State Management in MVC

State Management Technique in .net MVC application:

ASP.NET MVC offers us three options 

ViewData, ViewBag and TempData for passing data from controller to view and in next request. ViewData and ViewBag are almost similar and TempData performs additional responsibility. 

Similarities between ViewBag & ViewData:

1. Used to pass data from controller to view.
2. Its value becomes null when redirection occurs. 

Difference between ViewBag & ViewData:

1. ViewData is a dictionary of objects that is derived from ViewDataDictionary class and is accessible using strings as keys.
2. ViewBag is a dynamic property that takes advantage of the new dynamic features in C# 4.0.
3. ViewData requires typecasting for complex data type and check for null values to avoid error.
4. ViewBag doesn’t require typecasting for complex data type.

One more mechanism is the Session which works like the ViewData, like a Dictionary that takes a string for key and object for value. This one is stored into the client Cookie and can be used for a much more long time. It also needs more verification to never have any confidential information. Regarding ViewData or ViewBag, you should use it intelligently for application performance. Because each action goes through the whole life cycle of regular ASP.NET MVC request. You can use ViewData/ViewBag in your child action, but be careful that you are not using it to populate the unrelated data which can pollute your controller.

ViewBag:

If you want to send a small amount of data to the view. ViewBag can be used to send data from the controller to the view.

The viewBag is a dynamic type property of ControllerBase class which is the base class of all the controllers.

The following figure illustrates the ViewBag

On Controller->   ViewBag.Name = "Test";

On View-> @ViewBag.Name

Note: You can assign a primitive or a complex type object as a value to ViewBag property.

ViewBag only transfers data from controller to view, not visa-versa. ViewBag values will be null if redirection occurs.

The following example demonstrates how to transfer data from controller to view using ViewBag.

Example: Set ViewBag in Action method

namespace MVC_BasicTutorials.Controllers
{
    public class StudentController : Controller
    {
        IList<Student> studentList = new List<Student>() { 
                    new Student(){ StudentID=1, StudentName="Steve", Age = 21 },
                    new Student(){ StudentID=2, StudentName="Bill", Age = 25 },
                    new Student(){ StudentID=3, StudentName="Ram", Age = 20 },
                    new Student(){ StudentID=4, StudentName="Ron", Age = 31 },
                    new Student(){ StudentID=5, StudentName="Rob", Age = 19 }
                };
        // GET: Student
        public ActionResult Index()
        {
            ViewBag.TotalStudents = studentList.Count();

            return View();
        }

    }
}

In the above example, we have attached the TotalStudents property to the ViewBag and assigned the student count using studentList.Count().

Now, in the Index.cshtml view, you can access ViewBag.TotalStudents property and display all the student info as shown below.

Example: Acess ViewBag in a View

<label>Total Students:</label>  @ViewBag.TotalStudents

Output:

Total Students: 5

ViewBag doesn't require typecasting while retriving values from it.

Internally, ViewBag is a wrapper around ViewData. It will throw a runtime exception, if the ViewBag property name matches on with the key of ViewData.

Points to Remember :

1. ViewBag transfers data from the controller to the view, ideally temporary data which in not included in a model.
2. ViewBag is a dynamic property that takes advantage of the new dynamic features in C# 4.0
3. You can assign any number of propertes and values to ViewBag
4. The ViewBag's life only lasts during the current http request. ViewBag values will be null if redirection occurs.
5. ViewBag is actually a wrapper around ViewData.

ViewData:

ViewData is similar to ViewBag. It is useful in transferring data from Controller to View.

ViewData is a dictionary which can contain key-value pairs where each key must be string.

The following figure illustrates the ViewData.

ViewData["Name"]="Test" ; (on controller)
@ViewData["Name"]  (on View)

Note: ViewData only transfers data from controller to view, not vice-versa. It is valid only during the current request.

The following example demonstrates how to transfer data from controller to view using ViewData.

Example: ViewData in Action method

public ActionResult Index()
{
    IList<Student> studentList = new List<Student>();
    studentList.Add(new Student(){ StudentName = "Bill" });
    studentList.Add(new Student(){ StudentName = "Steve" });
    studentList.Add(new Student(){ StudentName = "Ram" });

    ViewData["students"] = studentList;
  
    return View();
}

In the above example, we have added a student list with the key "students" in the ViewData dictionary. So now, the student list can be accessed in a view as shown below.

