How can I implement static methods on an interface?

You can’t define static members on an interface in C#. An interface is a contract for instances.

I would recommend creating the interface as you are currently, but without the static keyword. Then create a class StaticIInterface that implements the interface and calls the static C++ methods. To do unit testing, create another class FakeIInterface, that also implements the interface, but does what you need to handle your unit tests.

Once you have these 2 classes defined, you can create the one you need for your environment, and pass it to MyClass‘s constructor.