What's the difference between ViewData and ViewBag?

It uses the C# 4.0 dynamic feature. It achieves the same goal as viewdata and should be avoided in favor of using strongly typed view models (the same way as viewdata should be avoided).

So basically it replaces magic strings:

ViewData["Foo"]

with magic properties:

ViewBag.Foo

for which you have no compile time safety.

I continue to blame Microsoft for ever introducing this concept in MVC.

The name of the properties are case sensitive.

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

Note: in most pre-release versions of MVC 3, the ViewBag property was named the ViewModel as noted in this snippet from MVC 3 release notes:

(edited 10-8-12) It was suggested I post the source of this info I posted, here is the source: http://www.asp.net/whitepapers/mvc3-release-notes#_Toc2_4

MVC 2 controllers support a ViewData property that enables you to pass data to a view template using a late-bound dictionary API. In MVC 3, you can also use somewhat simpler syntax with the ViewBag property to accomplish the same purpose. For example, instead of writing ViewData["Message"]="text", you can write ViewBag.Message="text". You do not need to define any strongly-typed classes to use the ViewBag property. Because it is a dynamic property, you can instead just get or set properties and it will resolve them dynamically at run time. Internally, ViewBag properties are stored as name/value pairs in the ViewData dictionary. (Note: in most pre-release versions of MVC 3, the ViewBag property was named the ViewModel property.)

ViewBag vs ViewData in MVC

http://royalarun.blogspot.in/2013/08/viewbag-viewdata-tempdata-and-view.html

Similarities between ViewBag & ViewData :

Helps to maintain data when you move from controller to view. Used to pass data from controller to corresponding view. Short life means value becomes null when redirection occurs. This is because their goal is to provide a way to communicate between controllers and views. It’s a communication mechanism within the server call.

Difference between ViewBag & ViewData:

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

ViewBag & ViewData Example:

public ActionResult Index()
{   
    ViewBag.Name = "Arun Prakash";   
    return View();
}

public ActionResult Index()
{  
    ViewData["Name"] = "Arun Prakash";  
    return View();
}   

Calling in View

@ViewBag.Name    
@ViewData["Name"]