C#我用Visual Studio 2008编了一个Windows登陆窗体，建了一个账号密码的数据库，当点登陆时怎么连接数据库

在此假设你用的数据库为SQL Server数据库，数据库名为“Test”，用户表名为“Users”，表结构为Users（UserName，Password），代码如下：

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

//引用命名空间

using System.Data.SqlClient;

namespace LoginTest

{

    public partial class LoginForm : Form

    {

        public LoginForm()

        {

            InitializeComponent();

        }

        private void btnLogin_Click(object sender, EventArgs e)

        {

            //创建Connection对象

            SqlConnection con = new SqlConnection("server=.;database=Test;uid=sa;pwd=");

            //创建Command对象

            SqlCommand cmd = new SqlCommand(string.Format("SELECT COUNT(*) FROM Users WHERE UserName='{0}' AND Password='{1}'",

                this.txtUserName.Text.Trim(), this.txtPwd.Text.Trim()), con);

            try

            {

                //打开数据库连接

                con.Open();

                //如果执行cmd,返回值为1,提示用户登录成功,否则提示登录失败

                if ((int)cmd.ExecuteScalar() == 1)

                {

                    MessageBox.Show("登录成功!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);

                }

                else

                {

                    MessageBox.Show("登录失败!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);

                }

            }

            catch (Exception ex)

            {

                //如果捕获到异常,提示用户数据库操作失败

                MessageBox.Show("数据库操作失败!" + ex.Message, "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);

            }

            finally

            { 

                //关闭数据库连接

                con.Close();

            }

        }

    }

}

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

public partial class admin_login : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}
protected void ImageButton1_Click(object sender, ImageClickEventArgs e)//登录按钮
{
string username = txtname.Text.Trim();
string pwd = txtpwd.Text.Trim();

string sql = "select * from Admin where Username='" + username + "' and Pwd='" + pwd + "' and Type=0";
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["LinXuan"].ConnectionString);
con.Open();//打开连接
SqlCommand cmd = new SqlCommand(sql, con);
SqlDataReader dr = cmd.ExecuteReader();//执行命令
if (dr.Read())
{
Session["myadmin"] = username;//保存登录名
Response.Redirect("manage.aspx", true);//登录成功跳转页面
}
else
{
Response.Write("");
}
dr.Close();
con.Close();
}
}

应该可以的
using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Web.Configuration;
using System.Data.SqlClient;

public partial class _Default : System.Web.UI.Page
{

protected void btnlogin_Click(object sender, EventArgs e)
{//获取用户输入
string username = txtname.Text.ToString();
string password = txtpwd.Text.ToString();
//判断用户是否输入用户名和密码
if (username == "" | password == "")
{
lblerr.Visible = true;
lblerr.Text = "请输入用户名和密码";
}
else
{//从配置文件中读取字符串
string settings = Convert.ToString(ConfigurationManager.ConnectionStrings["MySqlConnection"]);
//测试字符串是否正确
SqlConnection myconn = new SqlConnection(settings);
//打开数据库连接
myconn.Open();
//创建sql语句
string mysql = "select * from Employee where empName=" + "'"
+ username + "'" + "and empPwd=" + "'" + password + "'";
//创建命令对象
SqlCommand mycmd = new SqlCommand(mysql, myconn);
//执行命令，结果传递给适配器
SqlDataReader mydr = mycmd.ExecuteReader();
try
{
if (mydr.Read())
{//身份验证成功

lblerr.Visible = true;
lblerr.Text = "登录成功!";
}
else
{//身份验证失败
lblerr.Visible = true;
lblerr.Text = "登录失败!";
}
}
finally
{ //关闭连接
mydr.Close();
myconn.Close();
}
}
}
protected void btnclear_Click(object sender, EventArgs e)
{
txtname.Text = "";
txtpwd .Text= "";

}
protected void Page_Load(object sender, EventArgs e)
{
lblerr.Visible = false;
}
}