In and , and employ a technique called to enhance the quality of the software. Frequently, the final release software consists of a complex set of or interacting together to create the final result. In automated unit testing, it may be necessary to use objects or procedures that look and behave like their release-intended counterparts, but are actually simplified versions that reduce the complexity and facilitate testing. A test double is a generic (meta) term used for these objects or procedures.

 

Types of test doubles

Gerard Meszaros identified several different terms for what he calls, "Test Doubles." Using his vocabulary, there are at least five types of Test Doubles:

• (used for providing the tested code with "indirect input")
• (used for verifying "indirect output" of the tested code, by first defining the expectations before the tested code is executed)
• (used for verifying "indirect output" of the tested code, by asserting the expectations afterwards, without having defined the expectations before the tested code is executed. It helps in recording information about the indirect object created)
• (used as a simpler implementation, e.g. using an in-memory database in the tests instead of doing real database access)
• (used when a parameter is needed for the tested method but without actually needing to use the parameter)

For both manual and automated of or and use test doubles that communicate with the over a network protocol. These test doubles are called different names depending on the tool vendor. A commonly used term is . Other names used include , API mock, HTTP stub, HTTP mock, over the wire test double .

Another form of test double is the Verified Fake, a whose behavior has been verified to match that of the real object using a set of tests that run against both the Verified Fake and the real implementation.

While there is no for test double and the various types, there is momentum for continued use of these terms in this manner. used these terms in his article, Mocks Aren't Stubs referring to Meszaros' book. Microsoft also used the same terms and definitions in an article titled, Exploring The Continuum Of Test Doubles.