用户登录经典代码

private void btnLogin_ServerClick(object sender, System.EventArgs e)
        {
            SqlConnection con;
            string sql;
            SqlCommand cmd;
            string id;

            con = new SqlConnection("data source=(local)\\NetSdk;initial catalog=FriendsData;user id=sa");
            sql = "SELECT UserID FROM [User] WHERE Login='{0}' and Password='{1}'";
            
            // Format the string with the values provided
            sql = String.Format(sql, txtLogin.Value, txtPwd.Value);
            cmd = new SqlCommand(sql, con);
            con.Open();

            try 
            {
                // Retrieve the UserID
                id = (string) cmd.ExecuteScalar();
            }
            finally 
            {
                con.Close();
            }

            if (id != null)
            {
                // Set the user as authenticated and send him to the page originally requested.
                FormsAuthentication.RedirectFromLoginPage(id, chkPersist.Checked);
            }
            else
            {
                this.pnlError.Visible = true;
                this.lblError.Text = "Invalid user name or password!";
            }
        }