ViewState is the built-in structure for automatically retaining values between multiple requests for the same page in ASP.NET. In other words, ViewState technology saves/restores page state between postbacks. On the other hand, this technology comes with an overhead that affects performance especially during page load since the state data is maintained in a hidden field.

<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUJLTMzMTY4NDI5ZGRrYY+UdQNeb33gRiGcw2LoiMHduA==" />

Reducing ViewState Size

We can completely disable viewstate by setting EnableViewState to false in the page directive but you need extra programming effort for you to take care of the page state. It is a good idea to disable ViewState for the controls that do not actually need it such as Literals and Labels by setting EnableViewState to false. But this do not entirely solve the problem. 

Compressing ViewState

ASP.NET 2.0 comes with the System.IO.Compression namespace, which contains classes with functionality to compress/decompress streams. In ASP.NET 1.1, developers must use third party compression tools such as ICSharpCode.SharpZipLib to compress viewstate.

Compressing/Decompressing using GZipStream

The following class contains two methods for compressing and decompressing a stream.  

using System;
using System.Data;
using System.Configuration;
using System.IO;
using System.IO.Compression;

public static class CompressViewState
{
    public static byte[] Compress(byte[] data)
    {
        MemoryStream output = new MemoryStream();
        GZipStream gzip = new GZipStream(output,
                          CompressionMode.Compress, true);
        gzip.Write(data, 0, data.Length);
        gzip.Close();
        return output.ToArray();
    }

    public static byte[] Decompress(byte[] data)
    {
        MemoryStream input = new MemoryStream();
        input.Write(data, 0, data.Length);
        input.Position = 0;
        GZipStream gzip = new GZipStream(input,
                          CompressionMode.Decompress, true);
        MemoryStream output = new MemoryStream();
        byte[] buff = new byte[64];
        int read = -1;
        read = gzip.Read(buff, 0, buff.Length);
        while (read > 0)
        {
            output.Write(buff, 0, read);
            read = gzip.Read(buff, 0, buff.Length);
        }
        gzip.Close();
        return output.ToArray();
    }
} 

You need to save this class in a .cs file in the App_Code directory.

Utilizing the CompressViewState Class

In order to compress the ViewState of a web page, you have to override the two methods LoadPageStateFromPersistenceMedium and SavePageStateToPersistenceMedium.

The folowing code creates a BasePage class which inherits from System.Web.UI.Page, and web pages using the following Base Page class as the base class utilizes ViewState compression. The BasePage class adds an additional hidden field __COMPRESSEDVIEWSTATE, to store the compressed ViewState.

using System;
using System.IO;
using System.IO.Compression;
using System.Collections;
using System.ComponentModel;
using System.Web.UI;
using System.Configuration;
using System.Threading;
using System.Globalization;
using System.Text;

public abstract class BasePage : System.Web.UI.Page
{
    private ObjectStateFormatter _formatter =
        new ObjectStateFormatter();

    protected override void
        SavePageStateToPersistenceMedium(object viewState)
    {
        MemoryStream ms = new MemoryStream();
        _formatter.Serialize(ms, viewState);
        byte[] viewStateArray = ms.ToArray();
        ClientScript.RegisterHiddenField("__COMPRESSEDVIEWSTATE",
            Convert.ToBase64String(
            CompressViewState.Compress(viewStateArray)));
    }
    protected override object
        LoadPageStateFromPersistenceMedium()
    {
        string vsString = Request.Form["__COMPRESSEDVIEWSTATE"];
        byte[] bytes = Convert.FromBase64String(vsString);
        bytes = CompressViewState.Decompress(bytes);
        return _formatter.Deserialize(
            Convert.ToBase64String(bytes));
    }
}

Demo

Demo project contains two web pages. You may compare the compression performance using the demo project.

Download the viewstate compression demo VS2005 project. 

An online demo web page without viewstate compression.
An online demo web page with viewstate compression.

If you view the HTML of the page with viewstate compression, the __VIEWSTATE field is empty, while our __COMPRESSEDVIEWSTATE field contains the compressed ViewState, encoded in Base64.

ViewState Compression Performance

After few tests using the demo project, ViewState size is reduces by 40 - 60% resulting shorter response times for users and less bandwidth need for site owners.

Compression, decompressing, encoding and decoding data is a quite heavy work for the server so while you are saving from bandwidth and offering shorter response times for users, you are having a performance hit on the server's hardware.

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