Validation of viewstate MAC failed. System.Web.HttpException (0x80004005)

There are few workarounds provided by Microsoft back to 2008. See the original posting in here.

Personally I prefer solution#3 with Alex's improvement (see below). Luckily, .Net 4.0 or above has fix it within the framework.

 
public class BasePage : Page
{
private static string[] aspNetFormElements = new string[] 
{ 
"__EVENTTARGET",
"__EVENTARGUMENT",
"__VIEWSTATE",
"__EVENTVALIDATION",
"__VIEWSTATEENCRYPTED",
};

protected override void Render(HtmlTextWriter writer)
{
StringWriter stringWriter = new StringWriter();
HtmlTextWriter htmlWriter = new HtmlTextWriter(stringWriter);
base.Render(htmlWriter);
string html = stringWriter.ToString();
int formStart = html.IndexOf("<form");
int endForm = -1;
if (formStart >= 0)
endForm = html.IndexOf(">", formStart);

if (endForm >= 0)
{
StringBuilder viewStateBuilder = new StringBuilder();
foreach (string element in aspNetFormElements)
{
int startPoint = html.IndexOf("<input type=\"hidden\" name=\"" + element + "\"");
if (startPoint >= 0 && startPoint > endForm)
{
int endPoint = html.IndexOf("/>", startPoint);
if (endPoint >= 0)
{
endPoint += 2;
string viewStateInput = html.Substring(startPoint, endPoint - startPoint);
html = html.Remove(startPoint, endPoint - startPoint);
viewStateBuilder.Append(viewStateInput).Append("\r\n");
}
}
}

if (viewStateBuilder.Length > 0)
{
viewStateBuilder.Insert(0, "\r\n");
html = html.Insert(endForm + 1, viewStateBuilder.ToString());
}
}

writer.Write(html);
}
}