openTypeExtension resource type (open extensions)

Namespace: microsoft.graph

Represents open extensions (also known as open type extensions, and formerly known as Office 365 data extensions), an extensibility option that provides an easy way to directly add untyped properties to a resource in Microsoft Graph.

Any open extension added to a resource shows up in the extensions navigation property. Each extension has an extensionName property that is the only predefined, writable property for all extensions, along with your custom data. One way to help make sure extension names are unique is to use a reverse domain name system (DNS) format that is dependent on your own domain, for example, com.contoso.ContactInfo. Don't use the Microsoft domain ( or com.onmicrosoft) in an extension name.

Derives from the extension abstract type.

The following resources support open extension:

Note: * Due to an existing service limitation, delegates can't create open extension-appended events in shared mailbox calendars. Attempts to do so will result in an ErrorAccessDenied response.

For more information about Microsoft Graph extensibility including limits for open extensions, see Add custom properties to resources using extensions and Add custom data to users using open extensions.

Outlook-specific considerations

Each open extension on an Outlook resource (event, message, or personal contact) is stored in a MAPI named property. When you create open extensions for Outlook, consider that MAPI named properties are a finite resource in a user's mailbox. When a user's named property quota is exhausted, you can't create anymore named properties for that user. This can result in unexpected behavior from clients that rely on named properties to function.

Apply the following guidelines when you create open extensions on Outlook resources:

  • Create the minimum number of extensions required. Most applications should require no more than one extension. Extensions have no defined properties or structure, so you can store multiple values in a single extension.
  • Avoid naming extensions in a variable manner (such as based on user input, etc.). Each time an open extension is created with a new name that hasn't been used in a user's mailbox before, a new MAPI named property is created. Removing the extension doesn't remove the named property.

Use open extensions (for Outlook resources) or extended properties

Open extensions are the recommended solution for most scenarios involving storing and accessing custom data. If, however, you need to access custom data for Outlook MAPI properties that aren't already exposed through the Microsoft Graph API metadata, you can use extended properties and its REST API. You can verify which properties the metadata exposes at$metadata.


Method Return Type Description
Create open extension openTypeExtension(in an existing resource instance), or a new todoTask, todoTaskListcontact, event, message, post, todoTask, or todoTaskList that contains an openTypeExtension object. Create an openTypeExtension object in an existing or new resource instance.
Get open extension openTypeExtension Read properties and relationships of openTypeExtension object.
Update openTypeExtension Update openTypeExtension object.
Delete None Delete openTypeExtension object.


Property Type Description
extensionName String A unique text identifier for an open type data extension. Optional.
id String A fully qualified identifier that concatenates the extension type with the extensionName. Read-only.

The id must be defined during the Create operation via one of the following ways:
  • Explicitly define the id property in the request body.
  • Define an extensionName property in the request body, and Microsoft Graph automatically assigns the same value to the id property.
    • In subsequent updates, you can change the extensionName property value to one that's different from the id value.
    • In subsequent updates, specifying only the id property in the request body automatically deletes the extensionName property from the open extension.



JSON representation

The following JSON representation shows the resource type.

  "extensionName": "string",
  "id": "String (identifier)"