### PairwiseAttribute (NUnit 2.5)

The PairwiseAttribute is used on a test to specify that NUnit should generate test cases in such a way that all possible pairs of values are used. This is a well-known approach for combatting the combinatorial explosion of test cases when more than two features (parameters) are involved.

#### Example

Using the Combinatorial attribute, the following test would be executed 12 (3x2x2) times. With Pairwise it is executed only enough times so that each possible pair is covered..

    [Test, Pairwise]
    public void MyTest(
        [Values("a", "b", "c")] string a,
        [Values("+", "-")] string b,
        [Values("x", "y")] string c)
    {
        Console.WriteLine("{0} {1} {2}", a, b, c);
    }

For this test, NUnit currently calls the method seven times, producing the following output:

    	a - x
    	b + y
    	c + x
    	b - x
    	a - y
    	c - y
    	a + x

Note that this is not the optimal output. The pairs (a, -), (a, x), (+ x), (- x) and (- y) appear twice. NUnit uses a heuristic algorithm to reduce the number of test cases as much as it can. Improvements may be made in the future.

#### Limitations

When used on a generic method the programmer must ensure that all possible combinations of arguments are valid. When multiple parameters use the same generic type (e.g.: T) this may not be possible and the attribute may generate invalid test cases.

#### See also...

- SequentialAttribute
- CombinatorialAttribute