Friday, February 14, 2014

Generating immutable instances in C# with FsCheck

If you do any functional programming in C# you’ll probably have lots of classes that look like this:

class Person {
    private readonly string name;
    private readonly DateTime dateOfBirth;

    public Person(string name, DateTime dateOfBirth) { = name;
        this.dateOfBirth = dateOfBirth;

    public string Name {
        get { return name; }

    public DateTime DateOfBirth {
        get { return dateOfBirth; }

    // equality members...

I.e. immutable classes, similar to F# records.
Now let’s say we want to test some property involving this Person class. For example, let’s say we have a serializer:

interface IPersonSerializer {
    string Serialize(Person p);
    Person Deserialize(string source);

Never mind the unsafety of this serializer, we want to test that roundtrip serialization works as expected. So we grab FsCheck and write:

Spec.ForAny((Person p) => serializer.Deserialize(serializer.Serialize(p)).Equals(p))

Only to be greeted with:

System.Exception: Geneflect: type not handled Person

Ok, so we write the generator explicitly:

var personGen =
    from name in Any.OfType<string>()
    from dob in Any.OfType<DateTime>()
    select new Person(name, dob);

Spec.For(personGen, p => serializer.Deserialize(serializer.Serialize(p)).Equals(p))

And all is fine.

But the generator code is trivially derivable from the class definition. And when you have lots of immutable classes, this boilerplate becomes really annoying. Couldn’t we automatize that somehow?

As it turns out, FsCheck already does this for F# records, using reflection. With a bit of code we can extend the reflection-based generator (built into FsCheck) to make it generate instances of immutable classes. With this change, we can go back to writing Spec.ForAny and FsCheck will automatically derive the generator for our Person class!

The restrictions on the classes to make them generable by FsCheck are:

  • Must be a concrete class. No interfaces or abstract classes; FsCheck can’t guess a concrete implementation.
  • It has to have only one public constructor. Otherwise, which one would FsCheck choose?
  • All public fields and properties must be readonly. Otherwise, it creates the ambiguity of which ones to set and which ones not.
  • Must not be recursively defined. FsCheck doesn’t generate recursively defined F# records by reflection either. I assume this is to keep the implementation simple.
  • Must not have type parameters. This restriction could probably be relaxed, but since I haven’t needed it so far, I decided to play it safe. Left as an exercise for the reader :-)

This is available in FsCheck as of version 0.9.2.

Happy FsChecking in C# and VB.NET !


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