using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Configuration;
using System.Data.SqlClient;

namespace aa
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (txtName.Text == "")
            {
                MessageBox.Show("请输入用户名", "警告", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            }
            else
            {
                if (txtPassword.Text == "")
                {
                    MessageBox.Show("请输入密码", "警告", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                }
                else
                {
                    SqlConnection conn = new SqlConnection("server=.;Initial Catalog=ClassOnline;User ID=sa;Password=sa");
                    conn.Open();
                    SqlCommand cmd = new SqlCommand("select * from User where UserName=" + txtName.Text + "and UserPwd=" + txtPassword.Text + "", conn);
                    SqlDataReader sdr = cmd.ExecuteReader();
                    sdr.Read();
                    if (sdr.HasRows)
                    {
                        Form3 frm3 = new Form3();
                        frm3.Show();
                        this.Hide();
                    }
                    else
                    {
                        MessageBox.Show("用户名或密码错误");
                    }
                }
            }
        }
               
        private void button2_Click(object sender, EventArgs e)
        {
            Form2 frm2 = new Form2();
            frm2.Show();
            this.Hide();
        }

        private void button3_Click(object sender, EventArgs e)
        {
            txtName.Text = "";
            txtPassword.Text = "";
        }
    }
}
上面说 conn.Open();！未处理SqlException
         {在与 SQL Server 建立连接时出现与网络相关的或特定于实例的错误。未找到或无法访问服务器。请验证实例名称是否正确并且 SQL Server 已配置为允许远程连接。 (provider: Named Pipes Provider, error: 40 - 无法打开到 SQL Server 的连接)}
为什么会出现这种情况？求各位大神指导！！！

SqlConnection conn = new SqlConnection("server=.;Initial Catalog=ClassOnline;User ID=sa;Password=sa");
可能是这里错了,
你的数据库是express版的么,
试试这个
SqlConnection conn = new SqlConnection("server=.\sqlexpress;Initial Catalog=ClassOnline;User ID=sa;Password=sa");

我改后，出现了错误：无法识别的转义序列。

\这个是转义字符，写成SqlConnection conn = new SqlConnection("server=.\\sqlexpress;Initial Catalog=ClassOnline;User ID=sa;Password=sa");
或者SqlConnection conn = new SqlConnection(@"server=.\sqlexpress;Initial Catalog=ClassOnline;User ID=sa;Password=sa"); 你再试试

嗯，这样就可以了。谢谢了。
对了，还有一处错误： SqlDataReader sdr = cmd.ExecuteReader();关键字 'User' 附近有语法错误。还得麻烦一下各位大神们！！请多多指导

[ 本帖最后由 王祖品 于 2013-4-26 17:50 编辑 ]

sql接字符串要有'',你没用'';你用下边的试试再
SqlCommand cmd = new SqlCommand("select * from User where UserName='" + txtName.Text + "'and UserPwd='" + txtPassword.Text + "'", conn);