How to replace Assert.Fail() with FluentAssertions

Question

We are currently converting some code that was using Assert.IsTrue(), Assert.AreEqual(), Assert.IsNotNull(), etc. The basic unit test assert Library for C#

We want to use FluentAssertions, like value.Should().BeNull().

I'm stuck on a few tests using Assert.Fail() in some locations. What should I use to efficiently replace those, since we want to do away with every single "Assert.*", and I can't find an equivalent in FluentAssertions?

Here is an example

[TestMethod, TestCategory("ImportantTest")]
public void MethodToTest_Circumstances_ExpectedResult()
{
    // Arrange
    var variable1 = new Type1() { Value = "hello" };
    var variable2 = new Type2() { Name = "Bob" };

    // Act
    try
    {
        MethodToTest(variable1, variable2);
        // This method should have thrown an exception
        Assert.Fail();
    }
    catch (Exception ex)
    {
        ex.Should().BeOfType<DataException>();
        ex.Message.Should().Be(Constants.DataMessageForMethod);
    }

    // Assert
    // test that variable1 was changed by the method
    variable1.Should().NotBeNull();
    variable1.Value.Should().Be("Hello!");
    // test that variable2 is unchanged because the method threw an exception before changing it
    variable2.Should().NotBeNull();
    variable2.Name.Should().Be("Bob");
}

Len

Please post code samples of what you're stuck on. Unit testing is an art. We can't answer "What should I use to efficiently replace those..." without seeing what "those" are. – Catoptrics

Usually an Assert.Fail is used in a code path that was reached because of a problem like if(value == null) { Assert.Fail("should not be null");} so how you convert them is completely based on the circumstances that lead the code to reach them. – Slotnick

Sounds like you should be using Assert.Throws to start with, instead of the try/Assert.Fail/catch approach. – Jedediah

Answer 1

Restructure the test to utilize the .ShouldThrow<> assertion extension.

[TestMethod, TestCategory("ImportantTest")]
public void MethodToTest_Circumstances_ExpectedResult() {
    // Arrange
    var variable1 = new Type1() { Value = "hello" };
    var variable2 = new Type2() { Name = "Bob" };

    // Act
    Action act = () => MethodToTest(variable1, variable2);       

    // Assert
    // This method should have thrown an exception
    act.ShouldThrow<DataException>()
       .WithMessage(Constants.DataMessageForMethod);
    // test that variable1 was changed by the method
    variable1.Should().NotBeNull();
    variable1.Value.Should().Be("Hello!");
    // test that variable2 is unchanged because the method threw an exception before changing it
    variable2.Should().NotBeNull();
    variable2.Name.Should().Be("Bob");
}

In the above example, if the expected exception is not thrown the the assertion would fail, stopping the test case.

You should review the documentation on asserting exceptions to get a better understanding of how to use the library.

Teneshatenesmus

Answer 2

Following the example in here, he just dealt away with the Assert.Fail -- and use action and .ShouldThrow http://www.continuousimprover.com/2011/07/why-i-created-fluent-assertions-in.html

Menis

Answer 3

As others have mentioned, in your specific example you should probably use ShouldThrow<>().

However, in general, if no other FluentAssertions method applies, you can use:

Execute.Assertion.FailWith("[explain why this line should not be reached]");

Peccadillo

Answer 4

I think this is equivalent:

throw new AssertionFailedException("Reason...");

Pepillo