[2.0] Can't add a C# reference (dll) in my vb application

even checked "Register for COM Interop" under Build . When I add a reference to the comiled C# dll, I get "can't add reference".

using System;

using System.Runtime.InteropServices;

namespace Tester

{

[Guid("D6F88E95-8A27-4ae6-B6DE-0542A0FC7039")]

[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]

public interface _Numbers

{

[DispId(1)]

int GetDay();


[DispId(2)]

int GetMonth();

[DispId(3)]

int GetYear();

[DispId(4)]

int DayOfYear();

}

[Guid("13FE32AD-4BF8-495f-AB4D-6C61BD463EA4")]

[ClassInterface(ClassInterfaceType.None)]

[ProgId("Tester.Numbers")]

public class Numbers : _Numbers

{

public Numbers(){}

public int GetDay()

{

return(DateTime.Today.Day);

}

public int GetMonth()

{

return(DateTime.Today.Month);

}

public int GetYear()

{

return(DateTime.Today.Year);

}

public int DayOfYear()

{

return(DateTime.Now.DayOfYear);

}

}

}