Say I'm trying to test a simple Set class
public IntSet : IEnumerable<int>
{
Add(int i) {...}
//IEnumerable implementation...
}
And suppose I'm trying to test that no duplicate values can exist in the set. My first option is to insert some sample data into the set, and test for duplicates using my knowledge of the data I used, for example:
//OPTION 1
void InsertDuplicateValues_OnlyOneInstancePerValueShouldBeInTheSet()
{
var set = new IntSet();
//3 will be added 3 times
var values = new List<int> {1, 2, 3, 3, 3, 4, 5};
foreach (int i in values)
set.Add(i);
//I know 3 is the only candidate to appear multiple times
int counter = 0;
foreach (int i in set)
if (i == 3) counter++;
Assert.AreEqual(1, counter);
}
My second option is to test for my condition generically:
//OPTION 2
void InsertDuplicateValues_OnlyOneInstancePerValueShouldBeInTheSet()
{
var set = new IntSet();
//The following could even be a list of random numbers with a duplicate
var values = new List<int> { 1, 2, 3, 3, 3, 4, 5};
foreach (int i in values)
set.Add(i);
//I am not using my prior knowledge of the sample data
//the following line would work for any data
CollectionAssert.AreEquivalent(new HashSet<int>(values), set);
}
Of course, in this example, I conveniently have a set implementation to check against, as well as code to compare collections (CollectionAssert). But what if I didn't have either ? This is the situation when you are testing your real life custom business logic.
Granted, testing for expected conditions generically covers more cases - but it becomes very similar to implementing the logic again (which is both tedious and useless - you can't use the same code to check itself!). Basically I'm asking whether my tests should look like "insert 1, 2, 3 then check something about 3" or "insert 1, 2, 3 and check for something in general"
EDIT - To help me understand, please state in your answer if you prefer OPTION 1 or OPTION 2 (or neither, or that it depends on the case, etc )
