c# - How to sort a list of strings converted from nullable datetime -

i have list of strings have created nullable datetime objects. empty strings represent null entries. list can like

{ "23.06.2015", "", "01.07.2014", "" } 

i want sort list date descending. empty strings should @ bottom.

.orderbydescending(o => o.datestring) 

does wrong ordering. love use

.orderbydescending(o => datetime.parse(o.datestring)) 

but throw exception empty strings.

can format dates yyyymmdd rather dd.mm.yyyy? if can appear in numerical (also date) order when sort them.

once sorted, can convert them original dd.mm.yyyy format.