How to create a custom Html.ValidationMessage? - asp.net-mvc

Question

This is my Controller:
/// <summary>
/// Activity
/// </summary>
/// <returns></returns>
public ActionResult CreateActivity()
{
AriaCRMEntities aria = new AriaCRMEntities();
var unit = from u in aria.ActivityGroupIDs select u;
List<SelectListItem> lst = new List<SelectListItem>();
foreach (var u in unit)
{
lst.Add(new SelectListItem { Text = u.ActivityGroupName.ToString(), Value = u.ActivityGroupID_FK.ToString() });
}
ViewData["activity"] = lst;
return View();
}
[HttpPost]
public ActionResult CreateActivity(FormCollection model)
{
if (ModelState.IsValid)
{
Activity activ = new Activity();
if (!string.IsNullOrEmpty(model["ActivityGroupID_FK"]))
{
AriaCRMEntities aria = new AriaCRMEntities();
activ.ActivityGroupID_FK = Int32.Parse(model["ActivityGroupID_FK"]);
activ.ActivityType = model["ActivityType"];
activ.ActivityDes = model["ActivityDes"];
aria.Activities.AddObject(activ);
aria.SaveChanges();
return RedirectToAction("Create");
}
}
return View(model);
}
This is my View :
<% using (Html.BeginForm()) {%>
<%: Html.ValidationSummary(true,) %>
<fieldset>
<legend>Fields</legend>
<br />
<%:Html.DropDownListFor(model=>model.ActivityGroupID_FK , (IEnumerable<SelectListItem>)ViewData["activity"], "انتخاب نوع فعالیت")%><br />
<%Html.ValidationMessageFor (model=>model.ActivityGroupID_FK,"dddddddddd"); %>
<div class="editor-label">
<%: Html.LabelFor(model => model.ActivityType) %>
</div>
<div class="editor-field">
<%: Html.TextBoxFor(model => model.ActivityType) %>
<%: Html.ValidationMessageFor(model => model.ActivityType,"dfsaaa") %>
</div>
<div class="editor-label">
<%: Html.LabelFor(model => model.ActivityDes) %>
</div>
<div class="editor-field">
<%: Html.TextBoxFor(model => model.ActivityDes) %>
<%: Html.ValidationMessageFor(model => model.ActivityDes) %>
</div>
<p>
<input type="submit" value="Create" id="btn"/>
</p>
</fieldset>
<% } %>
But now I need to validate <%:Html.DropDownListFor%>. How do I create custom validation?

Answer 1

If you want to display a validation error message next to your drop-down list, you can do this from your controller like so:
ModelState.AddModelError("ActivityGroupID_FK", "The selected value is invalid.");
Update
I just noticed the validation message for your DropDownList looks like this:
<%Html.ValidationMessageFor (model=>model.ActivityGroupID_FK,"dddddddddd"); %>
You might want to change that like so:
<%: Html.ValidationMessageFor(model=>model.ActivityGroupID_FK,"dddddddddd") %>