CWMPPlayer4::GetCurrentMedia link error

[David van Kemenade]

I'm trying to find out the length of the file I've loaded into Windows
Media Player. I'm using Visual C++/MFC.

Some of the code:

#include "wmpcontrols.h"
#include "wmpmedia.h"

(...)

int iTrackLengthSecs = mMediaPlayer.GetCurrentMedia().GetDuration();

That last line causes a link error:
"error LNK2001: unresolved external symbol "public: class CWMPMedia
__thiscall CWMPPlayer4::GetCurrentMedia(void)"
(?GetCurrentMedia@CWMPPlayer4@@QAE?AVCWMPMedia@@XZ)"

With the line commented out, everything works perfectly (loading a
file and playing it, etc.).

Any idea what may be going on? Thanks in advance for any help!