Multiple interfaces

Classes can implement multiple interfaces. It might happen that the component under test expects the incoming dependency to implement more than one interface, like in the snippet below.

public class Dependency : IDependency, IDisposable { ... }
​
public class Service
{
    private readonly IDependency _dependency;
​
    public Service (IDependency dependency) { _dependency = dependency; }
​
    public void DoSomething()
    {
        using (_dependency as IDisposable) { ... }
    }
​
    public void Dispose() { ... }
}

While being a signal that responsibilities are not properly split across interfaces, it's something that developers need to be able to cope with while developing unit tests.

Moq offers a way to decorate a mock with additional interfaces using the construct As, giving developers the possibility to configure and verify methods of the added interface.

var mock = new Mock<IDependency>();
mock.As<IDisposable>().Setup(p => p.Dispose());

Alternatively, you can also store the reference returned by As into its own variable for later usage.

var mockDisposable = mock.As<IDisposable>();
mockDisposable.Verify(p => p.Dispose(), Times.Once());