I'm new to ASP.Net WebApi. Apologies therefore if this has been asked before (I'm unsure of the correct terminology), but I can only find this related answer which is slightly different to my problem.

I need to create a Post controller that accepts a complex type (a TestObject), containing:

1. An IEnumerable<Person>, where Person has properties Username and ExternalId
2. Options on how the controller should handle the data (a TestOptions object)

I have defined the classes below with data annotations to assist with validation:

public class TestObject
{
    public IEnumerable<TestPerson> TestPerson;
    public TestOptions Options;

    public void TestOject()
    {
        this.TestPerson = new List<TestPerson>();
        this.Options = new TestOptions();
    }
}

public class TestPerson
{
    [Required, MaxLength(50)]
    public string ExternalId;

    [Required, MaxLength(50)]
    public string Username;
}

public class TestOptions
{
    [Required]
    public bool Option1;

    [Required, MaxLength(50)]
    public string Option2;
}

The controller accepts TestObject and performs validation before doing anything:

    [Route("test")]
    public HttpResponseMessage Post([FromBody] TestObject t)
    {
        if (!ModelState.IsValid)
        {
            return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
        }
        else
        {
            return Request.CreateResponse(HttpStatusCode.OK, string.Format("Found {0} people", t.TestPerson.Count().ToString()));
        }
    }

If I use Fiddler to send a sample JSON object the with correct structures, it works fine. However, if I deliberately introduce errors (E.g. Username and Option2 missing) like this...

{
   "TestPerson":[
      { "ExternalId":"123", "Username":"Bob" },
      { "ExternalId":"123" }
   ],
   "Options":{"Option1":false}
}

I still get

Status 200

2 people found

Why is this happening please? Can I use data annotation validation on complex types?

Update

Debugging image with property values being set: