How to pass a lambda expression to a C# constructor from an IronPython script?

Question

I'm integrating an IronPython scritping engine into my C# raytracer which, so far, has been a breeze even though I'm completely new to Python. There is one particular thing, though, that I need help with. I have a C# class which defines a constructor like this:

public CameraAnimation(Action<Camera, float> animation)

In C#, I would instantiate this like so:

var camAnimation = new CameraAnimation((camera, time) => camera.Position += new Vector(1, 0, 0));

I can't quite figure out how to make a similar assignment for the Action object in IronPython, so how would the Python syntax look?

Would you list some of the failure attempts? I am sure that we can all learn from your failure. (codinghorror.com/blog/archives/000576.html) — dance2die
– dance2die, Commented Apr 28, 2009 at 23:40
Heh, if you insist. My approach to converting C# code to Python was to paste the C# into a .py file and remove stuff until it compiled. This mostly worked but unfortunately distracted me from the rather obvious point that I should just declare a function and pass it as a parameter. — Morten Christiansen
– Morten Christiansen, Commented Apr 29, 2009 at 7:15
Current URI for @Sung's link: blog.codinghorror.com/fail-early-fail-often — leewz
– leewz, Commented Mar 7, 2018 at 20:01

sll · Accepted Answer · 2012-03-16 20:28:22Z

2

Assuming I interpreted this right, and Action is a generic delegate, the below works (the stubs I used are included).

Python:

import clr
clr.AddReference("IronPythonDelegates")

import IronPythonDelegates

def camActionPy(camera, time):
  print "Camera: " + str(camera) + ", time: " + str(time)

IronPythonDelegates.CameraAnimation(camActionPy);

CSharp:

namespace IronPythonDelegates
{
    public class Camera{}

    public class CameraAnimation
    {
    private System.Action<Camera, float> animation;

    public CameraAnimation(System.Action<Camera, float> animation)
    {
        this.animation = animation;
        this.animation(new Camera(), 1.5f);
    }
    }
 }

I corrected the above to use System.Action, and it no longer requires explicit reflection. It's a bit weird though. For some reason, I could construct a user-created delegate like:

explicitTestAction = IronPythonDelegates.TestAction[IronPythonDelegates.Camera, System.Single](camActionPy);
IronPythonDelegates.CameraAnimation(explicitTestAction);

but could not do so with System.Action. E.g. with

explicitSystemAction = System.Action[IronPythonDelegates.Camera, System.Single](camActionPy)
IronPythonDelegates.CameraAnimation(explicitSystemAction);

explicitSystemAction is null. TestAction was just defined as:

public delegate void TestAction<T1, T2>(T1 one, T2 two);

But luckily either way it's fine to just do:

CameraAnimation(System.Action)

or

CameraAnimation(TestAction)

though for some reason I don't remember that working when I first tried...

edited Mar 16, 2012 at 20:28

sll

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answered Apr 28, 2009 at 23:03

Matthew Flaschen

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3 Comments

Matthew Flaschen Over a year ago

Note, I found System.Action, but unfortunately changing it to use that inexplicably broke the code... Looking at it now.

Srivatsn Narayanan Over a year ago

Did you add a reference to System.Core.dll? System.Action<T> is defined in mscorlib but System.Action<T1,T2> is defined in System.Core.

Matthew Flaschen Over a year ago

Yes, I have a reference to System.Core. As noted, I have it working now.

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