Get frame by frame index, use negative values for get next frames,
e.g. -1 - next frame, -2 - frame after next (skip one frame), etc.

Public Overridable Sub ReaderFrameGetByIdx( _
   ByVal _lIndex As Integer, _
   ByRef _ppFrame As IMFFrame, _
   ByVal _bsPropsList As String _
) 

public virtual void ReaderFrameGetByIdx( 
   int _lIndex,
   out IMFFrame _ppFrame,
   string _bsPropsList
)

public procedure ReaderFrameGetByIdx( 
    _lIndex: Integer;
   Out  _ppFrame: IMFFrame;
    _bsPropsList: String
); virtual; 

public: virtual void ReaderFrameGetByIdx( 
   int _lIndex,
   [PARAMFLAG::Out] IMFFrame* _ppFrame,
   BSTR* _bsPropsList
)