MVC 4 - Return error message from Controller - Show in View

The Return View(model) returns you error because you don't fill the model with the values in your post method and the model data for the dropdown is empty. Please provide the Get method to explain further how to manage displaying the error. In order to the error to be shown you should use this:

[HttpPost]
public ActionResult form_edit(FormModels model)
{          
   if(ModelState.IsValid())
   {
      --- operations 
      return Redirect("OtherAction", "SomeController");
   }

   // here you can use a little trick
   //fill the model property that holds the information for the dropdown with the data 

   // you haven't provided the get method but it should look something like this
   model.Countries = ... some data goes here;
   model.dd_value = ... some other data;
   model.dd_text = ... other data;

   ModelState.AddModelError("", "adfdghdghgdhgdhdgda");
   return View(model);
}

and then in the view just use :

@model mvc_cs.Models.FormModels
@using ctrlr = mvc_cs.Controllers.FormController


@using (Html.BeginForm("form_edit", "Form", FormMethod.Post))
{

    <table>
    <tr>
    <td>       
    @Html.ValidationSummary(true)             
    </td>
    </tr>
<tr>
    <th>
        @Html.DisplayNameFor(model => model.content_name)
        @Html.DropDownListFor(x => x.selectedvalue, new SelectList(Model.Countries, Model.dd_value, Model.dd_text), "-- Select Product--")

    </th>
</tr>
</table>

<table>
<tr>
    <td>
        <input  type="submit" value="Submit" />
    </td>
</tr>
</table>
}

This should work okay.

If you just use RedirectToAction it will redirect you to the get method --> you will have no error but the view will be just reloaded and no error would be shown.

other way around is that you can pass the error not by ModelState.AddError, but with ViewData["error"] like this:

[HttpPost]
public ActionResult form_edit(FormModels model)
{          
   TempData["error"] = "someErrorMessage";
   return RedirectToAction("form_Post", "Form");
}

[HttpGet]
public ActionResult form_edit()
{
    do stuff here ----
    ViewData["error"] = TempData["error"];
    return View();
}

@model mvc_cs.Models.FormModels
@using ctrlr = mvc_cs.Controllers.FormController


@using (Html.BeginForm("form_edit", "Form", FormMethod.Post))
{

    <table>
    <tr>
    <td>       
    <div>@ViewData["error"]</div>             
    </td>
    </tr>
<tr>
    <th>
        @Html.DisplayNameFor(model => model.content_name)
        @Html.DropDownListFor(x => x.selectedvalue, new SelectList(Model.Countries, Model.dd_value, Model.dd_text), "-- Select Product--")

    </th>
</tr>
</table>

<table>
<tr>
    <td>
        <input  type="submit" value="Submit" />
    </td>
</tr>
</table>
}

You can add this to your _Layout.cshtml:

@using MyProj.ViewModels;
...
    @if (TempData["UserMessage"] != null)
    {
        var message = (MessageViewModel)TempData["UserMessage"];
        <div class="alert @message.CssClassName" role="alert">
            <button type="button" class="close" data-dismiss="alert" aria-label="Close">
                <span aria-hidden="true">&times;</span>
            </button>
            <strong>@message.Title</strong>
            @message.Message
        </div>
    }

Then if you want to throw an error message in your controller:

    TempData["UserMessage"] = new MessageViewModel() { CssClassName = "alert-danger  alert-dismissible", Title = "Error", Message = "This is an error message" };

MessageViewModel.cs:

public class MessageViewModel
    {
        public string CssClassName { get; set; }
        public string Title { get; set; }
        public string Message { get; set; }
    }

Note: Using Bootstrap 4 classes.

Thanks for all the replies.

I was able to solve this by doing the following:

CONTROLLER:

[HttpPost]       
public ActionResult form_edit(FormModels model)
{
  model.error_msg = model.update_content(model);
  return RedirectToAction("Form_edit", "Form", model);
}

public ActionResult form_edit(FormModels model, string searchString,string id)
{
  string test = model.selectedvalue;
  var bal = new FormModels();
  bal.Countries = bal.get_contentdetails(searchString);
  bal.selectedvalue = id;
  bal.dd_text = "content_name";
  bal.dd_value = "content_id";

  test = model.error_msg;
  ViewBag.head = "Heading";

  if (model.error_msg != null)
  {
    ModelState.AddModelError("error_msg", test);
  }

  model.error_msg = "";
  return View(bal);
}   

VIEW:

@using (Html.BeginForm("form_edit", "Form", FormMethod.Post))
{
  <table>
    <tr>
      <td>
        @ViewBag.error
        @Html.ValidationMessage("error_msg")
      </td>
    </tr>
    <tr>
      <th>
        @Html.DisplayNameFor(model => model.content_name)
        @Html.DropDownListFor(x => x.selectedvalue, new SelectList(Model.Countries, Model.dd_value, Model.dd_text), "-- Select Product--")
      </th>
    </tr>
  </table>
}

If you want to do a redirect, you can either:

ViewBag.Error = "error message";

or

TempData["Error"] = "error message";