COM interop an interesting link

Here you can find an interesting link about COM interop

http://limbioliong.wordpress.com/2011/10/28/exposing-an-enumerator-from-managed-code-to-com/

This is one of his most important tips

The interface type should be either :

ComInterfaceType.InterfaceIsDual
or ComInterfaceType.InterfaceIsIDispatch
otherwise the GetEnumerator() method
will not be marked with dispid -4.
Without this dispid, this method
will not be recognized by COM as
returning an enumerator. It will not thus
not be usable in a VB6.0 For Each Next
statement.

Example of a COM Class Collection Written in C#

This is an example of a COM Class Collection written in C#.
You can use as starting point my template or use the Tools - Create Guid Tool on VS 2008.
You need to start a new project of type library, and set the project property Build -> Register for COM Interop
Do not check: Application, Assembly Information, Make Class COM Visible.
We are using the COMVisible attribute to decide which class is visible for us
The class will have both a defaul property (the indexers) and an iterator

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
using System.Collections;

namespace TestEmployeesCol
{
    //Wee first define the interface of the Collection
    [Guid("21C027E8-CF8C-4166-A63B-25D8E790F040"), InterfaceType(ComInterfaceType.InterfaceIsDual), ComVisible(true)]
    public interface INewEmployees : System.Collections.IEnumerable
    {


        [DispId(-4)]
        new IEnumerator GetEnumerator();
        //Iterator
        [DispId(1)]
        void Add(object key, object value);
        [DispId(2)]
        object Count { get; }
        [DispId(3)]
        void Remove(object key);
        [DispId(0)]
        object this[object key] { get; set; }
    }

    //We define the event interface
    [Guid("5C6B8153-D2D6-4e98-80EF-D13A53CC9CDD"), InterfaceType(ComInterfaceType.InterfaceIsIDispatch), ComVisible(true)]
    public interface INewEmployeesEvents
    {

    }



    //<ComClass(NewEmployees.ClassId, NewEmployees.InterfaceId, NewEmployees.EventsId)> _
    [Guid("1692DD4D-6F3E-4e77-AB50-5401F04306DC"), 
    ClassInterface(ClassInterfaceType.None),
    ComSourceInterfaces(typeof(INewEmployeesEvents)), 
    ComDefaultInterface(typeof(INewEmployees)), 
    ComVisible(true)]
    public class NewEmployees : INewEmployees
    {


        #region "COM GUIDs"
        // These  GUIDs provide the COM identity for this class 
        // and its COM interfaces. If you change them, existing 
        // clients will no longer be able to access the class.
        public const string ClassId = "1692DD4D-6F3E-4e77-AB50-5401F04306DC";
        public const string InterfaceId = "21C027E8-CF8C-4166-A63B-25D8E790F040";
        #endregion
        public const string EventsId = "5C6B8153-D2D6-4e98-80EF-D13A53CC9CDD";

        // A creatable COM class must have a Public Sub New() 
        // with no parameters, otherwise, the class will not be 
        // registered in the COM registry and cannot be created 
        // via CreateObject.
        SortedList _SortedList;
        public NewEmployees()
            : base()
        {
            _SortedList = new SortedList();
        }



        public object this[object key]
        {
            get { return _SortedList[key]; }

            set { _SortedList[key] = value; }
        }

        public object Count
        {
            get { return _SortedList.Count; }
        }


        public void Remove(object key)
        {
            _SortedList.Remove(key);
        }


        public void Add(object key, object value)
        {
            _SortedList.Add(key, value);
        }


        public System.Collections.IEnumerator GetEnumerator()
        {
            ICollection keys = _SortedList.Keys;
            return (IEnumerator)keys.GetEnumerator();
        }


    }



}