Property based testing in C#–Part 4

In this multipart blog post I want to introduce you in the world of property-based testing and how to do this in C#.

In the first part, I gave you an introduction on what property-based testing is, why it useful and how it can help you write better code. In the second post, I showed you a concrete example on how to start writing property-based tests in C# using FsCheck. In a third post I explained how property-based tests can help  to find edge cases and to understand a codebase better.

In this post I continue the journey by having a look at how to write our own generators. If you didn’t read my previous post, generators are the tool that helps you to select a value from an interval. For some of the available types in .NET, a default generator (and shrinker) exists but sometimes it is necessary to create your own generators.

Create your own FsCheck generator in C#

Creating your own generator for FsCheck is easy in C#, the only thing you need is a public static class with a public static method that returns an Arbitrary<T>.

In our previous post, we used a Filter() in our test to return only positive dates:

	[FsCheck.NUnit.Property(Verbose = true)]
	public Property ValidateShouldReturnTrueForDateTime()
	{
	var arb = Arb.Default.DateTime().Filter(d=> d < DateTime.Today);
	return Prop.ForAll<DateTime>(arb, x =>
	{
	var nationalRegisterNumber = NationalRegisterNumberGenerator.Generate(x);
	return NationalRegisterNumberGenerator.IsValid(nationalRegisterNumber);
	});
	}

view raw ValidateShouldReturnTrueForDateTime.cs hosted with ❤ by GitHub

Let’s see how we can do the same thing using a custom generator:

	public static class DatesInThePastGenerator
	{
	public static Arbitrary<DateTime> Generate()
	{
	return Arb.Default.DateTime().Filter(x=> x <DateTime.Today);
	}
	}

view raw DatesInThePastGenerator.cs hosted with ❤ by GitHub

To use this generator, we need to specify it as a parameter of the [FsCheck.NUnit.Property] attribute:

	[FsCheck.NUnit.Property(Arbitrary = new[] { typeof(DatesInThePastGenerator) }, Verbose = true)]
	public bool ValidateShouldReturnTrue(DateTime date)
	{
	var nationalRegisterNumber = NationalRegisterNumberGenerator.Generate(date);
	return NationalRegisterNumberGenerator.IsValid(nationalRegisterNumber);
	}

view raw ValidateShouldReturnTrue.cs hosted with ❤ by GitHub

As the library that we were testing allows you to generate National Registration Numbers, we can take our sample above one step further and create a generator for it:

	public static class NRNGenerator
	{
	public static Arbitrary<string> Generate()
	{
	return Arb.Default.DateTime()
	.Filter(x=> x <DateTime.Today)
	.Generator.Select(x => NationalRegisterNumberGenerator.Generate(x))
	.ToArbitrary();
	}
	}

view raw NRNGenerator.cs hosted with ❤ by GitHub

Now we can improve our test code to use this generator:

	[FsCheck.NUnit.Property(Arbitrary = new[] { typeof(NRNGenerator) },Verbose=true)]
	public bool ValidateShouldReturnTrue(string nrn)
	{
	return NationalRegisterNumberGenerator.IsValid(nrn);
	}

view raw ValidateShouldReturnTrue.cs hosted with ❤ by GitHub

Remark: It is possible to add multiple static Generator methods to one static class.

Generator utilities

In the example above, we started every time from an existing generator and expanded from there. But it is also possible to start from scratch. In that case, there are a few utility functions that can help you along the way.

One of the most used utility functions is Gen.Choose(), which makes a random choice of a value from an interval, with a uniform distribution.

Here is an example where we use this to generate a month in a specific year:

	public static class MonthOfYearGenerator
	{
	public static Gen<MonthOfYear> Generator =
	from month in Gen.Choose(1, 12)
	from year in Gen.Choose(1982, 2023)
	select new MonthOfYear()
	{
	Month = month,
	Year = year
	};
	public static Arbitrary<MonthOfYear> Generate() =>
	Arb.From(Generator);
	}

view raw MonthOfYearGenerator.cs hosted with ❤ by GitHub

Create your own FsCheck Shrinker in C#

So far the generators either used the default shrinker for the type or had no shrinker defined. Writing your own shrinker is easy thanks to the support of iterators in C# (through yield return).

One way to add a shrinker is to use an overload of the Arb.From() method:

	public static class MonthOfYearGenerator
	{
	public static Gen<MonthOfYear> Generator =
	from month in Gen.Choose(1, 12)
	from year in Gen.Choose(1982, 2023)
	select new MonthOfYear()
	{
	Month = month,
	Year = year,
	};
	public static IEnumerable<MonthOfYear> Shrinker(MonthOfYear monthOfYear)
	{
	yield return new MonthOfYear() { Month = monthOfYear.Month, Year = monthOfYear.Year - 1 };
	yield return new MonthOfYear() { Month = monthOfYear.Month, Year = monthOfYear.Year + 1 };
	yield return new MonthOfYear() { Month = monthOfYear.Month - 1, Year = monthOfYear.Year };
	yield return new MonthOfYear() { Month = monthOfYear.Month + 1, Year = monthOfYear.Year };
	}
	public static Arbitrary<MonthOfYear> Generate() =>
	Arb.From(Generator, Shrinker);
	}

view raw MonthOfYearGenerator.cs hosted with ❤ by GitHub

Another option is to create your own Arbitrary<T> instance and override the Shrinker method:

	public static class DateGenerator
	{
	public static Arbitrary<DateTime> Generate() =>
	new ArbitraryDateInThePast();
	public class DateInThePastArbitrary : Arbitrary<DateTime>
	{
	public DateInThePastArbitrary()
	{
	this.Generator = Arb.Default.DateTime().Filter(d => d < DateTime.Today).Generator;
	}
	public override Gen<DateTime> Generator { get; }
	public override IEnumerable<DateTime> Shrinker(DateTime dt)
	{
	yield return dt.AddDays(-1);
	yield return dt.AddMonths(-1);
	yield return dt.AddYears(-1);
	}
	}
	}

view raw DateGenerator.cs hosted with ❤ by GitHub

If we run our tests again, we see that the results are shrinked to 1/01/2000. This is a good indication that there is something wrong with National Registration Numbers generated after that date.