Unit-testing a simple collection class

Question

Consider the following class:

public class MyIntSet
{
    private List _list = new List();

    public void Add(int num)
    {
        if (         


        

Answer 1

Your test seems perfectly OK to me. You may have misunderstood a principle of unit testing.

A single test should (ideally) only test one thing, that is true, but that does not mean that it should test only one method; rather it should only test one behaviour (an invariant, adherence to a certain business rule, etc.) .

Your test tests the behaviour "if you add to a new set, it is no longer empty", which is a single behaviour :-).

To address your other points:

• Theoretically, there could be a bug in both, that only manifests in scenarios where they are not called one after the other.
  True, but that just means you need more tests :-). For example, add two numbers, then call Contains, or call Contains without Add.

• An alternative approach might suggest mocking out or exposing (possibly using InternalsVisibleTo) the private _list member and have the test inspect it directly, but that could potentially create test maintainability problems[...]
  Very true, so don't do this. A unit test should always be against the public interface of the unit under test. That's why it's called a unit test, and not a "messing around inside a unit"-test ;-).