How to use Fluent Assertions to test for exception in inequality tests?

Question

I'm trying to write a unit test for a greater than overridden operator using Fluent Assertions in C#. The greater than operator in this class is supposed to throw an exception if either of the objects are null.

Usually when using Fluent Assertions, I would use a lambda expression to put the method into an action. I would then run the action and use action.ShouldThrow<Exception>. However, I can't figure out how to put an operator into a lambda expression.

I would rather not use NUnit's Assert.Throws(), the Throws Constraint, or the [ExpectedException] attribute for consistencies sake.

That link is already dead. Right now it's changed to https://fluentassertions.com/exceptions/ but Kote's answer should steer anyone coming later correctly. — Steve Pettifer, Jul 15 '19 at 07:27

Kote · Accepted Answer · 2018-04-05T15:56:11.793

87

You may try this approach.

[Test]
public void GreaterThan_NullAsRhs_ThrowsException()
{
    var lhs = new ClassWithOverriddenOperator();
    var rhs = (ClassWithOverriddenOperator) null;

    Action comparison = () => { var res = lhs > rhs; };

    comparison.Should().Throw<Exception>();
}

It doesn't look neat enough. But it works.

Or in two lines

Func<bool> compare = () => lhs > rhs;
Action act = () => compare();

edited Apr 05 '18 at 15:56

answered Jan 26 '16 at 04:06

Kote

2,116
1
19
20

2

`act.Should().NotThrow();` to check for not exceptions. – dani herrera Jan 09 '20 at 10:11
I used it slightly different for my problem because I was testing an async method. So for async/await you can replace Action to Func – Spyros_Spy Apr 22 '21 at 12:16

score 0 · Answer 2 · answered Jul 22 '22 at 09:42

0

you can use Invoking too

 comparison.Invoking(()=> {var res = lhs > rhs;})
.Should().Throw<Exception>();

more information is here

answered Jul 22 '22 at 09:42

mostafa kazemi

514
6
7

How to use Fluent Assertions to test for exception in inequality tests?

2 Answers2