Drag and drop issue with 'Data' argument

Beginning a dragdrop operation is easy enough - call the DoDragDrop method.
I can't figure out what should best be put in the 'Data' argument for the
DoDragDrop method though.

Suppose I had a tree that showed a file/folder structure and I wanted a drop
target to be a textbox.  I could use the folder or file text as the 'Data'
object in the DoDragDrop method (or I could create another object based on
the file or folder selected for the 'Data' argument).

However, what should the textbox be looking for in its dragenter/dragover
events?  If I had another tree that showed some other information that also
could be used as a dragdrop source, how could I restrict the textbox from
being a target for the second tree while still allowing a drop from the
first tree?  It doesn't seem that I could just use text in the 'Data'
argument unless I did something (which seems like a hack) like prefacing the
string with the source tree name...

Peter