c# - Why should I not use AutoDual?

Question

Up to now, I've always decorated my .NET classes that I want to use from VB6 with the [AutoDual] attribute. The point was to gain Intellisense on .NET objects in the VB6 environment. However, the other day I googled AutoDual and the first answer is 'Do Not Use AutoDual'.

I've looked for coherent explanation of why I shouldn't use it, but could not find it.

Can someone here explain it?

Answer 1

I found a reliable way to both provide Intellisense for .NET objects in VB6, while at the same time not breaking the interface. The key is to mark each public method/property in the interface with DispatchID. Then the class must inherit from this interface - in the manner below.

[Guid("BE5E0B60-F855-478E-9BE2-AA9FD945F177")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface ICriteria
{
    [DispId(1)]
    int ID { get; set; }
    [DispId(2)]
    string RateCardName { get; set; }
    [DispId(3)]
    string ElectionType { get; set; }
}


[Guid("3023F3F0-204C-411F-86CB-E6730B5F186B")]    
[ClassInterface(ClassInterfaceType.None)]
[ProgId("MyNameSpace.Criteria")]
public class Criteria : ICriteria
{
    public int ID { get; set; }
    public string RateCardName { get; set; }
    public string ElectionType { get; set; }
}

What the dispatch ID gives you is the ability to move around items in the class, plus you can now add new things to the class and not break the binary compatibility.