Hooking up C# and VBA doesn't work

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C#

Hi,

I've been trying to implement a simple function addNumbers(double a, double b) in C# and call it from VBA. I've proceeded as here: Extend your VBA code with C#, VB.NET, or C++/CLI[^]

Code in C#:

using System
using System.Runtime.InteropServices; // Guid, ClassInterface, ClassInterfaceType
using System.Globalization; // CultureInfo
[Guid("97E1D9DB-8478-4E56-9D6D-26D8EF13B100")]
public interface IAddNumbers {
    double addNumbers(double a, double b)
};

[Guid("BBF87E31-77E2-46B6-8093-1689A144BFC6")]
[ClassInterface(ClassInterfaceType.None)]
public class TestClass : IAddNumbers {
    public double addNumbers(double a, double b) {
        return a + b;
    {
}

VBA:
first I registered the file TestClass.dll in the registry and then added a reference in the VBA environment as indicated in the website above. The problem is that VBA does not see TestClass at all. I cannot declare a variable like this:

Dim addNumObj as TestClass
or
Dim addNumObj as new TestClass

Does anyone know why this doesn't work/VBA is not seeing the .dll?
What are the Guids for and how are they generated? I took the Guids from the example on the website.

Many thanks for any help.