what is the ViewData and ViewBag inasp.net mvc

Use of ViewData and ViewBag in Asp.net:

Both ViewData and ViewBag are used to pass data from a controller to a view. ViewData is a dictionary of objects that are stored and retrieved using strings as keys. The syntax of ViewData is very similar to that of ViewState, SessionState and ApplicationState.

// Storing data in ViewData

ViewData["YourData"] = "SomeData";

// Retrieving data from ViewData
string strData = ViewData["YourData"].ToString();

ViewData does not provide compile time error checking. For example, if you mis-spell the key names you wouldn't get any compile time error. You get to know about the error only at runtime.

ViewBag uses the dynamic feature that was introduced in to C# 4.0. It allows an object to have properties dynamically added to it. Using ViewBag the above code can be rewritten as below.
// Storing data in ViewBag
ViewBag.YourData = "SomeData";

// Retrieving data from ViewBag
string strData = ViewBag.YourData;

Just like ViewData, ViewBag does not provide compile time error checking. For example, if you mis-spell the property name, you wouldn't get any compile time error. You get to know about the error only at runtime.

Internally ViewBag properties are stored as name/value pairs in the ViewData dictionary.

Please Note: To pass data from controller to a view, It's always a good practice to use strongly typed view models instead of using ViewBag & ViewData. Strongly typed view models provide compile time error checking. We will discuss view models in a later video session.