Unable to validate Dto with ModelState.IsValid

Question

I have a Dto class ContactInformationDto which contains the fields Phonenumber and Emailaddress. In my endpoint I want to use ModelState.IsValid to check whether both fields meet the requirements I have set.

The ContactInformationDto looks like this:

public class ContactInformationDto
{
    [Required(ErrorMessage = "Phonenumber is required")]
    [MaxLength(15, ErrorMessage = "Phonenumber is too long")]
    public string Phonenumber { get; set; }

    [Required(ErrorMessage = "Emailaddress is required")]
    public string Emailaddress { get; set; }
}

My endpoint:

[HttpPost]
public async Task<IActionResult> SaveContactInformation([FromBody] ContactInformationDto contactInformation, CancellationToken cancelllationToken)
{
    if(!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }
}

When I run the following unit test ModelState.IsValid is always true and the test fails.

public async Task SaveContactInformation_IfNoPhoneNumber_ThenBadRequest()
{
    var contactInformation = new ContactInformationDto
    {
        Emailaddress = "[email protected]"
    }

    var actionResult = await _controller.SaveContactInformation(contactInformation, _cancellationToken);
    
    Assert.IsBadRequest(actionResult);
}

My other test with a Phonenumber longer than the allowed number of characters alse fails.

---- Solution ---- @Xerillio points out correctly that modelstate validation does not happen in the unit test I'm using here. I added an error in my unit test to the modelstate to test how my method reacts to Model.IsValid and an integration test to test if my validation attributes are correct.