ASP.NET MVC : How to display error message from controller to view

This blog explain how to show validation error in MVC View from Controller.

There is different ways to pass error message from controller to View

·         ModelState

·         ViewBag

Use AddModelError() method of ModelState  to display error message :

Model State is a Key/Value dictionary, which is used to store state of Model, which is posed from Action and it includes validation information for each fields.

It expose the main properties/method, with help of this, you can validate the posted model by calling IsValid Property, if Model has any failed validation, it return false otherwise true .

if (ModelState.IsValid)

            { 

                // Call further API/Service or business object 

            }

            else

            { 

                // Show error message 

            }

You can add validation error message by using AddModelError() method and you can use @Html.ValidationSummary() to display the validation message on view, basically  the ValidationSummary helper method generates an unordered list (ul element) of validation messages that are in the ModelStateDictionary object and it can be used to display all the error messages for all the fields. It can also be used to display custom error messages

[HttpPost]

public ActionResult SavePerson(Person model)

{

  string Name = model.Name;   

  int firstOccurance = Name.IndexOf(',');

  int lastOcuurance =  Name.LastIndexOf(',');

  if (firstOccurance == 0)

  {

     ModelState.AddModelError("Name", "Name should not begin with Comma ','.");

  }

  else if (lastOcuurance == Name.Length - 1)

  {

     ModelState.AddModelError("Name", "Name should not end with Comma ','.");

  }

 return View("Person", person);

}

HTML Razor View:

@model Person

@Html.ValidationSummary()

<div>

    <div class="row">

        <div class="col-sm-12">

            @Html.Partial("_Person", Model })

        </div>

    </div>

</div>

HTML View:

Use Viewbag to display Error Message:

ViewBag is used to send data from controller to view and to display the error message, we can also use the ViewBag to transfer error or validation message from controller to view.

MVC Controller:

[HttpPost]

     public IActionResult SavePerson(Person person)

        {           

            string Name = person.Name;

            int firstOccurance = Name.IndexOf(',');

            int lastOcuurance = Name.LastIndexOf(',');

            if (firstOccurance == 0)

            {

                ViewBag.ErrorMessage = "Name should not begin with Comma ','.";

               // ModelState.AddModelError("Person.Name", "Name should not begin with Comma ','.");

            }

            else if (lastOcuurance == Name.Length - 1)

            {

                ViewBag.ErrorMessage = "Name should not begin with Comma ','.";

                //ModelState.AddModelError("Person.Name", "Name should not end with Comma ','.");

            }

            return View("Person", person);

        }

HTML Razor View:

<div class="alert alert-danger">

    <p>@ViewBag.ErrorMessage</p>

</div>