问C# SqlDataReader = null？

EN

            String sqlCheckPass = 
"Select * from Login where Username like @Username and Password like @Password";
        SqlCommand SqlCom = new SqlCommand(sqlCheckPass, myConnection);
        SqlCom.Parameters.Add(new SqlParameter("@Username", sUserName));
        SqlCom.Parameters.Add(new SqlParameter("@Password", sPassword));

        myConnection.Open();
        SqlDataReader myreader;
        myreader = SqlCom.ExecuteReader();
        int id = -1;

ErrorBox.InnerHtml = "Username:" + sUserName + ":" + sPassword + ":<br/>";
while (myreader.HasRows)
{
    id = (int)myreader["id"];
    String sUser = (String)myreader["Username"];
    String sPass = (String)myreader["Password"];
    ErrorBox.InnerHtml += "UserId is <b>" + id + "</b> " + sUser + ":" + sPass + ":<br >";
    Session["LoginID"] = id;
    Server.Transfer(ReturnPage);

}
if (id == -1)
{
    ErrorBox.InnerHtml = "Incorrect Password";
}
myConnection.Close();
catch (Exception err)
{
    ErrorBox.InnerHtml = "Error Getting  Option ID" + err.Message;
}