How to decode viewstate

Here's an online ViewState decoder:

http://ignatu.co.uk/ViewStateDecoder.aspx

Edit: Unfortunatey, the above link is dead - here's another ViewState decoder (from the comments):

http://viewstatedecoder.azurewebsites.net/

Use Fiddler and grab the view state in the response and paste it into the bottom left text box then decode.

Here is the source code for a ViewState visualizer from Scott Mitchell's article on ViewState (25 pages)

using System;using System.Collections;using System.Text;using System.IO;using System.Web.UI;namespace ViewStateArticle.ExtendedPageClasses{    /// <summary>    /// Parses the view state, constructing a viaully-accessible object graph.    /// </summary>    public class ViewStateParser    {        // private member variables        private TextWriter tw;        private string indentString = "   ";        #region Constructor        /// <summary>        /// Creates a new ViewStateParser instance, specifying the TextWriter to emit the output to.        /// </summary>        public ViewStateParser(TextWriter writer)        {            tw = writer;        }        #endregion        #region Methods        #region ParseViewStateGraph Methods        /// <summary>        /// Emits a readable version of the view state to the TextWriter passed into the object's constructor.        /// </summary>        /// <param name="viewState">The view state object to start parsing at.</param>        public virtual void ParseViewStateGraph(object viewState)        {            ParseViewStateGraph(viewState, 0, string.Empty);            }        /// <summary>        /// Emits a readable version of the view state to the TextWriter passed into the object's constructor.        /// </summary>        /// <param name="viewStateAsString">A base-64 encoded representation of the view state to parse.</param>        public virtual void ParseViewStateGraph(string viewStateAsString)        {            // First, deserialize the string into a Triplet            LosFormatter los = new LosFormatter();            object viewState = los.Deserialize(viewStateAsString);            ParseViewStateGraph(viewState, 0, string.Empty);            }        /// <summary>        /// Recursively parses the view state.        /// </summary>        /// <param name="node">The current view state node.</param>        /// <param name="depth">The "depth" of the view state tree.</param>        /// <param name="label">A label to display in the emitted output next to the current node.</param>        protected virtual void ParseViewStateGraph(object node, int depth, string label)        {            tw.Write(System.Environment.NewLine);            if (node == null)            {                tw.Write(String.Concat(Indent(depth), label, "NODE IS NULL"));            }             else if (node is Triplet)            {                tw.Write(String.Concat(Indent(depth), label, "TRIPLET"));                ParseViewStateGraph(((Triplet) node).First, depth+1, "First: ");                ParseViewStateGraph(((Triplet) node).Second, depth+1, "Second: ");                ParseViewStateGraph(((Triplet) node).Third, depth+1, "Third: ");            }            else if (node is Pair)            {                tw.Write(String.Concat(Indent(depth), label, "PAIR"));                ParseViewStateGraph(((Pair) node).First, depth+1, "First: ");                ParseViewStateGraph(((Pair) node).Second, depth+1, "Second: ");            }            else if (node is ArrayList)            {                tw.Write(String.Concat(Indent(depth), label, "ARRAYLIST"));                // display array values                for (int i = 0; i < ((ArrayList) node).Count; i++)                    ParseViewStateGraph(((ArrayList) node)[i], depth+1, String.Format("({0}) ", i));            }            else if (node.GetType().IsArray)            {                tw.Write(String.Concat(Indent(depth), label, "ARRAY "));                tw.Write(String.Concat("(", node.GetType().ToString(), ")"));                IEnumerator e = ((Array) node).GetEnumerator();                int count = 0;                while (e.MoveNext())                    ParseViewStateGraph(e.Current, depth+1, String.Format("({0}) ", count++));            }            else if (node.GetType().IsPrimitive || node is string)            {                tw.Write(String.Concat(Indent(depth), label));                tw.Write(node.ToString() + " (" + node.GetType().ToString() + ")");            }            else            {                tw.Write(String.Concat(Indent(depth), label, "OTHER - "));                tw.Write(node.GetType().ToString());            }        }        #endregion        /// <summary>        /// Returns a string containing the <see cref="IndentString"/> property value a specified number of times.        /// </summary>        /// <param name="depth">The number of times to repeat the <see cref="IndentString"/> property.</param>        /// <returns>A string containing the <see cref="IndentString"/> property value a specified number of times.</returns>        protected virtual string Indent(int depth)        {            StringBuilder sb = new StringBuilder(IndentString.Length * depth);            for (int i = 0; i < depth; i++)                sb.Append(IndentString);            return sb.ToString();        }        #endregion        #region Properties        /// <summary>        /// Specifies the indentation to use for each level when displaying the object graph.        /// </summary>        /// <value>A string value; the default is three blank spaces.</value>        public string IndentString        {            get            {                return indentString;            }            set            {                indentString = value;            }        }        #endregion    }}

And here's a simple page to read the viewstate from a textbox and graph it using the above code

private void btnParse_Click(object sender, System.EventArgs e)        {            // parse the viewState            StringWriter writer = new StringWriter();            ViewStateParser p = new ViewStateParser(writer);            p.ParseViewStateGraph(txtViewState.Text);            ltlViewState.Text = writer.ToString();        }