Pit falls in testing

Posted: June 13, 2008 in c#, Testing

Yesterday I wrote a post on default(T) not always being valid, that realization made us change the signature on the mentioned method.

Working on that rather simple method made me once again think about testing. We have Asserts like

Assert.AreEqual(default(Enumeration),Enum<Enumeration>.Parse(“not valid))

This gives 100% statement coverage and it might look as it gives a 100% branch coverage as well, which unfortunately is not true. You don’t necessarily have to have code to have a new branch.

object obj = “Valid”;

a more common example of a hidden branch is an if with no else. Even though you have not explicitly written an else clause you should test that branch none the less.

the code being tested might look like this:

class Guard{

public static void ArgumentNull(object argument,string name){
if(argument == null)
throw new ArgumentNullException(name);

we might then have an Assert like:
we have 100% statement coverage but the quality is not very high. At some point we want to log the call stack when we have a call with a null argument. However the implementation has an error which is not caught due to the lack of testing of the “invisible” branch.

class Guard{

public static void ArgumentNull(object argument,string name){
if(argument == null)
throw new ArgumentNullException(name);


We still have 100% statement coverage and our test still succeeds but unfortuantely any call to Guard.ArgumentNull now throws an ArgumentNullException no matter whether the argument is null or not.

When in doubt if more test cases are needed make a Cyclomatic Complexity Analysis of the code being tested. The number of tests needed is in most cases proportinal to the cyclomatic complexity of the code being tested.

for more information on how to apply CCA as a quality measuring mechanism for unit tests take a look at this blog. I do not agree on their actually “algorithmn” for figuring out the number of tests needed but the point of creating an algorithm based on CCA is well thought.

A rule of thumb says that you need 4 incorrect values for each correct value you need to test each decision point in your code.

Since CCA in essence is a measurement for the number of decision points in your code I go for a higher number than .5 * CCA. What that constant would be, would depend on the project. In my current project the constant is between 1 and 2 depending on factors such as source of the code (generated or written), the complexity (it’s not a linear relation ship for us but exponential) and the severity of an error in the tested code. (An error in the security is a lot more severe than in the part that does debug tracing)

  1. runefs says:

    I do agree that what I propose is very similar to the metrics you mention and I do think their algorithm comes closer to what I’m trying to describe than the artikel I’m linking to in the post. Thanks for linking to that article

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