[This documentation is preliminary and is subject to change.]

Files with a common file extension (.doc, .html, and so on) are the same type. When developing an application that uses files, you can use existing, well-defined file types. For example, if you create a new text editor, then you would use the existing .txt file type. In other cases, you may need to create a new file type.

This topic is organized as follows:

• Public Versus Private File Types
• Choose a File Type Extension
• Register a File Type
  • Set Optional Keys and File Type Extension Attributes
  • Define File Type Attributes
  • Exclude an Application from the Open With Dialog Box for Unassociated File Types
• Additional Resources
• Related Topics

Public Versus Private File Types

Public file types are also known as popular or contentious because competing applications might want to be associated with these file type.

Some characteristics of public file types include:

• Applications from multiple vendors are likely to handle them
• They are typically defined by standards bodies, or promoted by their defining organizations as interchange formats
• They are often exchanged between computers and users for diverse purposes
• They need to be supported on many different platforms

Some examples of file types that are considered public are the image formats .png, .gif, .jpeg, and .bmp, and the audio formats .wav, .mp3, and .au.

Unlike public file types, private or proprietary file types typically have a format that is implemented and understood by only one application or vendor. As a result, private file types are typically not prone to conflicts between applications. Some file types may start their life as private file types, but later become public file types.

Choose a File Type Extension

Your file type us identified by the file extension.

To define a new file type:

1. Choose a long file extension you want to use to identify the file type.

   For more information, see File Type Handlers.

2. Check the Internet Assigned Numbers Authority (IANA) MIME database to determine if the file extension is already associated with another application.

3. We recommend that you also define a MIME type when defining a new public file type, and register it with IANA MIME. There is no cost for registration.

   For more information, see the IANA  Web site.

Register a File Type

If you plan to associate the file with an existing application, locate an application ProgID in the registry. To associate the file type with a new application, define a ProgID for your application. For information about defining a new ProgID, see Programmatic Identifiers.

File extension keys have the following general form: extension=ProgID. The registry stores the file extension keys under HKEY_CLASSES_ROOT .

It is important to include the leading period when creating file type key entries in the registry. For example, if you want a file type with the short extension .myp and the long extension .myp-file to open with an application called MyProgram, use the following syntax:

As demonstrated in this example, if you also register a short file extension you should create a key for the short extension as well. For more information, see File Type Handlers.

Set Optional Keys and File Type Extension Attributes

File type extension entries in the registry have several optional sub keys and attributes.

The file type extension entries that are used by file associations are listed as follows:

The general form of a file extension key is as follows:

Important considerations regarding file types include:

• The HKEY_CLASSES_ROOT registry key is a view formed by merging HKEY_CURRENT_USER\Software\Classes and HKEY_LOCAL_MACHINE\Software\Classes . In general, HKEY_CLASSES_ROOT is intended to be read from but not written to. For more information, see HKEY_CLASSES_ROOT Key.
• To register a file type globally on a particular computer, create an entry for the file type in HKEY_LOCAL_MACHINE\Software\Classes .
• To make a file type registration visible to the current user only, create a file type entry in HKEY_CURRENT_USER\Software\Classes .
• To ensure that an application is included in the Open With dialog box for a file type, include a reference to the application in either the OpenWithProgids or OpenWithList key for the file type.
• Any time you create or change a file association, it is important to notify the system that you have made a change by calling SHChangeNotify, and specifying the SHCNE_ASSOCCHANGED event. If you do not call SHChangeNotify, then the change may not be recognized until after the system is rebooted.
• To retrieve information regarding a file association from the registry, use the IQueryAssociations interface.

For a sample scenario of this procedure, see File Association Sample Scenario.

Define File Type Attributes

Assigning file type attributes to an associated ProgID enables you to control some aspects of the file type's behavior. Prior to Windows Vista, these attributes could enable you to limit the extent to which the user can use the Folder Options property tab to modify various aspects of the file type, such as its icon or verbs.

File type attributes are binary flags specified as REG_DWORD values in the file type's associated ProgID key. To assign attributes for a file class, add an EditFlags REG_DWORD value to the file type's associated ProgID key and set it to the desired attribute value. The flags may be combined with a logical OR to form a single attribute value.

For a list of file class attributes and their hexadecimal values, see FILETYPEATTRIBUTEFLAGS.

The following example assigns the FTA_NoRemove (0x00000010) and FTA_NoNewVerb (0x00000020) attributes to the .myp file class.

Exclude an Application from the Open With Dialog Box for Unassociated File Types

In Windows 7 and later, when a user attempts to open a file that is not a member of any registered file type (that is, an unknown file type), or when a user selects Open With... or Open With -> Choose Default Program... from a file's shortcut menu, the Shell presents the shortcut menu, which enables the user to specify the program is used to open the file.

Prior to Windows 7, when a user attempts to open a file that is not a member of any registered file type, or when a user selects Open With... or Open With -> Choose Program... from a file's shortcut menu, the Shell presents the Open With dialog box, which enables the user to specify the program is used to open the file.

The following diagrams illustrate examples of the Open With dialog box.

Applications that are listed in the Open With dialog box are registered as subkeys of HKEY_CLASSES_ROOT\Applications . By default, an application with a key in HKEY_CLASSES_ROOT\Applications is listed in the Open With dialog box for any file the user attempts to open, including file types which the application is not registered to handle.

Sometimes an application should not be used to open files types it is not registered to handle. To exclude an application from the Open With dialog box for files that the application is not registered to handle, add an empty REG_SZ value named NoOpenWith to the application's key.

For example, assume that MyProgram.exe is registered to handle .myp and .txt files. The following sample registry entry excludes MyProgram.exe from being listed in the Open With dialog box for any file type other than .myp or .txt.

Additional Resources

• For a conceptual overview, see Verbs and File Associations.
• To execute a verb on an item, see InvokeVerb.
• To retrieve a collection of verbs that can be executed on an item, see Verbs Method.
• To perform an operation on a specified file, see either the ShellExecute or ShellExecuteEx functions.
• For conceptual information on creating a Shell data store, see Implementing the Basic Folder Object Interfaces.

Related Topics

• How File Associations Work
• File Type Handlers
• Programmatic Identifiers
• PerceivedTypes, SystemFileAssociations, and Application Registration