Testcases for NUnit 3 tests in F#

Question

I am trying to set up a test suite for an F# project using NUnit. It seems that especially when testing things like parsers and type checkers one typically has a list of valid input data and a list of invalid data. The tests itself are practically identical, so I am looking for a clever way to avoid writing a test function for each and every data item and instead seperate the test function from the data. Apparently, there seems to be something called test cases for that but I am having a hard time to find comprehensive documentation for the usage of NUnit 3 with F# in general and specifically a best practice example for my scenario.

Any pointers and hints are greately appreaciated!

Answer 1

In NUnit3 there is TestCaseSource and TestCaseData and for the best practices part I added FsUnit:

namespace NUnit3Demo

open NUnit.Framework
open FsUnit

[<TestFixture>]
module MyTest = 

    let methodToBeTested s = 
        if String.length s > 3 then failwith "Something's wrong"
        else String.length s

    let validData =
        [
            TestCaseData("   ").Returns(3)
            TestCaseData("").Returns(0)
            TestCaseData("a").Returns(1)
        ]

    let invalidData =
        [
            "    "
            "abcd"
            "whatever"
        ]

    let otherInvalidData =
        [
            "just"
            "because"
        ]

    [<TestCaseSource("invalidData");
      TestCaseSource("otherInvalidData")>]
    let ``More than 3 characters throws`` s = 
        (fun () -> methodToBeTested s |> ignore)
        |> should throw typeof<System.Exception>

    [<TestCaseSource("validData")>]
    let ``Less than 4 characters returns length`` s = 
        methodToBeTested s

Note that TestCaseData can take and return arbitrary objects (obviously they should match the test signatures). Also, the data can be written even nicer:

let validData =
    [
        "   ", 3
        "",    0
        "a",   1
    ] |> List.map (fun (d, r) -> TestCaseData(d).Returns r)