textBox框像百度搜索出现下拉列表的样式，如图：

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;
        }