I am currently trying to handle the same event handler for multiple richTextBox controls.
However, you do not know which control caused the event.

When right-clicking a rich text box
I am coding to call the context menu and process it.

Because it is called in the order of [Rich Text Box] → [Context Menu] → [Process],
I don't think it can be confirmed with the following code.

private void hoge_contextMenuStrip_Opening (object sender, CancelEventArgs e)
        {
            Console.WriteLine ((sender as ContextMenuStrip) .Name);
        }