Question

I need to force TryUpdateModel return false in unit testing in asp.net mvc web application, but it always return true.

my controller action is:

public JsonResult SaveStep(string path, string title, string definitionOfDone, int limit, bool isNewStep)
{
    Step step = new Step();

    if (TryUpdateModel(step)){
        // code    
    }
}

Answer 1

Simply add an error to the model state in the arrange phase of your unit test:

_controllerUnderTest.ModelState.AddModelError("key", "error message");

Now when you invoke the _controllerUnderTest.SaveStep in the act phase of your unit test the ModelState.IsValid will return false.