A simple TDD example

I recently posted a response to StackOverflow wrt TDD and Coverage and I thought it would be worth re-posting the response here. The example is simple but hopefully shows how writing the right tests using TDD gives you a better suite of tests for your code than you would probably write if you wrote the tests after the code (which may have been re-factored as you developed).

"As the [original] accepted answer has pointed out your actual scenario reduces to collection.Sum() however you will not be able to get away with this every time.

If we use TDD to develop this (overkill I agree but easy to explain) we would [possibly] do the following (I am also using NUnit in this example out of preference).

[Test]
public void Sum_Is_Zero_When_No_Entries()
{
    var bomManager = new BomManager();
    Assert.AreEqual(0, bomManager.MethodToTest(new Collection<int>()));
}

and then write the following code (note: we write the minimum to meet the current set of tests)

public int MethodToTest(Collection<int> collection)
{
    var sum = 0;
    return sum;
}

We would then write a new test e.g.

[Test]
[TestCase(new[] { 0 }, 0)]
public void Sum_Is_Calculated_Correctly_When_Entries_Supplied(int[] data, int expected)
{
    var bomManager = new BomManager();
    Assert.AreEqual(expected, bomManager.MethodToTest(new Collection<int>(data)));
}

If we ran our tests they would all pass (green) so we need a new test(cases)

[TestCase(new[] { 1 }, 1)]
[TestCase(new[] { 1, 2, 3 }, 6)]

In order to satisfy those tests I would need to modify my code e.g.

public int MethodToTest(Collection<int> collection)
{
    var sum = 0;
    foreach (var value in collection)
    {
        sum += value;
    }
    return sum;
}

Now all my tests work and if I run that through OpenCover I get 100% sequence and branch coverage - Hurrah!.... And I did so without using coverage as my control but writing the right tests to support my code.

BUT there is a 'possible' defect... what if I pass in null? Time for a new test to investigate

[Test]
public void Sum_Is_Zero_When_Null_Collection()
{
    var bomManager = new BomManager();
    Assert.AreEqual(0, bomManager.MethodToTest(null));
}

The test fails so we need to update our code e.g.

public int MethodToTest(Collection<int> collection)
{
    var sum = 0;
    if (collection != null)
    {
        foreach (var value in collection)
        {
            sum += value;
        }
    }
    return sum;
}

Now we have tests that support our code rather than tests that test our code i.e. our tests do not care about how we went about writing our code.

Now we have a good set of tests so we can now safely refactor our code e.g.

public int MethodToTest(IEnumerable<int> collection)
{
    return (collection ?? new int[0]).Sum();
}

And I did so without affecting any of the existing tests."