winfrom动态生成的combox控件 绑定数据源之后 无法设置默认值问题

原代码：

 List<student> ls = new List<student>();
            student st = null;
            for (int i = 0; i < 10; i++)
            {
                st = new student();
                st.Id = i.ToString();
                st.Name = "zj"+i;
                ls.Add(st);
            }
            cl = new ComboBox();
            ((ComboBox)cl).DataSource = ls;
            ((ComboBox)cl).ValueMember = "id";
            ((ComboBox)cl).DisplayMember = "name";
            (cl as ComboBox).SelectedIndex = 7;
            this.Controls.Add(cl);

// 这样写之后 当程序运行到   (cl as ComboBox).SelectedIndex = 7;时报错。跟踪下cl为空。明明已经绑定了，怎么还是为空呢。

改进之后：

     List<student> ls = new List<student>();
            student st = null;
            for (int i = 0; i < 10; i++)
            {
                st = new student();
                st.Id = i.ToString();
                st.Name = "zj"+i;
                ls.Add(st);
            }
            cl = new ComboBox();
            ((ComboBox)cl).DataSource = ls;
            ((ComboBox)cl).ValueMember = "id";
            ((ComboBox)cl).DisplayMember = "name";
            this.Controls.Add(cl);
            (cl as ComboBox).SelectedIndex = 7;

//仔细观察，把   (cl as ComboBox).SelectedIndex = 7;放到 this.Controls.Add(cl);之后就可以了，结果运行正常。