TUnit0001 error when tring to use a ClassDataSource on an IEnumerable<T> implementing class.

[elTRexx]

Hi, from the docs I saw that snippet :

public class UserTestData : IEnumerable<User>
{
    public IEnumerator<User> GetEnumerator()
    {
        yield return new User { Id = 1, Name = "Alice" };
        yield return new User { Id = 2, Name = "Bob" };
    }
    
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

public class UserTests
{
    [Test]
    [ClassDataSource<UserTestData>]
    public async Task ValidateUser_ShouldPass(User user)
    {
        var isValid = await UserValidator.ValidateAsync(user);
        await Assert.That(isValid).IsTrue();
    }
}

So I tried on my own project and it fail with TUnit0001 error Attribute argument types 'MyTest.UsersDataSource' don't match method parameter types 'MyTest.Users'

Here is my simple code base on the one above from your docs :

public record Users() { public int Id; public string Name; };

public class UsersDataSource : IEnumerable<Users>
{
  public IEnumerator<Users> GetEnumerator()
  {
    yield return new Users { Id = 1, Name = "1" }; 
    yield return new Users { Id = 2, Name = "2" };
  }

  IEnumerator IEnumerable.GetEnumerator()
  {
    return GetEnumerator();
  }
}

public class MyTest
  {
      [Test]
      public async Task Foo()
      {
          await Assert.That(true).IsTrue();
      }

  [Test]
  [ClassDataSource<UsersDataSource>]
  public async Task Bar(Users user)
  {
    await Assert.That(user.Name).Length().IsEqualTo(1);
  }
}

[thomhurst]

Hi! Thanks for the report — the docs were misleading.

ClassDataSource<T> injects a single instance of T into the test; it does not enumerate T even when T : IEnumerable<U>. The example in the docs was wrong. I have just fixed it on main (commit 17294ad).

For your scenario you have two clean options:

1. MethodDataSource (simplest) — works directly with IEnumerable<T>:

public static class UsersDataSource
{
    public static IEnumerable<Users> All()
    {
        yield return new Users { Id = 1, Name = "1" };
        yield return new Users { Id = 2, Name = "2" };
    }
}

public class MyTest
{
    [Test]
    [MethodDataSource(typeof(UsersDataSource), nameof(UsersDataSource.All))]
    public async Task Bar(Users user)
    {
        await Assert.That(user.Name).Length().IsEqualTo(1);
    }
}

2. Custom DataSourceGeneratorAttribute<T> — if you want a reusable attribute:

public sealed class UsersDataSourceAttribute : DataSourceGeneratorAttribute<Users>
{
    protected override IEnumerable<Func<Users>> GenerateDataSources(DataGeneratorMetadata _)
    {
        yield return () => new Users { Id = 1, Name = "1" };
        yield return () => new Users { Id = 2, Name = "2" };
    }
}

public class MyTest
{
    [Test]
    [UsersDataSource]
    public async Task Bar(Users user) { ... }
}

ClassDataSource<T> is for injecting fixtures (e.g. a WebApplicationFactory or DB connection) with optional sharing semantics (Shared = PerClass/PerAssembly/PerTestSession/Keyed) — not for emitting multiple test rows. Mixing the two would conflict with the sharing model, which is why we are keeping the behaviour as is and fixing the docs instead.

[elTRexx]

Ok, I was suspecting a docs deprecation indeed, explaining why I opened a discussion not an issue on GitHub.

For your interest @thomhurst , the same happen also here in the docs if you want to fix it too. It is also present in your TestProject project here, reason why I was still doubting if it was a docs deprecation or not.

Note : for this test file, the error is not reported because you implemented a converter operator

public static implicit operator string(EnumerableDataSource source) => "FromImplicitOperator";

meaning the test will use that single hard coded string value (FromImplicitOperator) and not the (what I supposed) 3 strings rows ("A", "B" and "C").

For my use case, I faced the problem that I needed both instance test rows and runtime initialization, meaning that both MethodDataSource and InstanceMethodDataSource didn't fit.

To be more precise I have a state machine to test, and I need to initialize my state machine with a First (fixture) IState, while my state machine having its own predefined Start and End IState already set up. And I didn't achieve to get all this 3 IState initialized via both ClassDataSource and InstanceMethodDataSource as your docs explicitly explaining why (not the same lifecycle step). I cannot fall back to MethodDataSource using static method as my state machine initialization didn't happen when the test are "populated", meaning my Start IState didn't have yet been connected to my First IState.