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What can be faked

What types can be faked?

FakeItEasy uses Castle DynamicProxy to create fakes. Thus, it can fake just about anything that could normally be overridden, extended, or implemented. This means that the following entities can be faked:

  • interfaces
  • classes that
    • are not sealed,
    • are not static, and
    • have at least one public or protected constructor whose arguments FakeItEasy can construct or obtain
  • delegates

Note that special steps will need to be taken to fake internal interfaces and classes.

Types whose methods have in parameters

Due to deficiencies in earlier .NET framework releases, generic types that contain methods having a parameter modified by the in keyword cannot be faked by FakeItEasy running on target frameworks earlier than .NET 6.

Where do the constructor arguments come from?

  • they can be supplied via WithArgumentsForConstructor as shown in creating fakes, or
  • FakeItEasy will use dummies as arguments

What members can be overridden?

Once a fake has been constructed, its methods and properties can be overridden if they are:

  • virtual,
  • abstract, or
  • an interface method when an interface is being faked

Note that this means that static members, including extension methods, cannot be overridden.

Methods that return values by reference

Methods that return values by reference (officially called "reference return values") cannot be invoked on a Fake. Any attempt to do so will result in a NullReferenceException being thrown.