Fluent Assertions

Remarks#

NUnit’s Assert.That() form supports the use of constraints as its second parameter. All constraints provided out of the box by NUnit are available through the static classes Is, Has and Does. Constraints can be combined into fluent expressions using the built in methods And, Or and With. Expressions can be conveniently expanded up using the many methods in ConstraintExpression, such as AtMost and Contains.

Basic fluent assertion

Assert.That(actual, Is.EqualTo(expected));

Advanced Constraint Usage

Large fluent assertions do become harder to read, but when combined with classes that have good implementations of ToString(), they can generate very useful error messages.

[Test]
public void AdvancedContraintsGiveUsefulErrorMessages() {
    Assert.That(actualCollection, Has
        .Count.EqualTo(4)
        .And.Exactly(1).Property("Age").GreaterThan(60)
        .And.Some.Property("Address").Null
        .And.No.Property("Age").LessThanOrEqualTo(17));
}

On failure, this assertion generates messages like this:

Expected: property Count equal to 4 and exactly one item property Age greater
than 60 and some item property Address null and not property Age less than or
equal to 17
But was:  < <Steve Taylor (23) lives in Newcastle
>, <Michelle Yung (65) lives in San Francisco
>, <Ranjit Saraman (49) lives in Milano
>, <LaChelle Oppenheimer (16) lives in 
> >

Collections

var a = new List<int> { 1, 2 };
var b = new List<int> { 2, 1 };

Assert.That (a, Is.EqualTo(b)); // fails
Assert.That (a, Is.EquivalentTo(b)); // succeeds