How to display Jquery modal dialog after server side validation error

You will need to use AJAX submit of the form. Here's how to proceed. As always start with a view model which will represent the information for the dialog form:

public class MyViewModel
{
    public string Foo { get; set; }

    [Required(ErrorMessage = "The bar is absolutely required")]
    public string Bar { get; set; }
}

then a controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        return View();
    }

    public ActionResult Create()
    {
        return PartialView("Create");
    }

    [HttpPost]
    public ActionResult Create(MyViewModel model)
    {
        if (!ModelState.IsValid)
        {
            return PartialView(model);
        }
        // TODO: the model is valid => do some processing with it
        // and return a JSON result confirming the success
        return Json(new { success = true });
    }
}

and a main view (~/Views/Home/Index.cshtml):

<script src="@Url.Content("~/Scripts/jquery-ui-1.8.11.js")" type="text/javascript"></script>
<script type="text/javascript">
    // Remark: all this javascript could be placed in a separate js file
    // to avoid cluttering the views
    $(function () {
        $('#modalLink').click(function () {
            $('#dialog').load(this.href, function () {
                $(this).dialog();
                bindForm(this);
            });
            return false;
        });
    });

    function bindForm(dialog) {
        $('form', dialog).submit(function () {
            $.ajax({
                url: this.action,
                type: this.method,
                data: $(this).serialize(),
                success: function (result) {
                    if (result.success) {
                        alert('thanks for submitting');
                        $('#dialog').dialog('close');
                    } else {
                        $('#dialog').html(result);
                        bindForm();
                    }
                }
            });
            return false;
        });
    }
</script>


@Html.ActionLink("open modal", "create", null, null, new { id = "modalLink" })
<div id="dialog"></div>

and a partial view (~/Views/Home/Create.cshtml) which will contain the form shown in the modal:

@model MyViewModel
@using (Html.BeginForm())
{
    <div>
        @Html.LabelFor(x => x.Foo)
        @Html.EditorFor(x => x.Foo)
        @Html.ValidationMessageFor(x => x.Foo)
    </div>
    <div>
        @Html.LabelFor(x => x.Bar)
        @Html.EditorFor(x => x.Bar)
        @Html.ValidationMessageFor(x => x.Bar)
    </div>
    <input type="submit" value="OK" />
}