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View Variables In .NET MVC

This series of articles will discuss View related issues of the MVC module.

Introduction

This will be the contents of this article,

  • ASP.NET MVC
    • Setup Environment for Code Testing
    • What ViewBag, ViewData, TempData, and Session are
    • How ViewBag, ViewData, TempData, and Session Work
      • ViewBag and ViewData
      • TempData
      • Session
  • ASP.NET Core MVC - will be discussed in the next article
    • What ViewBag, ViewData, TempData, and Session are
    • How ViewBag, ViewData, TempData, and Session Work

A: Setup MVC Environment for Code Testing

Step 1 - Create an ASP.NET MVC app

We use the current version of Visual Studio 2019 16.9.4 and .NET Framework 4.8 to build the app,

  • Start Visual Studio and select Create a new project.
  • In the Create a new project dialog, select ASP.NET Web Application (.NET Framework) > Next.
  • In the Configure your new project dialog, enter MVC for Project name > Create.
  • In the Create a new ASP.NET Web Application dialog, select MVC > Creat

Note
For beginners, you may see details from here.

Step 2 - Add ViewBag, ViewData, TempData, and Session

In Controller, update HomeController/Index action,

namespace MVC.Controllers
{
    public class HomeController : Controller
    {
        public ActionResult Index()
        {
            List<string> Student = new List<string>();
            Student.Add("Jignesh");
            Student.Add("Tejas");
            Student.Add("Rakesh");

            this.ViewData["Student"] = Student;
            this.ViewBag.Student = Student;
            this.TempData["Student"] = Student;
            this.HttpContext.Session["Student"] = Student;

            return View();
        }

        ......
    }
}

In View/Homw/Index.cshtml,

@{
    ViewBag.Title = "Home Page";
}

<br>
<br>

@*<ul>
    <b>ViewBag</b>
    @foreach (var student in ViewBag.Student)
    {
        <li>@student</li>
    }
</ul>*@
<ul>
    <b>ViewData</b>
    @foreach (var student in ViewData["Student"] as List<string>)
    {
        <li>@student</li>
    }

</ul>
<ul>
    <b>TempData</b>
    @foreach (var student in TempData["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>
<ul>
    <b>Session</b>
    @foreach (var student in Session["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>

The result,

B: What ViewBag, ViewData, TempData, and Session are

ViewBag, ViewData, TempData, and Session are the options to send value from controller to view or to other action method/pages.

1, They are the properties of Controller in MVC

Specifically, ViewBag, ViewData, and TempData are the properties of ControllerBase class, which is the parent of the Controller Class.

While the Session is the property of Controller class,

2, Type

They are all the Type of Dictionary with the string as key, except ViewBag is dynamic type:

  • ViewData --- public System.Web.Mvc.ViewDataDictionary ViewData { get; set; }
  • ViewBag --- public dynamic ViewBag { get; }
  • TempData --- public System.Web.Mvc.TempDataDictionary TempData { get; set; }
  • Session --- public System.Web.HttpSessionStateBase Session { get; }

Note:

ViewBag is a wrapper over ViewData and allows to store and retrieve values using object-property syntax rather than key-value syntax used by dictionary objects. It does so using the dynamic data type feature of .NET.

 

C: How ViewBag, ViewData, TempData and Session Work

1, ViewBag and ViewData

They can be used to pass data from controller to view, one way only in the same request.

The following example show if the ViewBag is assigned value in one action --- index, but try to retrieve from another view --- About (two trips, two requests), it will not work

Comment out the ViewBag from Index,

Add the code to View/Home/About.cshtml,

<ul>
    <b>ViewBag</b>
    @foreach (var student in ViewBag.Student)
    {
        <li>@student</li>
    }
</ul>

Run the app, it will go to the Index page, and then click the About button to navigate to the About page.

We got the error due to the value defined in ViewBag exists only in the same action, within the same request, i.e. in View/Home/About.cshtml retrieving ViewBag value, it must be defined in Action About,

The difference between ViewBag and ViewData,

  • ViewData
    • If passing string into ViewData then no need to typecast
    • If the passing object in ViewData then you need to typecast it but before that, you need to check if it is not null
  • ViewBag
    • ViewBag's a dynamic type so not necessary to typecast

2, TempData

TempData is used to transfer data from view to controller, controller to view, or from one action method to another action method of the same or a different controller.

