Passing objects from C# to VBA using COM Interop

I assume you're talking about Excel VBA to C# ...

here's a minimal C# class that does it, in a project w default name ClassLibrary1:

using System;
using System.Runtime.InteropServices;

namespace Tester
{
    [ClassInterface(ClassInterfaceType.AutoDual)]
    public class TestClass
    {
        public double D { get; set; }  // simple property to get, set a double
        public string S { get; set; }  // simple property to get, set a string
    }
}

and here's VBA to try the class out:

Private Sub foo()

Dim X As New ClassLibrary1.TestClass

X.S = "Hello"
Debug.Print X.S ' prints "hello"

X.D = 12
Debug.Print X.D ' prints a 12

End Sub

and here are the extra things you need to do to make this work:

(1) in C# Project...Properties...Build ==> check "Register for COM interop
(2) in C# Project...Properties...Application...Assembly Information ==> 
    check "Make assembly COM-visible"
(3) in VBA ... Tools ... References, browse to the C# bin output directory and select the "*.tlb" file

Note: this scheme may fail depending on what you add to the class - I don't think VBA will "see" static classes or classes w other than default constructors. You also cannot map VB collections to .NET collections, but you will be able to pass basic types (double, long) and arrays of the basic types back and forth. Also - the "Autodual" option used is a cheap way to get methods exposed ... easy to get started but less efficient and exposes all public methods. Better practice (but more work) would be to set up your own interfaces. If you expand the members of this TestClass to include instances of other classes you have defined, and if you likewise expose those class methods via AutoDual or via hand-coded interfaces, then those classes and their (non-overloaded) methods will likewise be visible in VBA (with Intellisense).

Hope this helps.