已有打开的与此命令相关联的 DataReader,必须首先将它关闭。解决办法

这是我的代码，我在网上找了很久，也知道它错的原因，但我还是不能改好它。还希望帮我优化一下我的代码。谢谢...        string cnsql = "server=.;database=wuliu;Integrated Security=SSPI";        SqlConnection mycon = new SqlConnection(cnsql);        mycon.Open();        string str = "select * from dingdan where  ID = '" + Session ["danhao"] + "'";        SqlCommand cmd = new SqlCommand();        cmd.Connection = mycon;        cmd.CommandText = "select * from dingdan where  ID = '" + Session["danhao"] + "'";        SqlDataReader dr = cmd.ExecuteReader();        if (dr.Read())        {            SqlDataAdapter sda = new SqlDataAdapter(str, mycon);            DataSet ds = new DataSet();            sda.Fill(ds, "mytb");            DataView dv = ds.Tables["mytb"].DefaultView;            GridView1.DataSource = dv;            GridView1.DataBind();            dr.Close();        }        else {            Response.Write("<script language='javascript'> alert('查询不到你的订单号，请电我们公司电话核实') ;window.location.href = 'index.aspx';</script>");        }        mycon.Close();

        string cnsql = "server=.;database=wuliu;Integrated Security=SSPI";        SqlConnection mycon = new SqlConnection(cnsql);        mycon.Open();        string str = "select * from dingdan where  ID = '" + Session ["danhao"] + "'";        SqlDataAdapter sda = new SqlDataAdapter(str, mycon);        DataSet ds = new DataSet();        sda.Fill(ds, "mytb");        if(ds.Tables["mytb"].Rows.Count>0)        {            DataView dv = ds.Tables["mytb"].DefaultView;            GridView1.DataSource = dv;            GridView1.DataBind();            dr.Close();        }                else {            Response.Write("<script language='javascript'> alert('查询不到你的订单号，请电我们公司电话核实') ;window.location.href = 'index.aspx';</script>");        }        mycon.Close();