My WPF Application instance implements a custom interface, but second DLL doesn't see the interface

I have an application written in WPF where one of the assemblies needs an

System.Net.Http.HttpClient

System.Net.Http.HttpClient

instance. The MSDN documentation says to have only one instance for your entire application. But I was using the client an assembly, here called

MyDLL

MyDLL

that wasn't the application's assembly. So I put in my application class a reference to a custom interface,

IHttpClientOwner

IHttpClientOwner

that's declared in an assembly that both projects can reference. (Otherwise, I'd get a circular reference.) I thought I'd find when the class inside

MyDLL

MyDLL

checks

System.Windows.Application.Current

System.Windows.Application.Current

, it would find

IHttpClientOwner

IHttpClientOwner

was implemented. But no. It isn't.

What's going on?