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ASP.NET MVC: Using Keep and Peek with TempData to Persist Data

Introduction: In this article you will learn the following

1) What is TempData and what is the use of TempData?
2) How to pass data from Controller to controller or action to action?
3) How to persist data in next request?
4) What is Keep() and Peek() methods and their use?


What is TempData and what is the use of TempData?

1) TempData is like ViewData, except that it persists for two successive requests thus helps to maintain data when we move from one controller to other controller or from one action to other action. So TempData is used to pass data from one HTTP request to next HTTP request.

2) TempData is derived from TempDataDictionary and is a property of BaseController class. 

3) Typecasting for complex data type is required and it is recommended to check for null values to avoid errors.

4) TempData uses session to store the data, behind the scenes.

5) It allows for persisting information for the duration of a single subsequent request. We store something inside TempData and then redirect. In the target controller action to which you redirected we can retrieve the value that was stored inside TempData.

6) It is mainly used to store only one time messages like error messages, validation messages etc.

7) TempData is meant to be a very short-lived instance thus lifetime of TempData is very short i.e. from the current request to the subsequent request.

8) Use TempData when you need data to be available for the next request only.

How to pass data from Controller to controller or action to action?

Example demonstrating the use of TempData

Right click on Models folder -> Add -> Class and name it "BookDetails.cs" and write the code as shown in image below:

Use of Keep and Peek with TempData in MVC


Now create the controller and add the code in the controller to store the data in the TempData as shown in image below: 

Use of Keep and Peek with TempData in MVC


In above example, we stored book object in TempData in Controller’s Index Action and retrieved that in Controller’s another Action "DisplayBookData" that will render the Book Details. Do you think it can be done using ViewBag or ViewData? No actually because if we try to do the same using ViewBag or ViewData, we will get null in Controller’s action "DisplayBookData" because only TempData object maintains data between controller actions.

Now create two view corresponding to Index() and DisplayBookData() actions defined in HomeController. There is no need to write any code in the index.cshtml view because that will not render any details instead we write the code on DisplayBookData.cshtml view as shown in image below:

Use of Keep and Peek with TempData in MVC


Now run the application. Output will be as shown in image below: 

Use of Keep and Peek with TempData in MVC

How to persist data in next request?

TempData’s behavior (i.e. whether it persists/preserves values for the next request or not) will be based on following 4 cases.

Case 1: Not Read in First Request.
Case 2: Read value in First Request.
Case 3: Read & persist value using Keep.
Case 4: Persist value using Peek and Read.

Let us discuss these four cases one by one in detail.

Not Read in First Request
If we set a value in TempData(e.g. TempData["BookName"]="Ramayana") in our controller’s action but don’t read that in view then TempData value will be persisted for the next request and will be available in next request.

Read in First Request
If we read TempData value as mentioned below in the current request then its value will not be persisted for the next request i.e. its value will be null on accessing in next request.

String bookName =TempData["BookName"].ToString()

Read & Persist using Keep
If we read TempData value in the current request and we use "keep" method as mentioned below then value will be persisted in TempData for the next request.

String bookName = TempData["BookName"].ToString()
TempData.Keep();  

Note: Simple version of Keep method preservers all TempData objects for next request. But if we want to preserve any specific object then we can use overloaded version of Keep method where we can pass the Object name e.g. BookName here in our example as following.

TempData.Keep("BookName");

Persist using Peek and Read
If we read TempData by using the "Peek" method, then the value will be persisted for the next request. This way we can read and retain the value of TempData in single statement as following.

String bookName = TempData.Peek("BookName").ToString();

Note: There is no overload of Peek method.


 Now over to you:
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