The Difference Between Unit Testing and Module Testing

A common source of confusion for new software testers is the difference between unit testing and module testing. In general, unit tests are a collection of tests written by a developer during the software development process. Module tests are a collection of tests written by a tester after some code has been written by a developer. There are many exceptions to this generalization but the key point is that unit testing is primarily a development related activity, and module testing is primarily a testing related activity. When a developer creates unit tests during development, this approach is sometimes called test driven development. So which is better, unit testing or module testing? The answer is that the two approaches are complementary, not exclusive. In most cases, neither unit testing nor module testing are sufficient by themselves, and both approaches should be used. Let me explain. Suppose you are creating a PokerLib library for some sort of card game. Imagine you design a Card object like:

 

public class Card {
 private string rank; // A,2,3, . . T,J,Q,K
 private string suit; // c,d,h,s

 public Card(string c) {
  this.rank = c[0].ToString();
  this.suit = c[1].ToString();
 }

 // Rank property, etc.
}

 

An associated unit test might look like:

 

[TestMethod()]
public void CardConstructorTest()
{
 Card card = new Card("Ac");
 Assert.IsTrue(card.Rank == "A");
}

 

This gives the developer confidence that he has written the Card constructor and Rank property correctly. However, there are a total of 52 legal inputs to the Card constructor (Ac, Ad, . . Kh, Ks) and all of them should be tested. In most cases, developers would not want to create 52 separate unit tests for such a simple constructor. But module testing could test all 52 legal inputs and might take the form of a separate test harness program and look like:

static void Main(string[] args)
{
 string[] testCases = { "001:Ac:A", "002:Ad:A", (etc.) "052:Ks:K" };
 foreach (string tc in testCases)
 {
  string[] tokens = tc.Split(‘:’);
  string caseID = tokens[0];
  string input = tokens[1];
  string expected = tokens[2];
  CardLib.Card c = new CardLib.Card(tokens[1]);
  if (c.Rank == expected)
   Console.WriteLine(caseID + " Pass");
  else
   Console.WriteLine(caseID + " FAIL");
 }

 

Module tessting is often used when the development environment includes dedicate software testers, such as most Microsoft product groups.