How to deal with tangled uses dependencies in order to start unit testing?

Question

I have a messy Delphi 7 legacy system to maintain and develop. I am already reading "Working effectively with legacy code" and I like this book very much.

In order to start following the advices in the book, I created a test project and tried to write a single test. To do this I need to add some unit to the test project, but here lies the problem: the system under test has horrific uses dependencies. One unit uses some other unit, that uses some other unit and so on, and so on. It seems that most units directly or indirectly use one particular unit, and this unit in turn has 170 dependencies in its uses clause. There are indirect circular dependencies also.

Currently I am trying to add all of the legacy system's units into the test project, but I am running into all kind of problems, like "unit xxx was compiled with a different version of xxx", and others.

So I wonder if I am doing something wrong. I have used unit testing before, but in my own projects, that were smaller and with better structure and modularization. What are the options I have in this situation? Am I missing something?

Answer 1

You will always have dependencies in your code. Well, as long as you have code re-use, you will have dependencies. Since you are testing a legacy system, wholesale re-structuring is out of the question.

So you simply need to accept the dependencies. The most convenient and practical approach is to have a single unit tests project. That project contains all your unit tests. Use the facilities of your runner program to run only specific tests at any one time.

This leads to your project have the same list of units in its .dpr file as the main project. That's what you have currently tried and it's the right approach.

Your problem sounds like you are sharing the DCU directory (unit output directory) between the main project and the unit tests project. And you have different compiler options for the two projects. That's the most likely explanation for the error you report.

There are a couple of obvious solutions:

1. Align the compiler options for both projects. Then they can share DCUs.
2. Have separate DCU directories for the two projects.

Option 2 is much more robust and is best practise. However, you should try to understand why the compiler options differ. It's quite possible that your compiler options in the new unit tests project will need to be changed so that the units under test compile and function as desired. In modern Delphi I would use option sets to ensure consistency of compiler options.

Now, there may be other technical problems that you are facing, and my explanation of the error may not be quite right since I'm having to guess a little. But the bottom line is that having the same list of units in your .dpr files is the way to go.