How to properly test Exceptions with FsUnit

Question

I'm trying to figure out how to properly test exceptions with FsUnit. Official documentation states, that to test for exceptions I have to right something like this:

(fun () -> failwith "BOOM!" |> ignore) |> should throw typeof<System.Exception>

But, if I don't mark my test method with [<ExpectedException>] attribute it will always fail. Sounds reasonable because if we want to test for exceptions we have to add such attribute in C# + NUnit.

But, as long as I've added this attribute it doesn't matter what kind of exception I'm trying to throw, it will be always handled.

Some snippets: My LogicModule.fs

exception EmptyStringException of string

let getNumber str =
    if str = "" then raise (EmptyStringException("Can not extract number from empty string"))
    else int str

My LogicModuleTest.fs

[<Test>]
[<ExpectedException>]
let``check exception``()=
    (getNumber "") |> should throw typeof<LogicModule.EmptyStringException>

Zymometer

FYI - with Unquote, code.google.com/p/unquote, you would assert that getNumber "" in your latter example raises the expected exception like raises<LogicModule.EmptyStringException> <@ getNumber "" @> – Symphonious

Answer 1

Answer has been found. To test that exception was thrown I should wrap my function call in the next style:

(fun () -> getNumber "" |> ignore) |> should throw typeof<LogicModule.EmptyStringException>

because underneath #fsunit uses NUnit's Throws constraint http://www.nunit.org/index.php?p=throwsConstraint&r=2.5 … which takes a delegate of void, raise returns 'a

Zymometer

Good answer - I don't think you want the ExpectedException attribute, though. – Lignite

Note that returning unit from the lambda (i.e., ending the fun definition with '|> ignore') is required. – Thermotensile

Answer 2

If you want to test that a specific exception type is raised by some code, you can add the exception type to the [<ExpectedException>] attribute like so:

[<Test; ExpectedException(typeof<LogicModule.EmptyStringException>)>]
let``check exception`` () : unit =
    (getNumber "")
    |> ignore

More documentation is available on the NUnit site: http://www.nunit.org/index.php?p=exception&r=2.6.2

Antihero

Thank you for the answer, but I don't like the idea of adding some additional attribute, because it doesn't look so great when you are using FsUnit. – Zymometer