.NET ICustomDoc interface - AxWebBrowser Control

Question

I'm trying to access my codebase with the COM AxWebBrowser control. In the .NET WebBrowser control you can do this with

mainWebBrowser.ObjectForScripting = this;

In the AxWebBrowser I found this:

var cDoc = (ICustomDoc)this;
cDoc.SetUIHandler((IDocHostUIHandler)this);

However, ICustomDoc is an interface I can't find anywhere. I've googled and found solutions like implementing it yourself or this link, but none of them seem to work.

Also System.Windows.Forms.UnsafeNativeMethods.IDocHostUIHandler gives me an error saying that I can't access internal class UnsafeNativeMethods.

After searching for an hour, I gave up. Maybe one of you guys can help me out here.