Invoking Custom Code

Sometimes a faked method's desired behavior can't be satisfactorily defined just by specifying return values, throwing exceptions, assigning out and ref parameters or even doing nothing. Maybe you need to simulate some kind of side effect, either for the benefit of the System Under Test or to make writing a test easier (or possible). Let's see what that's like.

A.CallTo(() => fakeShop.SellSmarties())
 .Invokes(() => OrderMoreSmarties()) // simulate Smarties stock falling too low
 .Returns(20);

Now when the System Under Test calls SellSmarties, the Fake will call OrderMoreSmarties.

If the method being configured has a return value, you should use Return to specify the return value, or it will return null (or a default value for a value type). This is true even if the return type of the method is such that an unconfigured method would not return null (for example, if the method returns a string or Task).

There are also more advanced variants that can invoke actions based on arguments supplied to the faked method. These act similarly to how you specify return values that are calculated at call time. For example

// Pass up to 4 original call argument values into the method that creates the exception.
// Since FakeItEasy 1.15.0, this works even if NumberOfSweetsSoldOn takes a ref DateTime 
// or an out DateTime.
A.CallTo(()=>fakeShop.NumberOfSweetsSoldOn(A<DateTime>._))
 .Invokes((DateTime when) => System.Console.Out.WriteLine("showing sweet sales for " + when)
 .Returns(17);

// Pass an IFakeObjectCall into the creation method for more advanced scenarios.
A.CallTo(() => fakeShop.NumberOfSweetsSoldOn(A<DateTime>._))
 .Invokes(callObject => System.Console.Out.WriteLine(callObject.FakedObject +
                                                     " is closed on " +
                                                     callObject.Arguments[0]))
 .Returns(0);