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Device Event Types

To determine the device event type when processing a WM_DEVICECHANGE message, examine the wParam parameter. The value of wParam determines the meaning of the event-specific data in the lParam parameter. In general, the event-specific data identifies the device and provides additional detail about the event. The format of this data depends on the device type, but the first few bytes always have the same format as the DEV_BROADCAST_HDR structure. To determine the format of the data, check the dbch_devicetype member.

The system broadcasts a device event of type DBT_DEVICEARRIVAL (that is, a WM_DEVICECHANGE message with wParam set to DBT_DEVICEARRIVAL) whenever a device has been inserted and is available for use. Applications typically check the device type and begin using the device immediately if it is appropriate.

The system broadcasts a DBT_DEVICEQUERYREMOVE device event to request permission to remove a device. To determine whether it needs the device, an application can display a dialog box to prompt the user for instructions. If an application determines that it needs the device, it can deny this request and cancel the removal by returning BROADCAST_QUERY_DENY. If the application does not need the device, it must return TRUE. The system immediately sends a DBT_DEVICEQUERYREMOVEFAILED message if any application or driver canceled a previous request to remove a device.

The system broadcasts a DBT_DEVICEREMOVEPENDING device event as a last warning before a device is removed. At this point, the application cannot cancel the removal, so if it is using the device it must prepare for its removal to prevent loss of data. This is especially important when a network connection is being removed. The application must determine whether any of its open files or pipes are on that connection. It can do this by comparing the network resource identifier in the event-specific data of the message with the resource identifiers previously obtained for the files and pipes. The system broadcasts a DBT_DEVICEREMOVECOMPLETE device event when a device has been removed and is no longer available.

The system broadcasts a DBT_QUERYCHANGECONFIG device event to request permission to change the current configuration (dock or undock). Any application can return BROADCAST_QUERY_DENY to deny the request and cancel the change. If an application denies the request, the system sends a DBT_CONFIGCHANGECANCELED message. If the current configuration has changed, due to a dock or undock, the system sends a DBT_CONFIGCHANGED message.

The system broadcasts a DBT_DEVICETYPESPECIFIC device event whenever a device-specific event occurs.

Drivers can create their own custom event types. Custom events are sent only to application that have registered for device-event notification on a particular device, and can only be initiated by kernel-mode drivers. For more information, see DBT_CUSTOMEVENT.



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