C# System.InvalidCastException

Question

Why I'm getting this error?

System.InvalidCastException was unhandled by user code
  Message=Specified cast is not valid.
  Source=System.Windows.Forms
  StackTrace:
       at System.Windows.Forms.UnsafeNativeMethods.IHTMLDocument2.GetLocation()
       at System.Windows.Forms.WebBrowser.get_Document()
       at System.Windows.Forms.WebBrowser.get_DocumentStream()
       at System.Windows.Forms.WebBrowser.get_DocumentText()
       at SiteBot.MainWindow.backgroundWorker1_DoWork(Object sender, DoWorkEventArgs e) in D:\Documents\Visual Studio 2010\Projects\SiteBot\MainWindow.cs:line 35
       at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
       at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
  InnerException: 

Solution

The following solves your cross thread issue.

public delegate string GetStringHandler();
public string GetDocumentText()
{
    if (InvokeRequired)
        return Invoke(new GetStringHandler(GetDocumentText)) as string;
    else
        return webBrowser.DocumentText;
}

if (regAddId.IsMatch(GetDocumentText()))
{
}