Example: Access ViewData in a Razor View

<ul>
@foreach (var std in ViewData["students"] as IList<Student>)
{
    <li>
        @std.StudentName
    </li>
}
</ul>

Please notice that we must cast ViewData values to the appropriate data type.

You can also add a KeyValuePair into ViewData as shown below.

Example: Add KeyValuePair in ViewData

public ActionResult Index()
{
    ViewData.Add("Id", 1);
    ViewData.Add(new KeyValuePair<string, object>("Name", "Bill"));
    ViewData.Add(new KeyValuePair<string, object>("Age", 20));

    return View();
}

ViewData and ViewBag both use the same dictionary internally. So you cannot have ViewData Key matches with the property name of ViewBag, otherwise it will throw a runtime exception.

Example: ViewBag and ViewData

public ActionResult Index()
{
    ViewBag.Id = 1;

    ViewData.Add("Id", 1); // throw runtime exception as it already has "Id" key
    ViewData.Add(new KeyValuePair<string, object>("Name", "Bill"));
    ViewData.Add(new KeyValuePair<string, object>("Age", 20));

    return View();
}




Points to Remember :

1. ViewData transfers data from the Controller to View, not vice-versa.
2. ViewData is derived from ViewDataDictionary which is a dictionary type.
3. ViewData's life only lasts during current http request. ViewData values will be cleared if redirection occurs.
4. ViewData value must be type cast before use.
5. ViewBag internally inserts data into ViewData dictionary. So the key of ViewData and property of ViewBag must NOT match.

TempData:

TempData in ASP.NET MVC can be used to store temporary data which can be used in the subsequent request. TempData will be cleared out after the completion of a subsequent request.

TempData is useful when you want to transfer non-sensitive data from one action method to another action method of the same or a different controller as well as redirects. It is dictionary type which is derived from TempDataDictionary.

You can add a key-value pair in TempData as shown in the below example.

Example: TempData

public class HomeController : Controller
{
    // GET: Student
    public HomeController()
    {

    }
    public ActionResult Index()
    {
        TempData["name"] = "Test data";
        TempData["age"] = 30;

        return View();
    }

    public ActionResult About()
    {
        string userName;
        int userAge;
        
        if(TempData.ContainsKey("name"))
            userName = TempData["name"].ToString();
    
        if(TempData.ContainsKey("age"))
            userAge = int.Parse(TempData["age"].ToString());
    
        // do something with userName or userAge here 

        return View();
    }
}

In the above example, we have added data into TempData and accessed the same data using a key inside another action method. Please notice that we have converted values into the appropriate type.

The following figure illustrates TempData.

TempData

Note: TempData internally uses session to store the data. So the data must be serialized if you decide you to switch away from the default Session-State Mode, and use State Server Mode or SQL Server Mode.


As you can see in the above example, we add test data in TempData in the first request and in the second subsequent request we access test data from TempData which we stored in the first request. However, you can't get the same data in the third request because TempData will be cleared out after second request.

Call TempData.Keep() to retain TempData values in a third consecutive request.

Example: TempData.Keep()

public class HomeController : Controller
{
    // GET: Student
    public HomeController()
    {

    }
    public ActionResult Index()
    {
        TempData["myData"] = "Test data";
        return View();
    }

    public ActionResult About()
    {
        string data;
        
        if(TempData["myData"] != null)
            data = TempData["myData"] as string;
        
        TempData.Keep();
        
        return View();
    }

    public ActionResult Contact()
    {
        string data;
        
        if(TempData["myData"] != null)
            data = TempData["myData"] as string;
            
        return View();
    }

}




Points to Remember :

1. TempData can be used to store data between two consecutive requests. TempData values will be retained during redirection.
2. TemData is a TempDataDictionary type.
3. TempData internaly use Session to store the data. So think of it as a short lived session.
4. TempData value must be type cast before use. Check for null values to avoid runtime error.
5. TempData can be used to store only one time messages like error messages, validation messages.
6. Call TempData.Keep() to keep all the values of TempData in a third request.