How do I unit test model validation in controllers decorated with [ApiController]?

Question

As pointed out in this anwer to Asp.Net Core 2.1 ApiController does not automatically validate model under unit test, the automatic ModelState validation that ASP.NET Core 2.1\'

Answer 1

If you want to validate that the api's are returning a badrequest when the data annotations are broken then you need to do an api integration test. One nice option is to run the integration tests via an in-memory client using the TestServer

Here's an example:

//arrange
var b = new WebHostBuilder()
    .UseStartup<YourMainApplication.Startup>()
    .UseEnvironment("development");

var server = new TestServer(b) { BaseAddress = new Uri(url) };
var client = server.CreateClient();
var json = JsonConvert.SerializeObject(yourInvalidModel);
var content = new StringContent(json, Encoding.UTF8, "application/json");

//act
var result = await client.PostAsync("api/yourController", content);

//assert
Assert.AreEqual(400, (int)result.StatusCode);

If you only need to make sure that the annotations is proper setup you can manually trigger the validation via the TryValidateObject method

var obj = new YourClass();
var context = new ValidationContext(obj);
var results = new List<ValidationResult>();
var valid = Validator.TryValidateObject(obj, context, results, true);