I'm trying to implement an interface I created, in a VisualAPL class.  When I instantiate the class directly, the methods and properties work.  When I instantiate the class and cast it to the interface, the methods and properties throw exceptions.

The code is below.  Any ideas what is wrong?

The interface, written in C#:

using System;
using System.Collections.Generic;
using System.Text;
namespace TestInterface3{
  public interface ITest{
    Int32 LeftArg {get; }
    Int32 RightArg {get; }
    Int32 AddArgs(Int32 dummy);
  }
}

The class, written in VisualAPL

#region Using directives
using System;
using System.Collections.Generic;
using System.Text;
refbyfile @"c:\temp\testinterface3.dll";
using TestInterface3;
#endregion
namespace VisualAPLTest{
  public class VisualClass : ITest{
    // Private variables.
    private Int32 m_leftarg;
    private Int32 m_rightarg;
 
    // The constructor.
    public VisualClass(Int32 left, Int32 right){
 
      // Store the variables.
      m_leftarg ← left
      m_rightarg ← right
    }
 
    // The two properties.
    public property LeftArg {
      get { return m_leftarg; }
    }
 
    public property RightArg {
      get { return m_rightarg; }
    }
 
    // The public method.
    public Int32 AddArgs(Int32 dummy){
      return m_leftarg + m_rightarg;
    }
  }
}
 
The calling code that works is:
  VisualAPLTest.VisualClass testClass = new VisualAPLTest.VisualClass(3, 5);
 
The calling code that fails is:
  TestInterface3.ITest testClass = new VisualAPLTest.VisualClass(3, 5);
 
I'm getting the error An unhandled exception of type 'System.Exception' 
occurred in VisualAPLTest.dll
 
Thanks for your help.  Chris.

I don't think I understand what you are doing; must look it up!

It is time this forum allowed file uploads so that problems can be easily reproduced.

At a guess, I think you are missing some casts.

   // The two properties.

    public property LeftArg {

      get { (int32) return m_leftarg; }

    }

    public property RightArg {

      get { return (Int32) m_rightarg; }

    }

    // The public method.

    public Int32 AddArgs(Int32 dummy){

      return (Int32) m_leftarg + m_rightarg;

The changes are on the 2  'get' and the one 'return' lines.

Ajay thanks for this advice.  You were right about the missing casts, but not quite right about where they needed to be.  I hadn't cast my property declarations.  The correct way to do this is:

    // The two properties.
    public property Int32 LeftArg {
      get { return m_leftarg; }
    }

    public property Int32 RightArg {
      get { return m_rightarg; }
    }

You can leave the casts in the { return } statements, but they are not necessary.

I've been learning a little about Design Patterns and am trying to use them.  These recommend programming to interfaces rather than implementations (concrete objects - instantiations of classes).