How to configure AutoFixture to use an enum value as seed when creating many of a certain type?

Question

I have the following types:

public enum Status
{
    Online,
    Offline
}

public class User
{
    private readonly Status _status;
    public User(Status status) { _status = status; }
    public Status Status {get {return _status; }}
    public string Name {get;set;}
}

Now, when executing fixture.CreateMany<User> I want AutoFixture to return two Users, one per status. All other properties - like Name - should be filled with anonymous data.

Question:
How to configure AutoFixture to do this?

---

I tried the following this:

1. Register collection that news up the User object:

   fixture.Register(
       () => Enum.GetValues(typeof(Status)).Cast<Status>().Select(s => 
           new User(s)));
   

   The problem with this approach is that AutoFixture doesn't fill the other properties like Name

2. Customize User to use a factory and register a collection that uses fixture.Create:

       f.Customize<User>(c => c.FromFactory((Status s) => new User(s)));
       f.Register(() =>
           Enum.GetValues(typeof(Status))
               .Cast<Status>()
               .Select(s => (User)f.Create(new SeededRequest(typeof(User), s),
                                           new SpecimenContext(f))));
   

   That didn't work either. The seed isn't being used.

Answer 1

You could do this:

var users = new Fixture().Create<Generator<User>>();

var onlineUser = users.Where(u => u.Status == Status.Online).First();
var offlineUser = users.Where(u => u.Status == Status.Offline).First();

If you're using AutoFixture.Xunit, the declarative equivalent is:

[Theory, AutoData]
public void CreateOneOfEachDeclaratively(Generator<User> users)
{
    var onlineUser = users.Where(u => u.Status == Status.Online).First();
    var offlineUser = users.Where(u => u.Status == Status.Offline).First();

    // Use onlineUser and offlineUser here...
}

Answer 2

You may declare and use a customization, e.g. StatusGenerator:

var fixture = new Fixture();
fixture.RepeatCount = 2;
fixture.Customizations.Add(new StatusGenerator());

var result = fixture.CreateMany<User>();

A hypothetical implementation of the StatusGenerator could be the following:

internal class StatusGenerator : ISpecimenBuilder
{
    private readonly Status[] values;
    private int i;

    internal StatusGenerator()
    {
        this.values =
            Enum.GetValues(typeof(Status)).Cast<Status>().ToArray();
    }

    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as ParameterInfo;
        if (pi == null || !pi.ParameterType.IsEnum)
            return new NoSpecimen(request);

        return this.values[i == this.values.Length - 1 ? i = 0 : ++i];
    }
}