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Instance ID

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An instance ID is a string reported by a device's enumerator (its bus driver) and distinguishes a device from other devices of the same type on a computer. An instance ID contains serial number information, if supported by the underlying bus, or some kind of location information. The string cannot contain any "\" characters or any other character disallowed in a response to a IRP_MN_QUERY_ID request; otherwise, the generic format of the string is bus-specific. The number of characters of an instance ID, excluding a NULL-terminator, must be less than MAX_DEVICE_ID_LEN. In addition, the instance ID is another input into the creation of the device instance ID, along with a device ID, and the maximum possible length of a device instance ID is MAX_DEVICE_ID_LEN. This requires that the length of the instance ID be enough less than MAX_DEVICE_ID_LEN that the device instance ID can be created as a string with length less than MAX_DEVICE_ID_LEN.

The UniqueID member of the DEVICE_CAPABILITIES structure for a device indicates if a bus-supplied instance ID is unique across the system, as follows:

  • If UniqueID is FALSE, the bus-supplied instance ID for a device is unique only to the device's bus. The Plug and Play (PnP) manager modifies the bus-supplied instance ID, and combines it with the corresponding device ID, to create a device instance ID that is unique in the system.

  • If UniqueID is TRUE, the device instance ID, formed from the bus-supplied device ID and instance ID, uniquely identifies a device in the system.

An instance ID is persistent across system restarts.

The PnP manager queries this string from the device's bus driver using an IRP_MN_QUERY_ID request with the Parameters.QueryId.IdType field set to BusQueryInstanceID.