文本框复制问题？？

先上代码       protected override void OnLoad(EventArgs e)
        {
            base.OnLoad(e);
            MenuItem CopySelection = new MenuItem("复制选中");
            MenuItem CopyAll = new MenuItem("复制全部");
            CopySelection.Click += new EventHandler(CopySelection_Click);
            CopyAll.Click += new EventHandler(CopyAll_Click);
            ContextMenu common_Menu = new ContextMenu(new MenuItem[] { CopySelection, CopyAll });
            richTextBox1.ContextMenu = common_Menu;
            richTextBox2.ContextMenu = common_Menu;
            richTextBox3.ContextMenu = common_Menu;        }        private void CopySelection_Click(object sender, EventArgs e)
        {
            if (this.tabControl1.SelectedIndex == 0)
                this.richTextBox1.Copy();
            else if (this.tabControl1.SelectedIndex == 1)
                this.richTextBox2.Copy();
            else if (this.tabControl1.SelectedIndex == 3)
                this.richTextBox3.Copy();            
        }        private void CopyAll_Click(object sender, EventArgs e)
        {
            if (this.tabControl1.SelectedIndex == 0)
            {
                this.richTextBox1.SelectAll();
                this.richTextBox1.Copy();
            }
            else if (this.tabControl1.SelectedIndex == 1)
            {
                this.richTextBox2.SelectAll();
                this.richTextBox2.Copy();
            }
            else if (this.tabControl1.SelectedIndex == 3)
            {
                this.richTextBox3.SelectAll();
                this.richTextBox3.Copy();
            }
        }以上能实现“复制选中”和“复制全部”的功能，我想完善下：当文本框为空时，“复制选中”和“复制全部”全灰显；            当文本框不为空时，若没选中任何字，“复制选中”灰显；            其他情况都不灰显。该怎么做？？