C# Process <instance>.StandardOutput InvalidOperationException "Cannot mix synchronous and asynchronous operation on process stream."

I tried this

        myProcess = new Process();

        myProcess.StartInfo.CreateNoWindow = true;
        myProcess.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;

        myProcess.StartInfo.FileName = "Hello.exe";

        myProcess.StartInfo.Arguments ="-say Hello";
        myProcess.StartInfo.UseShellExecute = false;  

        myProcess.OutputDataReceived += new DataReceivedEventHandler(myProcess_OutputDataReceived);
        myProcess.Er开发者_如何学编程rorDataReceived += new DataReceivedEventHandler(myProcess_OutputDataReceived);
        myProcess.Exited += new EventHandler(myProcess_Exited);
        myProcess.EnableRaisingEvents = true;

        myProcess.StartInfo.RedirectStandardOutput = true;
        myProcess.StartInfo.RedirectStandardError = true;
        myProcess.StartInfo.ErrorDialog = true;
        myProcess.StartInfo.WorkingDirectory = "D:\\Program Files\\Hello";

        myProcess.Start();

        myProcess.BeginOutputReadLine();
        myProcess.BeginErrorReadLine();

Then I am getting this error.. alt text http://img188.imageshack.us/img188/3759/errorstack.jpg

My process takes very long to complete, so I need to show progress in runtime.

You don't need to call ReadLine(), the line of text that was read is one of the properties passed to you in the DataReceivedEventArgs object.