The disclosure is generally directed towards automatically generating a mock object from a description of a real object, such as for use in testing. Mock object generation logic parses the description to determine interface(s) of the real object, which are replicated in the mock object, and to determine method(s) of the real object, which are simulated in the mock object. The mock object generation logic may generate a description of the mock object that is then compiled into the mock object for execution. Data types may be validated so that the arguments and/or return values from the mock object meet the expectations of a calling object.