Throwing Exceptions

When it's deployed, you may not want code to throw exceptions, but often it's necessary to test what happens when libraries your code interacts with throw them. You can configure a Fake to throw an exception like this:

A.CallTo(() => fakeShop.NumberOfSweetsSoldOn(DateTime.MaxValue))
 .Throws(new InvalidDateException("the date is in the future");

If the exception type has a parameterless constructor, you can use it like

A.CallTo(() => fakeShop.NumberOfSweetsSoldOn(DateTime.MaxValue))
 .Throws<InvalidDateException>();

There are also more advanced methods that can throw exceptions based on values calculated at runtime. These act similarly to how you specify return values that are calculated at call time. For example

// Generate the exception at call time.
A.CallTo(() => fakeShop.NumberOfSweetsSoldOn(A<DateTime>._))
 .Throws(() => new InvalidDateException(DateTime.UtcNow + " is in the future");

// 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 
// an out DateTime.
A.CallTo(() => fakeShop.NumberOfSweetsSoldOn(A<DateTime>._))
 .Throws((DateTime when)=>new InvalidDateException(when + " is in the future");

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