Question

I have a 3rd party C++ DLL that I call from C#. The methods are static.

I want to abstract it out to do some unit testing so I created an interface with the static methods in it but now my program errors with:

The modifier ‘static’ is not valid for this item

MyMethod cannot be accessed with an instance reference; qualify it with a type name instead

How can I achieve this abstraction?

My code looks like this

private IInterfaceWithStaticMethods MyInterface;

public MyClass(IInterfaceWithStaticMethods myInterface)
{
  this.MyInterface = myInterface;
}

public void MyMethod()
{
  MyInterface.StaticMethod();
}

Answer 1

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.