---
title: "Implementing a DataReader Class for a Data Processing Extension | Microsoft Docs"
ms.date: 03/06/2017
ms.prod: reporting-services
ms.prod_service: "reporting-services-native"
ms.technology: extensions


ms.topic: reference
helpviewer_keywords: 
  - "data processing extensions [Reporting Services], data readers"
  - "data readers [Reporting Services]"
  - "DataReader class"
  - "read-only data"
ms.assetid: 23e286e7-6074-4fbe-be29-203420d6c3d0
author: maggiesMSFT
ms.author: maggies
---
# Implementing a DataReader Class for a Data Processing Extension
  The **DataReader** object enables a client to retrieve a read-only, forward-only stream of data from a data source. Results are returned as the query executes and are stored in the network buffer on the client until you request them using the **Read** method of the **DataReader** class. To create a **DataReader** class, implement <xref:Microsoft.ReportingServices.DataProcessing.IDataReader> and optionally implement <xref:Microsoft.ReportingServices.DataProcessing.IDataReaderExtension>. Using a **DataReader** object increases application performance both by retrieving data as soon as it is available, rather than waiting for the entire results of the query to be returned, and (by default) storing only one row at a time in memory, reducing system overhead.  
  
 After creating an instance of your **Command** class, you create a **DataReader** object by calling **Command.ExecuteReader** to retrieve rows from the data source. The **DataReader** implementation must provide two basic capabilities: forward-only access over the result sets obtained by executing a command and access to the column types, names, and values within each row. Clients use the **Read** method of the **DataReader** object to obtain a row from the results of the query.  
  
 In Report Designer, your **DataReader** object is used to retrieve a list of fields as well as schema information about the result set. This is accomplished by implementing the **GetName**, **GetValue**, **GetFieldType,** and **GetOrdinal** methods of the <xref:Microsoft.ReportingServices.DataProcessing.IDataReader> interface.  
  
 The <xref:Microsoft.ReportingServices.DataProcessing.IDataReaderExtension> interface allows you to supply specific aggregation information about your result set. For a sample **DataReader** class implementation, see [SQL Server Reporting Services Product Samples](https://go.microsoft.com/fwlink/?LinkId=177889).  
  
## See Also  
 [Reporting Services Extensions](../../../reporting-services/extensions/reporting-services-extensions.md)   
 [Implementing a Data Processing Extension](../../../reporting-services/extensions/data-processing/implementing-a-data-processing-extension.md)   
 [Reporting Services Extension Library](../../../reporting-services/extensions/reporting-services-extension-library.md)