NUnit - ReusableConstraint

## ReusableConstraint (NUnit 2.5.6)

Normally constraints just work. However, attempting to reuse the same constraint in several places can lead to unexpected results.

Consider the following code as an example: ``

        Constraint myConstraint = Is.Not.Null;
        Assert.That("not a null", myConstraint); // Passes, of course
        Assert.That("not a null", myConstraint); // Fails! What's that about?

We'll save the technical explanation for later and show the solution first: ``

        ReusableConstraint myConstraint = Is.Not.Null;
        Assert.That("not a null", myConstraint); // Passes
        Assert.That("not a null", myConstraint); // Passes

Or alternatively.. `

        var myConstraint = new ReusableConstraint(Is.Not.Null);
        Assert.That("not a null", myConstraint); // Passes
        Assert.That("not a null", myConstraint); // Passes

`

### Technical Explanation

In the original example, the value assigned to myConstraint is known as an unresolved constraint. In fact, it's an unresolved NullConstraint, because that was the last constraint encountered in the expression. It's associated with a Not operator that has not yet been applied.

That's OK for use with Assert.That(), because the method knows how to resolve a constraint before using it. Assert.That() resolves this constraint to a NotConstraint referencing the original NullConstraint.

Of course, the original reference in myConstraint is left unchanged in all of this. But the EqualConstraint it points to has now been resolved. It is now a resolved constraint and can't be resolved again by the second Assert.That(), which only sees the NullConstraint and not the NotConstraint.

So, for reusability, what we want to save is the result of resolving the constraint, in this case

        NotConstraint => NullConstraint

That's what ReusableConstraint does for us. It resolves the full expression and saves the result. Then it passes all operations on to that saved result.

### When to Use It

Use this constraint any time you want to reuse a constraint expression and you'll be safe.

If you like to take chances, you'll find that you can avoid using it in the following cases...

1. With a simple constraint involving no operators, like...

           Constraint myConstraint = Is.Null;
           Constraint myConstraint = Is.EqualTo(42);

2. With any constraint you construct using new, without using the "dotted" constraint syntax...

           Constraint myConstraint = new NotConstraint(new NullConstraint());
           Constraint myConstraint = new AndConstraint(
               new GreaterThanConstraint(0),
               new LessThanConstraint(100));

   However, there is no significant penalty to using ReusableConstraint. It makes your intent much clearer and the exceptions listed are accidents of the internal implementation and could disappear in future releases.