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Default fake behavior

Fake objects come with useful default behavior as soon as they are created. Knowing the default behavior can make the fakes easier to work with and can lead to more concise tests.

Non-overrideable members cannot be faked

Methods and properties can only be faked if they are declared on a faked interface, or are declared abstract or virtual on a faked class. If none of these conditions hold, then a member cannot be faked, just as it could not be overridden in a derived class.

When such a member is invoked on the fake, the original behavior will be invoked.

Overrideable members are faked

When a method or property is declared on a faked interface, or is declared as abstract or virtual on a faked class, and the member is invoked on the fake, no action will be taken by the fake. It is as if the body of the member were empty. If the member has a return type (or is a get property), the return value will depend on the type T of the member:

  • If T can be made into a Dummy, then a Dummy T will be returned. Note that this may be a Fake or an instance of a concrete, pre-existing type;
  • otherwise, default(T) will be returned.

Read/write properties

By default, any overrideable property that has both a set and get accessor, and that has not been explicitly configured to behave otherwise, behaves like you might expect. Setting a value and then getting the value returns the value that was set.

var fakeShop = A.Fake<ICandyShop>();

fakeShop.Address = "123 Fake Street";

// prints "123 Fake Street"

This behavior can be used to

  • supply values for the system under test to use (via the getter) or to
  • verify that the system under test performed the set action on the Fake

Object members

Virtual methods inherited from System.Object are faked with a special default behavior:

  • Equals uses reference equality: it returns true if the argument is the fake itself, false for any other argument.
  • GetHashCode returns a hashcode consistent with the behavior of Equals.
  • ToString returns a string of the form "Faked <type of fake>"

This behavior applies to all fakes, including fakes of types that override these methods. Like any other method, the behavior can be explicitly configured.

Cancellation tokens

When a faked method that accepts a CancellationToken receives a canceled token (i.e. with IsCancellationRequested set to true), it will either:

  • return a canceled task, if it is asynchronous (i.e. if it returns a Task, Task<T>, ValueTask or ValueTask<T>);
  • throw an OperationCanceledException, if it is synchronous.


Suppose we have the following interface definition

public interface Interface
    bool BooleanFunction();
    int IntProperty { get; set; }
    string StringFunction();
    FakeableClass FakeableClassFunction();
    UnfakeableClass UnfakeableClassProperty { get; set; }
    Struct StructFunction();

Then the following test will pass

public void Members_should_return_empty_string_default_or_fake_another_fake()
    var fakeLibrary = A.Fake<Interface>();

    Assert.AreEqual(default(bool), fakeLibrary.BooleanFunction());

    Assert.AreEqual(default(int), fakeLibrary.IntProperty);

    Assert.AreEqual(typeof(string), fakeLibrary.StringFunction().GetType());
    Assert.AreEqual(string.Empty, fakeLibrary.StringFunction());

    Assert.IsInstanceOfType(fakeLibrary.FakeableClassFunction(), typeof(FakeableClass));
                    fakeLibrary.FakeableClassFunction().GetType().Name); // to show it's a fake


    Assert.AreEqual(default(Struct), fakeLibrary.StructFunction());