连接数据库判断用户是否存在

Question

 protected void Page_Load(object sender, EventArgs e)
    {
        string username = txtname.Text;
        string userpass = txtpass.Text;
    }
    protected void commit_Click(object sender, EventArgs e)
    {
       
        if (radteacher.Checked == false && radstudent.Checked == false && radvisiter.Checked == false)
        {
            Response.Write("请选择您的角色！");
        }
        if (radteacher.Checked == true)
        {
            //OleDbConnection strconn = new OleDbConnection();
            //string conn = Convert.ToString(strconn);
            string conn =@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=F:\ASP.NET\lab06\App_Data\teacher.mdb;Persist Security Info=True";
            
            OleDbConnection dbconn = new OleDbConnection(conn);
            //string sql = "select count(*) from Teacher where t_gh='{0}'";          
            OleDbCommand dbCmd = new OleDbCommand();
            dbCmd.CommandType = CommandType.Text;
            dbCmd.CommandText = "select * from Teacher where t_gh='username' and t_pass='userpass'";
            dbconn.Open();
            OleDbDataReader dbRead = dbCmd.ExecuteReader();
 
            if (dbRead.Read())
            {
                Response.Write("logsuccess.aspx");
            }
            else
            {
                Response.Write("此用户不存在，请输入您的工号！");
            }            
        }
        if (radstudent.Checked == true)
        {
            //OleDbConnection strconn1 = new OleDbConnection();
            //string conn1 = Convert.ToString(strconn1);
            string conn1 =@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=F:\ASP.NET\lab06\App_Data\student.mdb;Persist Security Info=True";
            
            OleDbConnection dbconn1= new OleDbConnection(conn1);
            //string sql1 = "select count(*) from student where s_xh='{0}'";
            OleDbCommand dbCmd1 = new OleDbCommand();
            dbCmd1.CommandType = CommandType.Text;
            dbCmd1.CommandText = "select * from student where s_xh='username' and s_pass='userpass'";
            dbconn1.Open();
            OleDbDataReader dbRead1 = dbCmd1.ExecuteReader();
            if (dbRead1.Read())
            {
                Response.Write("logsuccess.aspx");
            }
            else
            {
                Response.Write("此用户不存在，请输入您的学号！");            }
        }
        if (radvisiter.Checked == true)
        {
            //OleDbConnection strconn2 = new OleDbConnection();
            //string conn2 = Convert.ToString(strconn2);
            string conn2 =@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=F:\ASP.NET\lab06\App_Data\Users.mdb;Persist Security Info=True";            OleDbConnection dbconn2 = new OleDbConnection(conn2);
            //string sql2 = "select count(*) from student where s_xh='{0}'";
            OleDbCommand dbCmd2 = new OleDbCommand();
            dbCmd2.CommandType = CommandType.Text;
            dbCmd2.CommandText = "select * from Users where U_name='username' and U_pass='userpass'";
            dbconn2.Open();
            OleDbDataReader dbRead2 = dbCmd2.ExecuteReader();
            if (dbRead2.Read())
            {
                Response.Write("logsuccess.aspx");
            }
            else
            {
                Response.Write("此用户不存在，请输入您的学号！");            }
        }
    }
========================================================================================================================
小鸟一只，是在不会修改Connection未实例化的错误了，望大虾赐教，谢谢！

Answer 1

OleDbCommand dbCmd1 = new OleDbCommand(); 
dbCmd1.CommandType = CommandType.Text; 这里要加一个东西dbCmd1.Connection = dbconn;