无效操作，连接被关闭

        public DataTable GetExcelTable(string fileName)
        {
            string cmdText = "SELECT * FROM [Sheet1$]";
            DataTable tmp = new DataTable();
            OleDbDataAdapter da = new OleDbDataAdapter(cmdText, getExcelOleDbConnectionString(fileName));
            da.Fill(tmp);
            return this.FilterTable(tmp);
        }
        private DataTable FilterTable(DataTable dt)
        {
            //去除空行
            for (int i = dt.Rows.Count - 1; i >= 0; i--)
            {
                bool isNull = true;
                foreach (DataColumn col in dt.Columns)
                {
                    if (!(dt.Rows[i][col] is DBNull) && dt.Rows[i][col].ToString().Trim() != string.Empty)
                    {
                        isNull = false;
                        break;
                    }
                }
                if (isNull)
                    dt.Rows.RemoveAt(i);
            }
            return dt;
        }
        private string getExcelOleDbConnectionString(string fileName)
        {
            if (fileName.ToLower().EndsWith(".xls"))
                return "provider=Microsoft.Jet.OLEDB.4.0" +
                       ";data source=" + fileName +
                       ";Extended Properties=\"Excel 8.0;\"";
            else
                return "provider=Microsoft.ACE.OLEDB.12.0" +
                       ";data source=" + fileName +
                       ";Extended Properties=\"Excel 12.0 xml;\"";
        }
我想通过Excel将数据导入到数据库中，运行时总报“无效操作，连接被关闭”，还有“ServerVersion = “da.ServerVersion”引发了“System.InvalidOperationException”类型的异常”，麻烦各位大虾帮忙看看啊，小弟都快急死了！