Should, ShouldBe; a silly mistake using FluentAssertions

I use FluentAssertions for all of my asserting needs. I like it’s API better than the assert methods you get with the .Net framework, and the FluentAssertions library provides an overall more fully featured set of assertion options. It’s open-source and continually updated, too, which makes it all right in my book. Sometimes, though, while I’m furiously writing tests, I get this test failure signature and it catches me off guard.

Subject has property Subject that the other object does not have.

Here’s a sample NUnit test case that would cause this particular error.

internal class MistakesTests
    public void ThisDumbThingIKeepDoing()
        var thing = new Thing { FirstName = "Nelson" };
        var thing2 = new Thing { FirstName = "Nelson" };
        // This should definitely pass, right?
    private class Thing
        public string FirstName { get; set; }

As you can see, I’m just doing a very simple object graph comparison, but it isn’t working as I intended. Can you spot the error?

Whereas I should be calling ShouldBeEquivalentTo on my Thing instance, I’m actually calling it on an instance of ObjectAssertions type from the FluentAssertions library. Whoops! ?

Leave a comment ?


  1. Life saver!

  2. Tisk tisk. Wher is that wall of shame?

  3. Thank you,
    I had the same problem, you saved me ?

  4. I was scratching my head over this, as I had something like


    Thank you!

  5. Good man!!! Saved me as well – what a terrible error message.

  6. I feel so dumb right now. Thanks ?

  7. Thanks – just happened to me ?

  8. Silly mistake but apparently a lot of people have made it! Thanks for pointing me in the right direction.

  9. +1 Thank you ?

  10. What everyone else has said – it certainly had me scratching my head for 5 minutes.

  11. +1 your, -1 for this silly error message

  12. 3 years later, I just did the same thing. First google result, you rule. Thanks!

  13. Saved me!

  14. I’m another victim! Chers

  15. Nick Nijenhuis

    Perhaps it is an idea for some of you to create a pull request with a better error message, instead of giving the author of this fabulous library minus ones.

  16. amazing! saved me after wasting an hour

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