What is ViewData?

Introduction
 
This article demonstrates an interesting and very useful concept in ASP.NET MVC.
 
Question: What is ViewData?
 
In simple terms "ViewBag is a data holder that enables definition of a key-value pair and that holds the data that can be passed from controller to view". 

Step 1: Create a new ASP.NET MVC application




 
Step 2: Adding New Model

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Web;
namespace ViewDataDemoApp.Models
{
    public class Addition
    {
        [Required(ErrorMessage = "Please Enter FirstNumber")]
        public int FirstNumber { get; set; }
        [Required(ErrorMessage = "Please Enter SecondNumber")]
        public int SecondNumber
        {
            get;
            set;
        }
    }
}

Step 3: Adding New Controller

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using ViewDataDemoApp.Models;
namespace ViewDataDemoApp.Controllers
{
    public class AdditionController : Controller
    {
        //// GET: /Addition/public ActionResult Index(Addition addition)
        ViewData["AddMessage"] = "Addition Result is: ";
        ViewData["Result"] = addition.FirstNumber + addition.SecondNumber;
        if ((int)ViewData["Result"] < 1)
        {
           ViewData["AddMessage"] = string.Empty;
           ViewData["Result"] = string.Empty;
        }
        return View();
    }
  }
}

Step 4: Adding New View

@model ViewDataDemoApp.Models.Addition@
{ ViewBag.Title = "Addition Operation Using ViewData"; }<h2>
    Addition Operation Using ViewData</h2>
<br />
@using (Html.BeginForm()) {@Html.ValidationSummary(true)<fieldset>
    <legend>Addition</legend>
    <div class="editor-label">@Html.LabelFor(model => model.FirstNumber)</div>
    <div class="editor-field">@Html.EditorFor(model => model.FirstNumber)@Html.ValidationMessageFor(model => model.FirstNumber)</div>
    <div class="editor-label">@Html.LabelFor(model => model.SecondNumber)</div>
    <div class="editor-field">@Html.EditorFor(model => model.SecondNumber)@Html.ValidationMessageFor(model => model.SecondNumber)</div>
    <p>@ViewData["AddMessage"] @ViewData["Result"]</p>
    <br />
    <p>
        <input type="submit" value="Add" /></p>
    </fieldset> }<div>@Html.ActionLink("Back to List", "Index")
    </div>
@section Scripts {@Scripts.Render("~/bundles/jqueryval")
}

Step 5: The output for the application looks as in the following