Question

Inside my controller’s action I have the following code:

public ActionResult GridAction(string id)
{
    if (String.IsNullOrEmpty(id)) 
    {
        // add errors to the errors collection and then return the view saying that you cannot select the dropdownlist value with the "Please Select" option
    }

    return View(); 
}

UPDATE:

if (String.IsNullOrEmpty(id))
{
    // add error 
    ModelState.AddModelError("GridActionDropDownList", "Please select an option");
    return RedirectToAction("Orders"); 
}

UPDATE 2:

Here is my updated code:

@Html.DropDownListFor(x => x.SelectedGridAction, Model.GridActions,"Please Select") 
@Html.ValidationMessageFor(x => x.SelectedGridAction)  

The Model looks like the following:

public class MyInvoicesViewModel
{

    private List<SelectListItem> _gridActions;

    public int CurrentGridAction { get; set; }

    [Required(ErrorMessage = "Please select an option")]
    public string SelectedGridAction { get; set; }

    public List<SelectListItem> GridActions
    {
        get
        {
            _gridActions = new List<SelectListItem>();
            _gridActions.Add(new SelectListItem() { Text = "Export to Excel", Value = "1" });

            return _gridActions;
        }
    }
} 

And here is my controller action:

public ActionResult GridAction(string id)
{
    if (String.IsNullOrEmpty(id))
    {
        // add error 
        ModelState.AddModelError("SelectedGridAction", "Please select an option");
        return RedirectToAction("Orders"); 
    }

    return View(); 
}

Nothing happens! I am totally lost on this one!

UPDATE 3:

I am now using the following code but still the validation is not firing:

public ActionResult GridAction(string id)
{
    var myViewModel= new MyViewModel();
    myViewModel.SelectedGridAction = id; // id is passed as null           

    if (!ModelState.IsValid)
    {
        return View("Orders");
    }

UPDATE 4:

$("#linkGridAction").click(function () {
    alert('link grid action clicked'); 

    $.get('GridAction/', { SelectedGridAction: $("#SelectedGridAction").val() }, function (result) {
        alert('success');
    });
});

And the Controller looks like the following:

// OrderViewModel has a property called SelectedGridAction. 
public ActionResult GridAction(OrderViewModel orderViewModel)
{
    return View(); 
}

UPDATE 5: Validation is not firing:

public ActionResult GridAction(OrderViewModel orderViewModel)
{
    if (!ModelState.IsValid)
    {
        return View("Orders", orderViewModel); 
    }
    return View(); 
}

Answer 1

You could use a view model:

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

and then:

public ActionResult GridAction(MyViewModel model)
{
    if (ModelState.IsValid)
    {
        // the model is valid, the user has selected an id => use it
        return RedirectToAction("Success");
    }
    return View();
}

---

UPDATE:

After the hundreds of comments on my answer I feel in the necessity to provide a full working example:

As usual start with a view model:

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

    public IEnumerable<SelectListItem> Items 
    {
        get
        {
            // Dummy data
            return new SelectList(Enumerable.Range(1, 10)
                .Select(i => new SelectListItem 
                {
                    Value = i.ToString(),
                    Text = "item " + i 
                }), 
            "Value", "Text");
        }
    }
}

Then a controller:

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

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        if (!ModelState.IsValid)
        {
            // The user didn't select any value => redisplay the form
            return View(model);
        }
        // TODO: do something with model.SelectedItemId
        return RedirectToAction("Success");
    }
}

and finally the view:

<% using (Html.BeginForm()) { %>
    <%= Html.DropDownListFor(
        x => x.SelectedItemId, 
        Model.Items, 
        "-- Select Item --"
    ) %>
    <%= Html.ValidationMessageFor(x => x.SelectedItemId) %>
    <input type="submit" value="OK" />
<% } %>