You can listen to the PreviewKeyDown
event like:
<RichTextBox PreviewKeyDown="RichTextBox_PreviewKeyDown"/>
and in the handler:
private void RichTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
{
if (e.Key == Key.Return)
{
// DO YOUR WORK HERE and then set e.Handled to true on condition if you want to stop going to next line//
e.Handled = true;
}
}