processing large text in C#
public void Process()
        {

               string[] lines;
                string line;
                string foo = " baa";
                if (richTextBox1.InvokeRequired)
                {

                    lines = (string[])Invoke((ReadLines)delegate
                    {
                        return /* read .Lines[] from richTextBox1 */
                    });
                    int max = lines.Length;

                    for (int i = 0; i < max; i++)
                    {
                        line = lines[i];
                        if (..)
                        {
                            lines[i] += "foo";
                        }
                    }

                    Invoke((Update)delegate
                    {
                        /* set new lines to RichTextBox1 */
                    });

              }
       }

Thread th1 = new Thread(Process);
 th1.Start();