Detailed description of Viewstate in ASP.Net

The detailed view of viewstate concept in asp.net

How ViewState Works?

Viewstate is a hidden form field managed by the ASP.NET page framework.hen ASP.NET executes a page, the ViewState values from the page and all of the controls are collected and formatted into a single encoded string, and then assigned to the value attribute of the hidden form field (specifically, ). Since the hidden form field is part of the page sent to the client, the ViewState value is temporarily stored in the client's browser. If the client chooses to post the page back to the server, the ViewState string is posted back too. 

Upon postback, the ASP.NET page framework parses the ViewState string and populates the ViewState properties for the page and each of the controls. The controls, in turn, use the ViewState data to rehydrate themselves to their former state.

There are three more small, but useful things to know about ViewState.

1. You must have a server-side form tag (
   ) in your ASPX page if you want to use ViewState. A form field is required so the hidden field that contains the ViewState information can post back to the server. And, it must be a server-side form so the ASP.NET page framework can add the hidden field when the page is executed on the server.
2. The page itself saves 20 or so bytes of information into ViewState, which it uses to distribute PostBack data and ViewState values to the correct controls upon postback. So, even if ViewState is disabled for the page or application, you may see a few remaining bytes in ViewState.
3. In cases where the page does not post back, you can eliminate ViewState from a page by omitting the server side
   tag.

How to get best performance with viewstate?

Each object must be serialized going into ViewState and then deserialized upon post back, so the performance cost of using ViewState is definitely not free. However, there's usually no significant performance impact if you follow some simple guidelines to keep your ViewState costs under control.

• Disable ViewState when you don't need it. The next section, Getting Less from ViewState, covers this in detail.
• Use the optimized ViewState serializers. The types listed above have special serializers that are very fast and optimized to produce a small ViewState footprint. When you want to serialize a type not listed above, you can greatly improve its performance by creating a custom TypeConverter for it.
• Use the fewest number of objects, and if possible, reduce the number of objects you put into ViewState. For example, rather than a two-dimensional string array of names/values (which has as many objects as the length of the array), use two string arrays (only two objects). However, there is usually no performance benefit to convert between two known types before storing them in ViewState—then you're basically paying the conversion price twice.

How to make viewstate more secure?

Because it's not formatted as clear text, folks sometimes assume that ViewState is encrypted—it's not. Instead, ViewState is merely base64-encoded to ensure that values are not altered during a roundtrip, regardless of the response/request encoding used by the application.

There are two levels of ViewState security you may wish to add to your application:

1. Tamper-proofing
2. Encryption

Tamper proofing

A hashcode will not secure the actual data within the ViewState field, but it will greatly reduce the likelihood of someone tampering with ViewState to try to spoof your application, that is, posting back values that your application would normally prevent a user from inputting.

You can instruct ASP.NET to append a hashcode to the ViewState field by setting the EnableViewStateMAC attribute:

<%@Page EnableViewStateMAC=true %>

EnableViewStateMAC can be set at the page or application level. Upon postback, ASP.NET will generate a hashcode for the ViewState data and compare it to the hashcode store in the posted value. If they don't match, the ViewState data will be discarded and the controls will revert to their original settings.

By default, ASP.NET generates the ViewState hashcode using the SHA1 algorithm. Alternatively, you can select the MD5 algorithm by setting  in the machine.config file as follows:


Encryption

You can use encryption to protect the actual data values within the ViewState field. First, you must set EnableViewStatMAC="true", as above. Then, set the machineKeyvalidation type to 3DES. This instructs ASP.NET to encrypt the ViewState value using the Triple DES symmetric encryption algorithm.