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  1. PO
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    <p><strong>You don't need mocking for this.</strong> You use mocking when two classes work together and you want to replace one class with a mock (fake) one so you only have to test the other one. This is not the case in your example.</p> <p><strong>There is however a way you could use mocks, and that fixes your issue with the 2bln instances.</strong> If you separate the ID generation from the <code>Primitive</code> class and use a generator, you can mock the generator. An example:</p> <p>I've changed <code>Primitive</code> to use a provided generator. In this case it's set to a static variable, and there are better ways, but as an example:</p> <pre><code>public abstract class Primitive { internal static IPrimitiveIDGenerator Generator; protected Primitive() { Id = Generator.GetNext(); } internal int Id { get; private set; } } public interface IPrimitiveIDGenerator { int GetNext(); } public class PrimitiveIDGenerator : IPrimitiveIDGenerator { private int? _previousId; public int GetNext() { try { _previousId = checked(++_previousId) ?? 0; return _previousId.Value; } catch (OverflowException ex) { throw new OverflowException("Cannot instantiate more than (int.MaxValue) unique primitives.", ex); } } } </code></pre> <p>Then, your test case becomes:</p> <pre><code>[Fact(DisplayName = "Test Primitive count limit")] public void TestPrimitiveCountLimit() { Assert.Throws(typeof(OverflowException), delegate() { var generator = new PrimitiveIDGenerator(); for (; ; ) { generator.GetNext(); } }); } </code></pre> <p>This will run a lot faster and now you're only testing whether the ID generator works.</p> <p>Now, when you e.g. want to test that creating a new primitive actually asks for the ID, you could try the following:</p> <pre><code>public void Does_primitive_ask_for_an_ID() { var generator = new Mock&lt;IPrimitiveIDGenerator&gt;(); // Set the expectations on the mock so that it checks that // GetNext is called. How depends on what mock framework you're using. Primitive.Generator = generator; new ChildOfPrimitive(); } </code></pre> <p>Now you have separated the different concerns and can test them separately.</p>
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    1. USPieter van Ginkel
    1. USPieter van Ginkel
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    1. COI though of a different idea and expanded my answer. This also solves your 2bln instances. Concerning testing abstract classes: inherit it for the unit test. Just create a dummy inherited class.
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    2. COWow excellent answer. Thank you man. I like the idea of separating the ID generator which I could then reuse somewhere else. I'll need to read up on mocks I think though!
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