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<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>
<p>All files have an associated compound identifier, a <xref href="GUID-B6D6B0AD-B15C-339A-8540-40540885089A.dita"><apiname>TUidType</apiname></xref>,
comprising three component UIDs that together identify the file. A compound
identifier is also commonly known as a UID type.</p>
<p>For a polymorphic DLL, the first UID component always has the value <codeph>KDynamicLibraryUidValue</codeph>.
The second UID component is used to verify that the DLL satisfies the protocol
defined by the programming interface. Indeed, this UID <i>identifies</i> that
protocol. The third UID component is used to identify a specific implementation;
the use of the third component UID depends on the protocol.</p>
<p>For a UI application, the second UID component takes the value <codeph>KAppUidValue</codeph>.
The third UID component must be defined and must be unique in order to differentiate
it from all other applications. In contrast, a DLL which implements a printer
driver does not require a third UID component.</p>
<p>The use of UIDs in DLLs provides a guarantee that the loaded DLL is the
correct type. This prevents files which just happen to have the right name
from being loaded inadvertently.</p>
<p>A polymorphic interface DLL is loaded dynamically. The handle to the DLL
is encapsulated by an <codeph>RLibrary</codeph> object. The <codeph>Type()</codeph> member
function of <codeph>RLibrary</codeph> is used to return the DLL's <codeph>TUidType</codeph> from
which the individual UID components can be extracted.</p>
<p><codeph>KDynamicLibraryUidValue</codeph> is defined in <filepath>e32uid.h</filepath> and <codeph>KAppUidValue</codeph> is
defined in <filepath>apadef.h</filepath>.</p>
</conbody></concept>