Parts Overview

This topic describes the basics of using Packaging APIs to interact with parts.

This topic contains the following sections.


In the analogy of a package to a real world filing system, the parts in a package are like the folders in a filing cabinet. Parts store application data, like folders store personal or business information. To further extend the analogy, the content of a part is data in the form of a byte stream like the content of a folder is information in the form of paperwork.

In the logical model of a package, parts are resources that are URI-addressable data components inside a package. A part consists of a byte stream of data and properties which are specified in the ECMA-376 OpenXML, 1st Edition, Part 2: Open Packaging Conventions (OPC).

To interact with a specific part, the caller uses the Packaging APIs to get a pointer to a part object that represents the part (a conceptual file-folder). All interactions are performed using this part object.


For a table of prerequisites, see Packaging.

Part Objects

Windows 7 provides the part object implementation of the IOpcPart interface.

A part object is instantiated to represent a part. The object is created by calling the IOpcPartSet::CreatePart method and accessed by calling the IOpcPartSet::GetPart or IOpcPartEnumerator::GetCurrent method.

A part object is contained in a part set object that implements the IOpcPartSet interface. This set can be retrieved by calling the IOpcPackage::GetPartSet method, and it represents all the parts that are not Relationships parts in a package. In addition to providing support for creating part objects, the set also provides support for deleting part objects and for getting an enumerator of the part objects contained in the set. For more information about part set objects, see the IOpcPartSet topic.

A pointer to the IOpcPart interface of a part object provides access to part information and an IOpcRelationshipSet interface. An IOpcPart provides access to part information through methods that allow access to the following properties: content, content type, name, and compression option. These properties are listed in the table that follows.

Property Description
content The stream of bytes that makes up a part, as it is defined in the OPC.
content type The media type, specified by the format designer, of part content.
name The URI to a part in the package, relative to the package root.
compression The degree to which the part content is compressed.

The part name is represented by a part URI object, which implements the IOpcPartUri interface. The following diagram shows a part object and the part URI and relationship set objects that can be retrieved from it.

illustration showing a part object and the part uri and relationship set objects that can be retrieved from it

The relationship set object retrieved with the IOpcPart::GetRelationshipSet represents the Relationships part that stores relationships that have the current part as their source. The object is an unordered set of IOpcRelationship interface pointers to relationship objects. Each relationship object represents one part relationship. For more information about relationship objects, see the Relationships Overview topic.

Mapping Parts to a Physical Model

In the logical model of a package, parts are data components that are URI-addressable resources inside the package. When a package format is designed, the package format designer defines how parts are mapped to physical features of the physical format. How a part is mapped can influence the part name. For example, in a ZIP-based physical package, parts map to ZIP file items and part names map to the path of the part, which is relative to the root of the ZIP archive.

Finding a Part by Relationship Type

To find a part in a package, use a relationship type to identify a relationship that targets the part. Then, resolve the part name from the relationship target's URI and retrieve the part.

Why use a relationship type?

The part name is closely tied with the package format in use. This name varies according to how the logical package model is mapped to a physical format. Mapping allows package format designers to optimize the package format to the necessary software. As a side effect, the part name is unpredictable from one package to another. For this reason, a part is found by first identifying a relationship, which is a logical link to the part, and then resolving the name of the part to be found in the current package.

Relationship information is tracked with several properties, which are represented as attributes in the following example of a Relationship element. The relationship identifier property (the Id attribute) is arbitrary and local to a Relationships part. The target URI property (the Target attribute) is a relative URI to the target part and is not known before a relationship is identified. The relationship type property (the Type attribute), however, is defined by the package format designer or in the OPC and is, therefore, known and predictable across a package format.

The following code shows example markup for a relationship, including the relationship identifier, target URI, and relationship type properties.


The example uses the default TargetMode: "Internal".

                Target="word/document.xml" />

Procedure for Finding a Part

Use the following steps to find a part by using a relationship type.

For an example of how an application can use a relationship type to find a part, see the Finding the Core Properties Part topics or the Set Author Sample.

  1. Identify and define a constant for the relationship type and, if required, for the content type of the part to be found. For definitions and requirements for the relationship type and content type, see the specification provided by the package format designer or the OPC.

    The content type can be used to ensure that part content is the expected content type, which is described in the package-format specification or the OPC.

  2. Access the package where the part to be found is stored.

  3. Find the part.

    1. Get the Relationships part that contains relationships that target the part.

      • If the part is targeted by a package relationship, identify the Relationships part that stores all package relationships.

      • If the part is targeted by a part relationship, identify the Relationships part that stores a part relationship that targets the part.

    2. Identify the relationships of the specified relationship type in the set.

      If the package format designer describes an expected number of relationships of the specified relationship type that can be stored in a Relationships part, check that the number of relationships retrieved conforms to that expectation. For information about the expected number of relationships for a specified relationship type, see the specification provided by the package format designer or the OPC.

  4. To find the part, iterate through relationships of the specified relationship type.

    1. If the target of a retrieved relationship is a part, resolve the part name against the URI of the relationship's source.

      For an example of how to resolve a part name from the URI targeted by a relationship, see Resolving a Part Name from a Target URI.

    2. Ensure that a part with the resolved part name exists in the package.

    3. Get the part.

    4. If an expected content type is defined for the part by the package designer or the OPC, ensure that the part has the correct content type. If the part has the expected content type, or if no expected content type is defined, the part has been found.

Additional Resources

While not required to use the Packaging API, knowledge of the following technologies will advance your understanding of the Packaging API.

Technology Description
Markup Compatibility requirements in the ECMA-376 OpenXML standard If your application interacts with packages that comply with different editions of the OPC, familiarity with markup compatibility requirements will be helpful in developing your application. For more information, see 1st edition, Part 5: Markup Compatibility in the ECMA-376 OpenXML (

Parts Fundamentals


Open Packaging Conventions Fundamentals

Parts How-To Topics

Getting Started with the Packaging API


Packaging API Reference

Packaging API Samples


ECMA-376 OpenXML