Exception:There Is An Open DataReader Associated With This Command Which Must Be Closed First.

the class code:

Dataase.cs:
using System;
using System.Data;
using System.Configuration;
using System.Linq;
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.Xml.Linq;
using System.Data.SqlClient;
using System.Data.Common;
using System.Web;

/// <summary>
/// Summary description for DataBase
/// </summary>
public class DataBase
{
private SqlConnection con=new SqlConnection();

private void Open()
{
if (con==null)
{
con = new SqlConnection("Data Source=58.17.30.81;Initial Catalog=a1230192748;Persist Security Info=True;User ID=a1230192748;Password=***");
}
if (con.State == System.Data.ConnectionState.Closed)
{
con.ConnectionString = "Data Source=58.17.30.81;Initial Catalog=a1230192748;Persist Security Info=True;User ID=a1230192748;Password=****";
con.Open();
}
}
public void Close()
{
if (con != null && con.State != System.Data.ConnectionState.Open)
con.Close();
}
public DataBase()
{
//
// TODO: Add constructor logic here
//
}

public string liuyan(string id,string sign)
{
string com=string.Empty;
switch(sign)
{
case "xiaobiaoti":
com="Select subject from liuyan where liuyanid='"+id+"'";
break;
case "def_message":
com="Select message from liuyan where liuyanid='"+id+"'";
break;
}

SqlCommand myCommand=new SqlCommand(com,con);
Open();
try
{
SqlDataReader sdr=myCommand.ExecuteReader();
if (sdr.Read())
{
return sdr[0].ToString();
}
else
{
return "";
}

sdr.Close();    //what i have written.

}
catch (Exception ex)
{
HttpContext.Current.Response.Write("<script>alert('error:" + ex.Message + "')</script>");
return "";
}
finally
{
myCommand.Dispose();
Close();
}
}

}


 

it was instantiated once in  aspx.cs code.I invoke liuyan(string id,string sign) twice.The first one is OK and the second one makes an exception.