Replace a .NET object instance with an IronPython object

I'm trying to do the following:

• Get an instance of an object defined in C#
  • It implements a certain interface
  • It is a field on another C# class instance
• Wrap it with an IronPython proxy object that logs method calls
• Put the wrapper back in place of the original object

Here's an example showing what I am trying to do. In reality, the IWoof interface has many more methods on it.

The C# classes and interface:

namespace Foo
{
    public interface IWoof { string DoSomething(); }

    public class Bar
    {
        public Bar() { Woof = new WoofImp(); }
        public IWoof Woof { get; set; }
    }

    public class WoofImp : IWoof
    {
        public string DoSomething()
        {
            return "DoSomething executing in C#";
        }
    }

}

I want to take the bar.Woof and replace it with an IronPython proxy. I want to do this from an IronPython Script. I have tried various things.

First attempt: Overriding getattr

I defined a Wrapper like this:

class Wrapper(object):

    def __init__(self, obj):
        self._obj = obj

    def __getattr__(self, name):
        print '__getattr__ called'
        if name == "DoSomething":
           return self.__DoSomething
        else:
            return object.__getattr__(self, name)

    def __DoSomething(self):
        result = self._obj.DoSomething()
        return result + " AND DoSomething executing in Python"

This works fine, as you would expect:

import Foo
bar = Foo.Bar()
woof = Wrapper(bar.Woof)
print woof.DoSomething()

> DoSomething executing in C# AND DoSomething executing in Python

However, when I try to replace bar.Woof with the wrapped object, it doesn't work:

bar.Woof = woof_wrapper

> TypeError: expected IWoof, got Object_1$1
> Microsoft.Scripting.ArgumentTypeException: expected IWoof, got Object_1$1
   at CallSite.Target(Closure , CallSite , Object , Object )
   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
   at Microsoft.Scripting.Interpreter.DynamicInstruction`3.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)

Second attempt: Implementing the C# interface

So obviously my wrapper needs to implement the IWoof interface, so I tried that.

class Wrapper_IWoof(object, Foo.IWoof): # Inherits from IWoof
... etc

Now I can add the wrapper instance and try to call it like this:

woof = Wrapper_IWoof(bar.Woof)
bar.Woof = woof
print bar.Woof.DoSomething()

But I get a new problem:

> AttributeError: 'Wrapper_IWoof' object has no attribute 'DoSomething'
> System.MissingMemberException: 'Wrapper_IWoof' object has no attribute 'DoSomething'
   at IronPython.Runtime.Operations.PythonOps.MissingInvokeMethodException(Object o, String name)
   at IronPython.NewTypes.System.Object#IWoof_4$4.DoSomething()
   at Microsoft.Scripting.Interpreter.FuncCallInstruction`2.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.LightLambda.Run3[T0,T1,T2,TRet](T0 arg0, T1 arg1, T2 arg2)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
   at Microsoft.Scripting.Interpreter.DynamicInstruction`3.Run(InterpretedFrame frame)
   at Microsoft.Scripting.Interpreter.Interpreter.Run(InterpretedFrame frame)

Third and fourth attempts: Dynamic Proxy

I also tried following this approach described here (and again implementing IWoof): http://www.ronnie-midnight-oil.net/2008/11/checking-type-contract-from-ironpython.html However, they resulted in the same problems found in the two attempts above.

Alternate solution

The other way to solve the same problem is to generate a python class via reflection and use that. I have done this and it works. This gives me the explicit function declaration as described by Jeff Hardy below.

But, I would really like to know why the attempts above aren't working. Am I doing something wrong or missing something?