This question has just come up on the .NetCF newsgroup. P/Invoke is again to the rescue:

public void SaveTextBox(TextBox textBox, string fileName)

{

      StringBuilder buffer = new StringBuilder(" ", 255);

      // Get TextBox's native handle

      textBox.Capture = true;

      IntPtr handle = GetCapture();

      textBox.Capture = false;

      // The EM_GETLINE requires the length of the buffer in the first byte.

      buffer[0] = (char)255;

      using(StreamWriter writer = new StreamWriter(fileName))

      {

            // Get line count

            int lineCount = SendMessage(handle, EM_GETLINECOUNT, 0, 0);

            for(int i=0;i<lineCount;i++)

            {

                  SendMessage(handle, EM_GETLINE, i, buffer);

                  writer.WriteLine(buffer.ToString());

            }

            writer.Flush();

      }

}

Required P/Invoke declarations:

const int EM_GETLINECOUNT    =  0x00BA;

const int EM_GETLINE         =  0x00C4;

[DllImport("coredll.dll")]

static extern IntPtr GetCapture();

[DllImport("coredll.dll")]

static extern int SendMessage(IntPtr hwnd, int msg, int wParam, int lParam);

[DllImport("coredll.dll")]

static extern int SendMessage(IntPtr hwnd, int msg, int wParam, StringBuilder lParam);