导出为pdf时，它会给出缺少参数值的错误

Question

请告诉我为什么会出现这些错误，但是在上层回发代码中，当导出异常时，它会显示参数值：

var _exportOptions = _reportDocument.ExportOptions;

            _diskFileDestinationOptions.DiskFileName = obj.File_Storage_Path;
            _exportOptions.ExportDestinationType = ExportDestinationType.DiskFile;
            _exportOptions.ExportFormatType = exportFormat;
            _exportOptions.ExportDestinationOptions = _diskFileDestinationOptions;
            _exportOptions.ExportFormatOptions = _reportFormatOptions;
           // _exportOptions.SetParameterValue("Parameter1", Convert.ToString(Session["myname"]));
            _reportDocument.Export();

            fileToOpen = obj.File_To_Open;
           // filePath = obj.File_Storage_Path;
        }
        catch (Exception ex)
        {
            throw new Exception("Error as " + ex.Message);
        }
        finally
        {
            _reportDocument.Close();                
        }
        return fileToOpen;