DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(xxx));
通过FindByValue没有找到指定项返回值为null,而Items.IndexOf(null)会返回-1
DropDownList1.SelectedIndex = DropDownList1.Items.IndexOf(DropDownList1.Items.FindByValue(xxx));
通过FindByValue没有找到指定项返回值为null,而Items.IndexOf(null)会返回-1