Advanced usage

FakeItEasy exposes a few APIs that aren't commonly needed, but can be useful in certain scenarios.

Clearing the configuration of a fake

To "unconfigure" a fake so it has default behavior, discard the fake and create a new one. If this is not feasible, for example because you wish to remove the configuration in the middle of a test and the system under test already holds the fake, see Changing behavior between calls.

Clearing a fake's recorded calls

The Fake.ClearRecordedCalls method clears all recorded calls from a fake. Subsequent call assertions won't see these calls.

var foo = A.Fake<IFoo>();
foo.Bar();
A.CallTo(() => foo.Bar()).MustHaveHappened();
Fake.ClearRecordedCalls(foo);
A.CallTo(() => foo.Bar()).MustNotHaveHappened();

Getting the list of calls made on a fake

Sometimes, the call assertion API offered by FakeItEasy isn't enough, and you need to manually check the calls made on a fake. The Fake.GetCalls method returns a fake's recorded calls as a list of ICompletedFakeObjectCall objects that you can examine yourself:

var foo = A.Fake<IFoo>();
foo.Bar();
foo.Baz();
var calls = Fake.GetCalls(foo).ToList();
Assert.Equal(2, calls.Count);
Assert.Equal("Bar", calls[0].Method.Name);
Assert.Equal("Baz", calls[1].Method.Name);

The FakeManager object

The Fake.GetFakeManager method returns a FakeManager object that can be used to get information on the fake and manipulate its call rules.

Fake.GetFakeManager throws an exception if the provided object is not a fake. To test if an object is a fake you can call Fake.IsFake or try get the FakeManager with Fake.TryGetFakeManager which will return true if the provided object is a fake and also give you the FakeManager for that object via an out parameter.

Getting the type of the fake

The FakeManager.FakeObjectType property returns the type of the fake, i.e. the type that was passed to A.Fake. This can be useful if you're writing code that dynamically manipulates fakes.

var foo = A.Fake<IFoo>();
var manager = Fake.GetFakeManager(foo);
Assert.Equal(typeof(IFoo), manager.FakeObjectType);

Getting the fake from the fake manager

The FakeManager.Object property returns the fake object managed by this FakeManager.

var foo = A.Fake<IFoo>();
var manager = Fake.GetFakeManager(foo);
Assert.Equal(foo, manager.Object);

Manipulating a fake's call rules

The FakeManager.Rules property returns all the rules configured on a fake.

var foo = A.Fake<IFoo>();
A.CallTo(() => foo.Bar()).Returns(42);
A.CallTo(() => foo.Baz()).DoesNothing();
var manager = Fake.GetFakeManager(foo);
var rules = manager.Rules.ToList();
Assert.Equal(2, rules.Count);

It is also possible to add custom rules for advanced scenarios, using AddRuleFirst and AddRuleLast. AddRuleFirst adds a rule at the beginning of the rule list, so that it's considered before any other rule (which is the normal behavior when configuring a fake with A.CallTo(...)). AddRuleLast adds a rule at the end of the rule list, so that it's considered after any other rule.

class MyRule : IFakeObjectCallRule
{
    public int? NumberOfTimesToCall => null;

    public void Apply(IInterceptedFakeObjectCall fakeObjectCall)
    {
        fakeObjectCall.SetReturnValue(42);
    }

    public bool IsApplicableTo(IFakeObjectCall fakeObjectCall)
    {
        return fakeObjectCall.Method.DeclaringType == typeof(IFoo)
            && fakeObjectCall.Method.Name == "Bar";
    }
}

...

var foo = A.Fake<IFoo>();
var manager = Fake.GetFakeManager(foo);
manager.AddRuleFirst(new MyRule());
Assert.Equal(42, foo.Bar());

You can also remove a rule using the RemoveRule method.

Intercepting calls

Using the FakeManager.AddInterceptionListener method, you can add a listener that is called every time a fake method is called.

class MyListener : IInterceptionListener
{
    public void OnBeforeCallIntercepted(IFakeObjectCall interceptedCall)
    {
        Console.WriteLine($"A call to '{interceptedCall.Method}' is about to be processed");
    }

    public void OnAfterCallIntercepted(ICompletedFakeObjectCall interceptedCall)
    {
        Console.WriteLine($"A call to '{interceptedCall.Method}' has been processed");
    }
}

...

var foo = A.Fake<IFoo>();
A.CallTo(() => foo.Baz()).Invokes(() => Console.WriteLine("Hello world"));
var manager = Fake.GetFakeManager(foo);
manager.AddInterceptionListener(new MyListener());
foo.Baz();

The code above prints:

A call to 'Void Baz()' is about to be processed
Hello world
A call to 'Void Baz()' has been processed