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# sharedContainerIdentifier

The identifier for the shared container into which files in background URL sessions should be downloaded.

```
var sharedContainerIdentifier: String? { get set }
```

## Discussion

To create a URL session for use by an app extension, set this property to a valid identifier for a container shared between the app extension and its containing app.

> Important:
> If you try to create a URL session from your app extension but fail to set this property to a valid value, the URL session is invalidated upon creation.

For information about app extensions, see [App Extension Programming Guide](https://developer.apple.com/library/archive/documentation/General/Conceptual/ExtensibilityPG/index.html#//apple_ref/doc/uid/TP40014214).

## See Also

[`NSURLErrorBackgroundSessionRequiresSharedContainer`](/documentation/Foundation/NSURLErrorBackgroundSessionRequiresSharedContainer-swift.var)

The shared container identifier of the URL session configuration is needed but hasn’t been set.

[`NSURLErrorBackgroundSessionRequiresSharedContainer`](/documentation/Foundation/NSURLErrorBackgroundSessionRequiresSharedContainer-c.enum.case)

The shared container identifier of the URL session configuration is needed but hasn’t been set.


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