Assume I have opened a worksheet and marked exactly one cell (e.g. D47).

Now I want to run a VB macro. How can I find out from inside this macro
which cell is currently marked in the current worksheet?

This should look similar to:

Set baseCell = ActiveSheet.Range(getcurrentlymarkedcell())

Obviously "getcurrentlymarkedcell()" does not exists. Which function can I use instead?

If the user accidentially marked a range of cells instead of a single cell then only the
first/upper+leftmost cell should be returned.

Claudia