winform中textbox提示框

在winform中向textbox输入内容时下面有提示信息，效果如图所示：

private void Form1_Load(object sender, EventArgs e){    AutoCompleteStringCollection strings = new AutoCompleteStringCollection();    strings.Add("A1");//不区分大小写    strings.Add("A2");    strings.Add("Aas");    strings.Add("B1");    strings.Add("Beijing");    strings.Add("bb-cims");    textBox1.AutoCompleteCustomSource = strings;    textBox1.AutoCompleteSource = AutoCompleteSource.CustomSource;    textBox1.AutoCompleteMode = AutoCompleteMode.Suggest;}