asp.net数据库问题

写了一个登录界面的登录按钮代码，但是总是有错，请大家指正一下，谢谢。
代码如下：

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 denglu : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {

            SqlConnection conn = new SqlConnection("server=.;database=user1;uid=sa;pwd=;");

            SqlCommand cmd = conn.CreateCommand();

            string sql = "select count(*) from user1 where 用户名='" + TextBox1.Text + "' and 密码='" + TextBox2.Text + "'";

             = sql;

            conn.Open();

            int count = (int)cmd.ExecuteScalar();

            if (count == 1)
            {
                Response.Redirect("./web/index.aspx");
            }
            else
            {
                Response.Write("<script>alert('你输入的用户名或密码错误！');</script>");
            }

            conn.Close();
        
    }
    protected void Button3_Click(object sender, EventArgs e)
    {
        Response.Redirect("login.aspx");
    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        TextBox1.Text = "";
        TextBox2.Text = "";
    }
}

运行后出现错误如下：