gistfile1.cs

private void textControl1_KeyPress(object sender, KeyPressEventArgs e)
{
    switch (Convert.ToInt32(e.KeyChar))
    {
        // Enter key is pressed
        case 13:

            // get the currently active TextPart
            IFormattedText textPart =
              (IFormattedText)textControl1.TextParts.GetItem();

            // get the current line
            TXTextControl.Line line =
              textPart.Lines.GetItem(textPart.Selection.Start);

            // remove the numbered list format type, if the line is empty
            if (line.Text == "" || line.Text == "\r\n")
            {
                if (textPart.Selection.ListFormat.Type != 
                    TXTextControl.ListType.None)
                {
                    textPart.Selection.ListFormat.Type =
                        TXTextControl.ListType.None;
                }
            }

            break;
    }
}