Re: [IronPython] C# code calling IP subclass weirdness

msurel Mon, 14 Nov 2005 11:58:29 -0800

In C#, if you want somebody to be able to override a method in a base class, I 
think you need the virtual keyword. I believe if you define the DoSomething 
method in Helper with the virtual keyword, then you should be ok.


That would be about the only suggestion I have. Other than that, the code looks 
ok to me. In C# if you want to override a method in a base class I believe it 
has to be declared virtual and the method in the subclass needs to have the 
override keyword.

Since you don't have the equivalent of the 'override' keyword in Python, I'm 
not 100% sure you will be able to do this anyway.


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A question about subclassing, virtual methods, C#/IP interop, etc in IronPython-0.9.4. I have a library of two classes Helper & Master (greatly simplified from my actual situation, but analagous):

using System;

namespace TestLib

{

    public class Helper

    {

        int _id;

        public Helper(int id)

        {

            _id = id;

        }

        public void DoSomething()

        {

            Console.WriteLine("Helper”);

        }

    }

    public class Master

    {

        protected virtual Helper CreateHelper(int id)

        {

            return new Helper(id);

        }

        public void DoSomething()

        {

            Helper helper = CreateHelper(0);

            helper.DoSomething();

        }

    }

}

Master has a factory-style method that creates a helper, and then invokes a method on it. I want to insert some throw-away behavior, by subclassing both Master and Helper, so that FooMaster creates a FooHelper, and then invokes the FooHelper version of DoSomething.

from TestLib import *

class FooHelper(Helper):

    def __init__(self, id):

        self._id = id

    def DoSomething(self):

        print "FooHelper"

class FooMaster(Master):

    def CreateHelper(self, id):

        return FooHelper(id)

# === Main ==================

fooMaster = FooMaster()

fooMaster.DoSomething()

I expect this to print out “FooHelper”. Instead it prints out “Helper”.

Note that if I were to define the superclasses Helper/Master in Python instead of C#, everything works like a charm, and it prints out “FooHelper”:

class Helper:

    _id = 0

    def __init__(self, id):

        self._id = id;

    def DoSomething(self):

        print "Helper"

class Master:

    def CreateHelper(self, id):

        return Helper(id)

    def DoSomething(self):

        self.CreateHelper(10).DoSomething()

This leads me to believe it is impossible for a C# object to call a Python method that overrides a virtual method. However, also note that if I define the Helper class to be abstract in C#, it also works fine and I see “FooHelper”.

using System;

using System.Text;

namespace TestLib

{

    public abstract class Helper

    {

        int _id;

        public Helper(int id)

        {

            _id = id;

        }

        public abstract void DoSomething();

    }

    public class Master

    {

        protected virtual Helper CreateHelper(int id)

        {

            return null;

        }

        public void DoSomething()

        {

            Helper helper = CreateHelper(0);

            helper.DoSomething();

        }

    }

}

Furthermore, when I simplify things even further, it also works the way I’d expect:

public class Helper

{

    public Helper(int id) { }

    public virtual void DoSomething() { Console.WriteLine("Helper"); }

}

public class Master

{

    public Master(Helper helper) { helper.DoSomething(); }

}

class FooHelper(Helper):

    def __init__(self, id): pass

    def DoSomething(self): print "FooHelper"

Can anybody tell me what’s going on? Is this a bug in IronPython or am I doing something wrong?

Many thanks,

Michael

--- End Message ---

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Re: [IronPython] C# code calling IP subclass weirdness

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