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    10 <!DOCTYPE concept

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    12 <concept id="GUID-C135B8D8-DA5A-5852-9C2D-18622404FE99" xml:lang="en"><title>UID Protection</title><shortdesc>Describes the identifiers used in a polymorphic DLL.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

    13 <p>All files have an associated compound identifier, a <xref href="GUID-B6D6B0AD-B15C-339A-8540-40540885089A.dita"><apiname>TUidType</apiname></xref>,

    14 comprising three component UIDs that together identify the file. A compound

    15 identifier is also commonly known as a UID type.</p>

    16 <p>For a polymorphic DLL, the first UID component always has the value <codeph>KDynamicLibraryUidValue</codeph>.

    17 The second UID component is used to verify that the DLL satisfies the protocol

    18 defined by the programming interface. Indeed, this UID <i>identifies</i> that

    19 protocol. The third UID component is used to identify a specific implementation;

    20 the use of the third component UID depends on the protocol.</p>

    21 <p>For a UI application, the second UID component takes the value <codeph>KAppUidValue</codeph>.

    22 The third UID component must be defined and must be unique in order to differentiate

    23 it from all other applications. In contrast, a DLL which implements a printer

    24 driver does not require a third UID component.</p>

    25 <p>The use of UIDs in DLLs provides a guarantee that the loaded DLL is the

    26 correct type. This prevents files which just happen to have the right name

    27 from being loaded inadvertently.</p>

    28 <p>A polymorphic interface DLL is loaded dynamically. The handle to the DLL

    29 is encapsulated by an <codeph>RLibrary</codeph> object. The <codeph>Type()</codeph> member

    30 function of <codeph>RLibrary</codeph> is used to return the DLL's <codeph>TUidType</codeph> from

    31 which the individual UID components can be extracted.</p>

    32 <p><codeph>KDynamicLibraryUidValue</codeph> is defined in <filepath>e32uid.h</filepath> and <codeph>KAppUidValue</codeph> is

    33 defined in <filepath>apadef.h</filepath>.</p>

    34 </conbody></concept>