Constraint Model (Assert.That)

The constraint-based Assert model uses a single method of the Assert class for all assertions. The logic necessary to carry out each assertion is embedded in the constraint object passed as the second parameter to that method.

Here's a very simple assert using the constraint model:

Assert.That(myString, Is.EqualTo("Hello"));

The second argument in this assertion uses one of NUnit's syntax helpers to create an EqualConstraint. The same assertion could also be made in this form:

Assert.That(myString, new EqualConstraint("Hello"));

Using this model, all assertions are made using one of the forms of the Assert.That() method, which has a number of overloads...

Assert.That(bool condition);
Assert.That(bool condition, string message, params object[] params);
Assert.That(bool condition, Func<string> getExceptionMessage);

Assert.That(Func<bool> condition);
Assert.That(Func<bool> condition, string message, params object[] params);
Assert.That(Func<bool> condition, Func<string> getExceptionMessage);

Assert.That<TActual>(ActualValueDelegate<TActual> del, IResolveConstraint constraint)
Assert.That<TActual>(ActualValueDelegate<TActual> del, IResolveConstraint constraint,
    string message, object[] params)
Assert.That<TActual>(ActualValueDelegate<TActual> del, IResolveConstraint expr,
    Func<string> getExceptionMessage)

Assert.That<TActual>(TActual actual, IResolveConstraint constraint)
Assert.That<TActual>(TActual actual, IResolveConstraint constraint, string message,
    params object[] params)
Assert.That<TActual>(TActual actual, IResolveConstraint expression,
    Func<string> getExceptionMessage)

Assert.That(TestDelegate del, IResolveConstraint constraint)
Assert.That(TestDelegate code, IResolveConstraint constraint, string message,
    params object[] args)
Assert.That(TestDelegate code, IResolveConstraint constraint,
    Func<string> getExceptionMessage)

The overloads that take a bool work exactly like Assert.IsTrue, except that Assert.IsTrue does not accept a Func<string> for the exception message.

For overloads taking a constraint, the argument must be an object implementing the IResolveConstraint interface, which supports performing a test on an actual value and generating appropriate messages. This interface is described in more detail under Custom Constraints.

NUnit provides a number of constraint classes similar to the EqualConstraint used in the example above. Generally, these classes may be used directly or through a syntax helper. The valid forms are described on the pages related to each constraint.

See also