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# Data Containers

A few constants identify messages as data containers. The system currently uses these constants to mark the containers it constructs for drag-and-drop operations.

### B_MIME_DATA

This message constant indicates that all the data in the message is identified by MIME type names. The type code of every data field is `B_MIME_TYPE` and the name of each field is the MIME type string.

As an example, a BTextView object puts together a `B_MIME_DATA` message for drag-and-drop operations. The message has the text itself in a field named "text/plain"; the `text_run_array` structure that describes the character formats of the text is in a field named "application/x-vnd.Be-text_run_array".

### B_SIMPLE_DATA

This message is a package for a single data element. If there are multiple data fields in the message, they present the same data in various formats.

For example, when the user drags selected files and directories from a Tracker window, the Tracker packages `entry_ref` references to them in a `B_SIMPLE_DATA` message. The references are in a "refs" array with a type code of `B_REF_TYPE`. In other words, the message has the same structure as a `B_REFS_RECEIVED` message, but a different `what` constant.

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