TempData stores the data temporarily and automatically removes it after retrieving a value. i.e., It can be retrieved once, and only once.

We demo how TempData transfers info by the following example (you need to run the example from step 1 to step 4, one by one).

Step 1 - Go to Action Index to define the TempData

public ActionResult Index()
{
    List<string> Student = new List<string>();
    Student.Add("Jignesh");
    Student.Add("Tejas");
    Student.Add("Rakesh");

    this.ViewData["Student"] = Student;
    this.ViewBag.Student = Student;
    this.TempData["Student"] = Student;
    this.HttpContext.Session["Student"] = Student;

    return View();
}

while in the View: the TempData comments out,

@{
    ViewBag.Title = "Home Page";
}

<br>
<br>

<ul>
    <b>ViewBag</b>
    @foreach (var student in ViewBag.Student)
    {
        <li>@student</li>
    }
</ul>
<ul>
    <b>ViewData</b>
    @foreach (var student in ViewData["Student"] as List<string>)
    {
        <li>@student</li>
    }

</ul>
@*<ul>
    <b>TempData</b>
    @foreach (var student in TempData["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>*@
<ul>
    <b>Session</b>
    @foreach (var student in Session["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>

Result: not showing TempData as expected,

Step 2 - Go to Action Contact to do nothing, but in the View

Retrieve TempData["Student"], also redefine TempData["name"] = "Steve"

}
<h2>@ViewBag.Title.</h2>
<h3>@ViewBag.Message</h3>

<address>
    One Microsoft Way<br />
    Redmond, WA 98052-6399<br />
    <abbr title="Phone">P:</abbr>
    425.555.0100
</address>

<address>
    <strong>Support:</strong>   <a href="mailto:[email protected]">[email protected]</a><br />
    <strong>Marketing:</strong> <a href="mailto:[email protected]">[email protected]</a>
</address>

<ul>
    <b>TempData</b>
    @foreach (var student in TempData["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>
@{
    TempData["name"] = "Steve";
}
<ul>
    <b>Session</b>
    @foreach (var student in Session["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>

Result: showing retrieved TempData["Student"]

Step 3 - Go to Action About

Retrieve TempData["name"], and redefine TempData["Student"]

public ActionResult About()
{
    ViewBag.Message = "Your application description page.";

    if (TempData["name"].ToString() == "Steve")
    {
        List<string> Student = new List<string>();
        Student.Add("Jignesh");
        Student.Add("Tejas");
        Student.Add("Rakesh");

        this.ViewData["Student"] = Student;
        this.ViewBag.Student = Student;
        this.TempData["Student"] = Student;
        this.HttpContext.Session["Student"] = Student;
    }
    return View();
}

View: desplay TempData["Student"]

<ul>
    <b>TempData</b>
    @foreach (var student in TempData["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>
<ul>
    <b>Session</b>
    @foreach (var student in Session["Student"] as List<string>)
    {
        <li>@student</li>
    }
</ul>

Result: as expected: showing TempData["Student"]

Step 4

Finally, Go back to Click Contact: try to a second time to Retrieve TempData["Student"], we got an error message,

Finally, we should mention,

  • TempData Saves into the session so on the expiration of session data loss;
  • TempData removes a key value once accessed, you can still keep it for the subsequent request by calling TempData.Keep() method.
  • TempData is usually used to pass error messages or something similar.

3, Session

  • Session stores data into session
  • The session is not similar to TempData to access one but you can read as many time as you want
  • The session never becomes null until or unless session timeout or session expires.
  • The session is not a good practice to use session very frequently or store big data, it hits performance

Summary

This article actually discussed ASP.NET MVC ViewBag, ViewData, TempData, and Session. We will leave ASP.NET Core MVC for the same discussion in the next article.

  • ASP.NET MVC
    • Setup Environment for Code Testing
    • What ViewBag, ViewData, TempData, and Session are
    • How ViewBag, ViewData, TempData, and Session Work
      • ViewBag and ViewData --- pass data from controller to view, one way only in the same request.
        • ViewBag is a dynamic type so not necessary to typecast
      • TempData --- pass data from view to controller, controller to view, or from one action method to another action method of the same or a different controller. It can retrieve once, and only once.
      • Session --- Session is similar to TempData but can be retrieved multiple times.

Reference


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