Test with invalid model

PostController.cs

[HttpPost]
public ActionResult Write(PostInputViewModel input)
{
    //bool val = TryValidateModel(input);
   
    if (!ModelState.IsValid)
        return View("Write", input);

    var post = new Post
    {
        Title = input.Title,
        ContentWithHtml = input.ContentWithHtml,
        DateCreated = DateTime.Now,
        DateModified = DateTime.MaxValue        
    };
    
    dbContext.Posts.Add(post);
    dbContext.SaveChanges();

    return RedirectToAction("Index", "Home");
}

PostController_Test.cs

[TestMethod]
public void write_should_validate_model()
{
    var input = new PostInputViewModel();
    input.Title = null;
    input.ContentWithHtml = null;

    // #1 test-validation
    //var actionResult = controller.Write(input) as ViewResult;
    //Assert.IsFalse(actionResult.ViewData.ModelState.IsValid);

    // #2 test-data input failure and view result when model is invalid
    controller.ModelState.AddModelError("InvalidModel", "Title and Content are empty");
    var actionResult = controller.Write(input) as ViewResult;
    Assert.AreEqual("Write", actionResult.ViewName);
    Assert.AreEqual(input, actionResult.Model);
    Assert.AreEqual(0, context.Posts.Count());
    
}

PostInputViewModel.cs

public class PostInputViewModel
{
    [Required]
    [MinLength(1)]
    [MaxLength(125)]
    public string Title { get; set; }

    [Required]
    [AllowHtml]
    [Display(Name="Content")]
    public string ContentWithHtml { get; set; }
}