textBox框像百度搜索出现下拉列表的样式，如图：
重点就是要关注：AutoCompleteMode和AutoCompleteSource两种属性。
具体实现代码如下：
将此方法在frmLogin_Load事件里加载，
private void bindTxt()
{
txtUser.AutoCompleteCustomSource = null;
User user = new User();
List<UserInfo> userList = user.GetUserList();
if (userList.Count > 0)
{
foreach (UserInfo var in userList)
{
txtUser.AutoCompleteCustomSource.Add(var.UserId);
}
}
txtUser.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
txtUser.AutoCompleteSource = AutoCompleteSource.CustomSource;
}