This tutorial provides a quick and easy guide for basic communication between Visual Basic and C# utilizing the COM Interoperability in .NET.

We’ll focus on building a C# class library designed to receive messages from a VB6 application. The VB6 component features a straightforward form housing a button that dispatches a message to the C# library.

Check the repository for the full source code

https://github.com/nikosportolos/COM-Interoperability-in-.NET/tree/main/part-i

• .NET class library
  • Setup .NET project
  • C# source code
• VB6 winform app
  • Setup VB6 project
  • VB6 winform app
• Demo

1. Create a new C# Project in Visual Studio

   

2. Select Class Library

   

3. Make assembly COM Visible

• Right-Click on Project

• Properties (Alt+Enter)

  
  • Application tab
  • Assembly Information button
  
  • Check option “Make assembly COM Visible”
  

4. Register for COM Interop

   • Build tab
   • Output section
   • Check option “Register for COM interop”
   

1. Access InteropServices Namespace

using System.Runtime.InteropServices

2. Create your Class interface

[Guid("123456789-1234-1234-1234-123456789123")]
[ComVisible(true)]
public interface IVB6Interop
{
    [DispId(1)]
    void SampleMethod(string Message);
}

3. Implement the IVB6Interop interface

[Guid("123456789-1234-1234-1234-123456789123")]
[ProgId("VB6Interop")]
[ClassInterface(ClassInterfaceType.None)]
[ComSourceInterfaces(typeof(IVB6InteropEvents))]
[ComVisible(true)]
public class VB6Interop : IVB6Interop
{
    public void SampleMethod(string Message)
    {
        try
        {
            MessageBox.Show(Message);
        }
        catch (Exception ex)
        {
            throw new Exception("Exception occured in SampleMethod(): ", ex);
        }
    }
}

1. Create a new VB6 Project

• Select Standard EXE

• Add VB6Interop.dll reference

  • Project

  • References

    

  • Check VB6Interop Reference

    

1. Declare a VB6Interop object

Public VB6 As VB6Interop.VB6Interop

2. Initialize VB6Interop object

Private Sub Form_Load()
    Set VB6 = New VB6Interop.VB6Interop
End Sub

3. Use SampleMethod() of VB6Interop on button click

Private Sub Command1_Click()
    VB6.SampleMethod "Hello VB6!"
End Sub

…and we’re done!