// コピペ用ショートカットメニューの表示
private void openContextMenu(TextBox txt, double posX, double posY)
{
    ContextMenu cm = new ContextMenu();

    var miCut = new MenuItem()
    {
        Header = "切り取り"
    };
    miCut.Click += (s, e) =>
    {
        txt.Focus();
        Clipboard.SetText(txt.SelectedText);
        txt.SelectedText = "";
    };
    cm.Items.Add(miCut);

    var miCopy = new MenuItem()
    {
        Header = "コピー"
    };
    miCopy.Click += (s, e) =>
    {
        txt.Focus();
        Clipboard.SetText(txt.SelectedText);
    };
    cm.Items.Add(miCopy);

    var miPaste = new MenuItem()
    {
        Header = "貼り付け"
    };
    miPaste.Click += (s, e) =>
    {
        txt.Focus();
        txt.SelectedText = Clipboard.GetText();
    };
    cm.Items.Add(miPaste);

    if (txt.SelectedText == "")
    {
        miCut.IsEnabled = false;
        miCopy.IsEnabled = false;
    }

    cm.HorizontalOffset = posX;
    cm.VerticalOffset = posY;
    cm.IsOpen = true;
}

// 利用例
private void TextBox1_MouseRightButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
    e.Handled = true;
    openContextMenu((TextBox)sender, e.GetPosition(owner).X, e.GetPosition(owner).Y);
}