ASP.NET MVC - 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
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.