How do I write CustomAssertion using FluentAssertions?

Question

There is official example how to create CustomAssertion at FluentAssertions docs, however my attempt to apply it fails. Here's the code:

public abstract class BaseTest
{
    public List<int> TestList = new List<int>() { 1, 2, 3 };
}

---

public class Test : BaseTest { }

---

public class TestAssertions
{
    private readonly BaseTest test;

    public TestAssertions(BaseTest test)
    {
        this.test = test;
    }

    [CustomAssertion]
    public void BeWorking(string because = "", params object[] becauseArgs)
    {
        foreach (int num in test.TestList)
        {
            num.Should().BeGreaterThan(0, because, becauseArgs);
        }
    }
}

---

public class CustomTest
{
    [Fact]
    public void TryMe()
    {
        Test test = new Test();
        test.Should().BeWorking(); // error here
    }
}

I'm getting compile error:

CS1061 'ObjectAssertions' does not contain a definition for 'BeWorking' and no accessible extension method 'BeWorking' accepting a first argument of type 'ObjectAssertions' could be found (are you missing a using directive or an assembly reference?)

I also tried to move BeWorking from TestAssertions to BaseTest but it still won't work. What am I missing and how do I make it work?

Answer 1

You did a very good job actually :) The most important thing you are missing is the Extension class. I'll guide you through.

Add this class:

public static class TestAssertionExtensions
{
    public static TestAssertions Should(this BaseTest instance)
    {
        return new TestAssertions(instance);
    }
}

Fix your TestAssertions class like this:

public class TestAssertions : ReferenceTypeAssertions<BaseTest, TestAssertions>
{
    public TestAssertions(BaseTest instance) => Subject = instance;

    protected override string Identifier => "TestAssertion";

    [CustomAssertion]
    public AndConstraint<TestAssertions> BeWorking(string because = "", params object[] becauseArgs)
    {
        foreach (int num in Subject.TestList)
        {
            num.Should().BeGreaterThan(0, because, becauseArgs);
        }

        return new AndConstraint<TestAssertions>(this);
    }
}

Your TryMe() test should be working fine now. Good luck.