Two Paragraphs About Unit Testing

Create in-process test doubles for every out-of-process dependency. Instantiate the object to be tested. Establish pure preconditions. Establish impure preconditions. Assert that impure postconditions are not satisfied. Invoke the method to be tested. Assert that impure postconditions are now satisfied. Assert that pure postconditions are now satisfied.

Pure preconditions constrain method parameters. Pure postconditions constrain the return value. Impure preconditions constrain the state of your test doubles before the test. Impure postconditions constrain the state of your test doubles after the test.