Example using FakeItEasy; using FakeItEasy.Auto; using NUnit.Framework; public interface IBar { void DoSomething(); } public class Foo { private readonly IBar bar; public Foo(IBar bar) { this.bar = bar; } public void MakeBarDoSomething() { bar.DoSomething(); } } public class FooTests { [Test] public void Can_make_bar_do_something() { // Given var foo = An.AutoFaked<Foo>(); var bar = TheFake<IBar>.UsedBy(foo); // When foo.MakeBarDoSomething(); // Then A.CallTo(() => bar.DoSomething()).MustHaveHappened(); } }