Consider the problem writing a unit test for the IList.Add method. At first sight this is trivial (as an example, we check that IList.Add supports null reference):

[TypeFixture(typeof(IList))]
public class ListTest
{
   [Test("List accepts null values")]
   public void AddNull(IList list)
   {
       list.Add(null);
       ...
   }
}

Now what about the Readonly and the FixedSize wrapper of ArrayList. They implement IList, but calling IList.Add on them will throw a NotSupportedException. Using the classic ExpectedExceptionAttribute technique, the only solution is to create a special fixture for read-only and fixed size wrappers, and that involves a lot of code duplication.

To tackle this problem, I have added ConditionalExceptionAttribute, which expects an exception if a predicate is true. The predicate is define by a method of the fixture that returns a boolean. This solution is simple and avoids the duplicate of unit tests when possible. Let's addapt the example with the new decorator:

[TypeFixture(typeof(IList))]
public class ListTest
{
   public bool IsReadOnly(IList list)
   {
       return list.ReadOnly;
   }

   [Test("List accepts null values")]
   [ConditionalException(typeof(NotSupportedException),"IsReadOnly")]
   public void AddNull(IList list)
   {
       list.Add(null);
       ...
   }
}

When the test will be executed, if list.ReadOnly is true, the framework will expect to receive a NotSupportedException, but if it is false, he will not check for exception. Note that the parameters of the predicate must match the parameters of the test method and it must return a bool.