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Dummies

A Dummy is an object that FakeItEasy can provide when an object of a certain type is required, but the actual behavior of the object is not important.

How to use them in your tests

Consider this example. Say that you want to test the following class:

public class Library
{
    public bool Checkout(Card patronCard, Book someBook);
}

Maybe in one of your tests you want to invoke Checkout with an expired library card. The checkout should fail, regardless of the book being checked out—only the status of the card matters. Instead of writing

library.Checkout(MakeExpiredCard(),
                 new Book { Title = "The Ocean at the End of the Lane" } );

You can write:

library.Checkout(MakeExpiredCard(), A.Dummy<Book>());

This signals that the actual value of the Book is really not important. The code is intention-revealing.

You can also create a collection of dummies by writing:

A.CollectionOfDummy<Book>(10);

This will return an IList containing 10 dummy Book instances.

For cases where the type of dummy isn't statically known, non-generic methods are also available. These are usually only required when writing extensions for FakeItEasy, so they live in the FakeItEasy.Sdk namespace: 

using FakeItEasy.Sdk;
...
var type = GetTypeOfDummy();
object dummy = Create.Dummy(type);
IList<object> dummies = Create.CollectionOfDummy(type, 10);

How FakeItEasy uses them

When creating Fakes or Dummies of class types, FakeItEasy needs to invoke the classes' constructors. If the constructors take arguments, FakeItEasy needs to generate appropriate argument values. It uses Dummies.

How are the Dummies made?

When FakeItEasy needs to access a Dummy of type T, it tries a number of approaches in turn, until one succeeds:

  1. If there's a user-supplied dummy factory for T, return whatever it makes.
  2. If T is String, return an empty string.
  3. If T is Task or ValueTask, the returned Dummy will be an actual Task or ValueTask that is already completed.
  4. If T is Task<TResult> or ValueTask<TResult>, the returned Dummy will be an actual Task<TResult> or ValueTask<TResult> that is already completed and whose Result is a Dummy of type TResult, or a default TResult if no Dummy can be made for TResult.
  5. If T is a Lazy<TValue> the returned Dummy will be an actual Lazy<TValue> whose Value is a Dummy of type TValue, or a default TValue if no Dummy can be made for TValue.
  6. If T is a tuple type (Tuple<> or ValueTuple<>), the Dummy will be a tuple whose elements are dummies, or default values when dummies can't be made. Activator.CreateInstance.
  7. If T is a value type, the Dummy will be a T created via Activator.CreateInstance.
  8. If T is fakeable, the Dummy will be a Fake T.
  9. If nothing above matched, then T is a class. Loop over all its public constructors in descending order of argument list length. For each constructor, attempt to get Dummies to satisfy the argument list. If the Dummies can be found, use Activator.CreateInstance to create the Dummy, supplying the Dummies as the argument list. If the argument list can't be satisfied, then try the next constructor.

If none of these strategies yield a viable Dummy, then FakeItEasy can't make a Dummy of type T.