Question

Is it possible to add an error for input via ModelState.AddModelError(inputId) to have error highlight on UI and to have it to have to behave as like client validation, i.e. when user changes smth in the input error class would be removed.

Answer 1

Model:

public class MyViewModel
{
    [Required]
    public string Foo { get; set; }
}

Controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        ModelState.AddModelError("foo", "Foo is required");
        return View(new MyViewModel());
    }
}

View:

@model MyViewModel

<script src="@Url.Content("~/Scripts/jquery.validate.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.js")" type="text/javascript"></script>

@using (Html.BeginForm())
{
    @Html.EditorFor(x => x.Foo)
    @Html.ValidationMessageFor(x => x.Foo)
    <button type="submit">OK</button>
}

Now when the page is rendered, the Foo field will be highlighted red with an error and when the user types something into the field and blurs out, the error will be removed.