Testing First

I’m not a TDD guy, but I am intrigued by it. With TDD, you write a test that fails and then write code to make it pass. Then you write another test that fails and write code to make it pass. Every so often you stop to refactor the code. I’m not an expert, so this may be overly simplistic or just plain wrong. If I used this methodology, I would end up with a crap load more tests than I do now. Maybe that’s a good thing.

What I like, in principal, about TDD is that nothing slips through the cracks. What I don’t like about it is that implementing it in VBA is probably a big headache. Mike Woodhouse has done some good work in this area, but I wanted to trying something simple from scratch to see what there is to see.

In my scenario, I’m computing commission for salesguys. A salesguy has a standard rate that he earns up to $100k of sales. From $100k to $500k, he gets 1.5 times that rate. And from $500k up, he gets 2x that rate. My first test is to check the commission on the first dollar.

This doesn’t even compile. Literally all I did was add a module called MTest and write this test. I get no intellisense because I haven’t even created the classes yet. The first compile error is on the csalesguy line, so I’m going to create that class.

Now I need a CSales class. I create a CSale class with SaleDate and Amount and use my VBHelpers add-in to create the CSales parent class. The next compile error is that I don’t have a Commission property of the CSalesGuy class.

And in the CSales class:

Everything compiles, I run my test, and it works. So far so good. Next I want to test just below the first threshold, so I add another test and a procedure to run all tests.

And they pass. I guess I didn’t write a test that failed, but I will this time. Next I want to test just over the first tier of sales.

That fails. So I fix my commission property.

Note that I changed the data type to Currency to eliminate the precision errors. This is obviously not optimal code, but I think the idea is to write the minimum code to make the test pass, then refactor. Next up, I test just under the next threshold.

That passes as expected. My last test case is over the second threshold.

Now to fix up Commission to pass the test

And they all pass. Now I have only to refactor. That’s not insignificant as this is some really crappy code. The first thing I want to do is create a CRates class to hold all of the rates. The business rule of 1.5x and 2x is fine, but that could change so I need some flexibility. I create a CRate class with Rate and Threshold properties and make the parent CRates class a child of the CSalesGuy class. In CSalesGuy:

Then I create a test setup function to instantiate the SalesGuy

Finally, I need to change the Commission property to use the Rates rather than the (now eliminated) StandardRate property.

That requires an IncrementalRate property of the CRate class.

All of my tests pass, so I should be good right? It’s nice to be able to refactor the code and see that all the results are still correct. I made the commission rates model more flexible to allow for an unlimited number of sales tiers, but I’m still only testing my original business rules. Maybe I should add more tests, but then it’s not really the tests that drive the development. Do you write tests first? If so, how would you do it differently?

You can download TDD_VBA.zip

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