Not empty string generator with FsCheck using c# fluent interface

Question

I'm trying to create a not empty string generator but when running the test I still have empty strings as inputs.

Here is the code I wrote:

    [Test]
    public void MyTest()
    {
        Func<Gen<string>> generateNotEmptyString = () =>
        {
            var gen = Any.OfType<string>()
                         .Where(name => !string.IsNullOrEmpty(name));
            return gen;
        };

        Action<string> assertIdIsNeverEmpty = name =>
        {
                var id = MyService.CreateId(name);
                id.Should().NotBeNullOrEmpty();
        };

        Spec.For(generateNotEmptyString(), assertIdIsNeverEmpty)
            .QuickCheckThrowOnFailure()
    }

I'm using NUnit v. 2.6.2.12296, FsCheck v. 0.9.4.0 and FluentAssertions v. 3.0.90.0.

What if I also want to modify the generator so that it creates not empty strings that match a regular expression?

[EDIT] This is the code I'm using to create strings that contain letters but don't contain special characters:

    private static Gen<NonEmptyString> GenerateValidNames()
    {
        return
            Any.OfType<NonEmptyString>()
                .Where(s =>
                    !s.Get.Contains("\r") &&
                    !s.Get.Contains("\n") &&
                    !s.Get.Contains("\t"))
                .Where(s =>
                {
                    var regEx = new Regex(@"^[A-Za-z]*$");
                    return regEx.Match(s.Get).Success;
                });            
    }

score 2 · Accepted Answer · edited May 23 '17 at 12:11

Maybe you have a bug :) The following works for me:

    public void MyTest()
    {
        Gen<string> generateNotEmptyString = Any.OfType<string>()
                                                .Where(name => !string.IsNullOrEmpty(name));

        Action<string> assertIdIsNeverEmpty = name =>
        {
                Assert.False(String.IsNullOrEmpty(name));
        };

        Spec.For(generateNotEmptyString, assertIdIsNeverEmpty)
            .QuickCheckThrowOnFailure();
    }

Note you don't need the extra Func around the generator. A generator is a function already, under the covers, so just creating one won't execute any code (more or less), much like an IEnumerable.

Alternatively, use the built in non-empty string generator:

        Action<NonEmptyString> assertIdIsNeverEmpty = name =>
        {
                Assert.False(String.IsNullOrEmpty(name.Get));
        };

        Spec.For(Any.OfType<NonEmptyString>(), assertIdIsNeverEmpty)
            .QuickCheckThrowOnFailure();

Generating strings that match a regexp is a solved problem, but not straightforward. FsCheck currently has no support for it directly. Though I think it would make a great addition! See How to generate random strings that match a given regexp?

Indeed this is an interesting topic (generate strings from regex) - if your regex is just a DFA in disguise this is (almost) trivial - but alas most modern regex implementations can do much more than this - are you working on this already? — Random Dev, Jul 11 '14 at 08:35
Nope. Open an issue to discuss if you're interested. Actually I think "just" supporting really regular expressions would give the most bang for the buck anyway. — Kurt Schelfthout, Jul 11 '14 at 08:48
Actually I wasn't receiving empty strings but not printable characters that were printed as empty strings in the output window. — , Jul 11 '14 at 13:50

Not empty string generator with FsCheck using c# fluent interface

1 Answers1