Getting linker error in Visual studio while trying to use class ActiveSelectionObserver

I am trying to implement a selection observer. I am using public specifier to derive from class ActiveSelectionObserver and have included the header file SelectionObserver.h.

But I am getting the below error from linker while trying to do that.

Below is the my code:

class CSDTSelectionObserverImpl : public ActiveSelectionObserver

{

/* some code here**/

}

Below is the linker error:

CSDTSelectionObserverImpl.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: __cdecl ActiveSelectionObserver::ActiveSelectionObserver(class IPMUnknown *,class IDType<struct PMIID_tag>)" (__imp_??0ActiveSelectionObserver@@QEAA@PEAVIPMUnknown@@V?$IDType@UPMIID_tag@@@@@Z) referenced in function "public: __cdecl CSDTSelectionObserverImpl::CSDTSelectionObserverImpl(class IPMUnknown *)" (??0CSDTSelectionObserverImpl@@QEAA@PEAVIPMUnknown@@@Z)