Enable copy, cut, past window in a rich text box

If you have more than one RichTextBox then you can use this extension method:

public static void AddContextMenu(this RichTextBox rtb)
{
    if (rtb.ContextMenuStrip == null)
    {
        ContextMenuStrip cms = new ContextMenuStrip()
        {
            ShowImageMargin = false
        };

        ToolStripMenuItem tsmiUndo = new ToolStripMenuItem("Undo");
        tsmiUndo.Click += (sender, e) => rtb.Undo();
        cms.Items.Add(tsmiUndo);

        ToolStripMenuItem tsmiRedo = new ToolStripMenuItem("Redo");
        tsmiRedo.Click += (sender, e) => rtb.Redo();
        cms.Items.Add(tsmiRedo);

        cms.Items.Add(new ToolStripSeparator());

        ToolStripMenuItem tsmiCut = new ToolStripMenuItem("Cut");
        tsmiCut.Click += (sender, e) => rtb.Cut();
        cms.Items.Add(tsmiCut);

        ToolStripMenuItem tsmiCopy = new ToolStripMenuItem("Copy");
        tsmiCopy.Click += (sender, e) => rtb.Copy();
        cms.Items.Add(tsmiCopy);

        ToolStripMenuItem tsmiPaste = new ToolStripMenuItem("Paste");
        tsmiPaste.Click += (sender, e) => rtb.Paste();
        cms.Items.Add(tsmiPaste);

        ToolStripMenuItem tsmiDelete = new ToolStripMenuItem("Delete");
        tsmiDelete.Click += (sender, e) => rtb.SelectedText = "";
        cms.Items.Add(tsmiDelete);

        cms.Items.Add(new ToolStripSeparator());

        ToolStripMenuItem tsmiSelectAll = new ToolStripMenuItem("Select All");
        tsmiSelectAll.Click += (sender, e) => rtb.SelectAll();
        cms.Items.Add(tsmiSelectAll);

        cms.Opening += (sender, e) =>
        {
            tsmiUndo.Enabled = !rtb.ReadOnly && rtb.CanUndo;
            tsmiRedo.Enabled = !rtb.ReadOnly && rtb.CanRedo;
            tsmiCut.Enabled = !rtb.ReadOnly && rtb.SelectionLength > 0;
            tsmiCopy.Enabled = rtb.SelectionLength > 0;
            tsmiPaste.Enabled = !rtb.ReadOnly && Clipboard.ContainsText();
            tsmiDelete.Enabled = !rtb.ReadOnly && rtb.SelectionLength > 0;
            tsmiSelectAll.Enabled = rtb.TextLength > 0 && rtb.SelectionLength < rtb.TextLength;
        };

        rtb.ContextMenuStrip = cms;
    }
}

And use it like this: richTextBox1.AddContextMenu();

Screenshot: