What does the C# CoClass attribute do?

Question

I found code something like the following in a 3rd party library we're using.

[CoClass(typeof(BlahClass))]
public interface Blah : IBlah
{
}

What is this doing exactly? The msdn documentation didn't illuminate the subject sufficiently for me to follow.

Answer 1

It declares that the interface Blah is intended to be implemented by a specific class. It means that you can conveniently say new Blah and the runtime will know what object to create - something that is not normally possible with an interface.

If you look at the generated declaration for BlahClass, it will presumably have a Guid associated with it which can be used to create an instance of a COM object.