegate. Obviously this implies:

1. All the passed delegates must have the same signature
2. you can pass Python function with at most 16 parameters (because Action support 16 params maximum)

For the first problem I think you need to write your own "Delegates invoker", something like:

//
// WARNING: very very rough code here !!
//
public class DynamicEvent
{
    List<Delegate> delegates;

    public DynamicEvent()
    {
        delegates = new List<Delegate>();
    }
    public void AddHandler(Delegate dlgt)
    {
        delegates.Add(dlgt);
    }
    public void Invoke(params object[] args)
    {
        foreach (var del in delegates)
        {
            var parameters = del.Method.GetParameters();
            // check parameters number
            if (args.Length != parameters.Length)
                continue; // go to next param

            // check parameters assignability
            bool assignable = true;
            for (int i = 0; i < args.Length; i++)
            {
                if (!parameters[i].ParameterType.IsInstanceOfType(args[i]))
                {
                    assignable = false;
                    break; // stop looping on parameters
                }
            }
            if (!assignable)
                continue; // go to next param

            // OK it seems compatible, let's invoke
            del.DynamicInvoke(args);
        }
    }
}

N.B.:

the part checking delegate-parameters compatibility is wrong, is just to give you an idea.

The problem is that I don't know how to check, at runtime, whether a list of object args is compatible with a delegate signature... maybe we should check IronPython source code :)

Answer 2

My first idea was as digEmAll suggested but I came up with a better solution that doesn't require the casting to System.Action or any invoke-time type checking or branching. Just overload addHandler to take a PythonFunction as well as a Delegate, meaning you can use your original code:

def foo(a, b):
     print a, b

def bar(a, b, c = None):
     print a, b, c

p = DynamicEvent(engine)
p.addHandler(foo)
p.addHandler(bar)

p.invoke("a", "b")
p.invoke("a", "b", "c")

With this C# code:

public class DynamicEvent
{
    private Dictionary<int, Action<object[]>> delegates = new Dictionary<int, Action<object[]>>();
    public ScriptEngine Engine { get; set; }


    public DynamicEvent(ScriptEngine engine)
    {
        Engine = engine;
    }

    public void addHandler(PythonFunction pythonFunction)
    {
        int args = (int) pythonFunction.func_code.co_nlocals;
        delegates.Add(args, a => Engine.Operations.Invoke(pythonFunction, a));
    }

    public void addHandler(Delegate d)
    {
        int args = d.Method.GetParameters().Length;
        delegates.Add(args, a => d.DynamicInvoke(a));
    }

    public void invoke(params object[] args)
    {
        Action<object[]> action;
        if(!delegates.TryGetValue(args.Length, out action))
            throw new ArgumentException("There is no handler that takes " + args.Length + " arguments!");

        action(args);
    }
}

Note that you need to add the engine to the script's scope so you can use it in the constructor.

Hope that helped!

---

Note: It is possible to get a Delegate and execute it from a PythonFunction like this:

Delegate target = pythonFunction.__code__.Target;
var result = target.DynamicInvoke(new object[] {pythonFunction, args});

but it is more platform dependant than using Engine.Operations.Invoke() and the Delegate signature is different to a 'regular' Delegate.