private void button1_Click(object sender, EventArgs e)
        {
            if (txtUserName.Text.Trim() == "" || txtUserPassword.Text.Trim == "")
            {
                MessageBox.Show("用户名或密码不能为空！");

                txtUserName.Focus();
                return;
            
            
            }

            string connString = @"Data Source=windows-PC;Initial Catalog=AddressList;Integrated Security=True";
            string sqlStr = string.Format("select count(*) from [User] where UserName='{0}' and Password='{1}'",txtUserName.Text.Trim,txtUserPassword.Text.Trim());
            using (SqlConnection conn = new SqlConnection(connString))
            {
                SqlCommand amd = new SqlCommand(sqlStr, conn);
                conn.Open();
                int  n=Convert.ToInt32(cmd.ExecuteScalar());
                if (n == 1)
                {
                    MessageBox.Show("登录成功！");


                }
                else
                {
                    MessageBox.Show("用户名或密码错误！","错误");
                    txtUserName.Text = "";
                    txtUserPassword.Text = "";
                    txtUserName.Focus();
               
               
                 }

if (txtUserName.Text.Trim() == "" || txtUserPassword.Text.Trim == "")
            {
                MessageBox.Show("用户名或密码不能为空！");
                //注释掉吧.提示就没有这个名叫:txtUserName的textbox
               // txtUserName.Focus();
                return;
            
            
            }

肯定是你最初的时候名命名的，然后删掉了，可是这样应该提示（下面有红线啊），何况这行代码没有什么意义   删了吧

新手路过.楼主加油！！