How to determine if a selected face is 2D (or a plane)

I'm kind of new to SolidWorks API programming.

I need a method of determining if a face that is selected on a solid is 2D (i.e. a selected square that comprises a cube).  I've done some research and noticed a couple commands that might be useful: isPlane and is3D, but it looks like I have to somehow convert what I have selected into either a "Surface" or an "IFace" (depending on which command I use) and I need help on how to convert the selected face into the proper type to work with those commands.  Or, is there a simpler method for what I'm trying to accomplish.

Any help is appriciated.

UPDATE:

I think I have found a method for which I could use isPlane, but I'm not sure why one part of this isn't working.  Here is what I have come up with:

Code Sample

'Declarations

Dim isThisAPlane As Boolean

Dim WorkFace As Face2

Dim WorkSurface As Surface

Dim SelMgr As SldWorks.SelectionMgr

'Determine if the selected face is 2D
isThisAPlane = False  'initalize variable to false
WorkFace = SelMgr.GetSelectedObjectsFace(1, 0)  'obtain the face from what is selected
WorkSurface = WorkFace.GetSurface               'obtain the surface from the face
isThisAPlane = WorkSurface.IsPlane              'check to see if surface is 2D

The problem I'm having (aside from not understanding everything that's going on... I'm new at this) is that my line to get the face from what I have selected:

WorkFace = SelMgr.GetSelectedObjectsFace(1,0) 'gets the face from the selection

generates the following error when I run the macro:

What is it that I'm doing wrong?

Thanks for any help!

SolidworksApi macros