cmbSelect.SelectedIndex

（cmbSelect）为自定义的下拉框name；cmbSelect.SelectedIndex表示下拉框内集合的序号，由0开始计起。

例如：

int i = cmbSelect.SelectedIndex;
                switch (i)
                {
                    case 0:
                        sql1 = "select * from Student where sno='" + texSelect.Text.Trim() + "'";
                        break;
                    case 1:
                        sql1 = "select * from Student where sname like   '%" + texSelect.Text.Trim() + "'";
                        break;
                    case 2:
                        sql1 = "select * from Student where sex like   '%" + texSelect.Text.Trim() + "'";
                        break;

   自己真的是好菜啊，呃啊啊啊啊。。。