public  static class DBHelper
    {
        private static SqlConnection connection;
   //private static SqlDataAdapter da;
        public static SqlConnection Connection
        {
            get
            {
                string connectionString = ConfigurationManager.ConnectionStrings["XCgo"].ConnectionString;
                if (connection == null)
                {
                    connection = new SqlConnection(connectionString);
                    connection.Open();
                }
                else if (connection.State == System.Data.ConnectionState.Closed)
                {
                    connection.Open();
                }
                else if (connection.State == System.Data.ConnectionState.Broken)
                {
                    connection.Close();
                    connection.Open();
                }
                return connection;
            }
        }
      public static DataTable GetDataTable(string safeSql)
        {
            DataSet ds = new DataSet();
            SqlCommand cmd = new SqlCommand(safeSql, Connection);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            cmd.Dispose();

                da.Fill(ds);//异常出在这里，提示There is already an open DataReader associated with this Command which must be closed first. 


      
            da.Dispose();
          
            connection.Close();
            return ds.Tables[0];
        }
}