Window functions to count distinct records

Question

The query below is based on a complicated view and the view works as I want it to (I\'m not going to include the view because I don\'t think it will help with the question a

Answer 1

Why would something like this not work?

SELECT 
   IDCol_1
  ,IDCol_2
  ,Count(*) Over(Partition By IDCol_1, IDCol_2 order by IDCol_1) as numDistinct
FROM Table_1