LINQ OrderBy on inner object's property -

below hierarchies , required function

public class friend {     public string name { get; set; }     public list<message> messages { get; set; } }  public class message {     public string text { get; set; }     public datetime time { get; set; } } 

now required function:

public list<string> whatsapp(list<friend> friends)     {          throw new notimplementedexception();     } 

i need list of friend names in descending order of there message time stamp. whats app or other im matter.

i getting feeling can done in 1 or 2 lines using linq since new linq, unable drill down problem.

thanks in advance help.

if idea order last (i.e. max) message timestamp, following should job:

return friends.orderbydescending(f => f.messages.max(m => (datetime?)m.time))     .select(f => f.name)     .tolist(); 

casting datetime? needed avoid exception when there no messages friend.

in general when need order parent having multiple children based on children properties, should aggregate value (like sum, count, min, max etc.).