Delete item from dictionary, where value is empty list

Question

What is the best way to remove an item from a dictionary where the value is an empty list?

IDictionary<int,Ilist<T>> 

Answer 1

 var foo = dictionary .Where(f => f.Value.Count > 0) .ToDictionary(x => x.Key, x => x.Value); 

This will create a new dictionary. If you want to remove in place, John will do the trick.

Answer 2

Well, if you need to do this in place, you can use:

 var badKeys = dictionary.Where(pair => pair.Value.Count == 0) .Select(pair => pair.Key) .ToList(); foreach (var badKey in badKeys) { dictionary.Remove(badKey); } 

Or if you are happy to create a new dictionary:

 var noEmptyValues = dictionary.Where(pair => pair.Value.Count > 0) .ToDictionary(pair => pair.Key, pair => pair.Value); 

Note that if you have a chance to change the way you build a dictionary, you might consider creating ILookup instead of ToLookup . This is usually simpler than a dictionary, where each value is a list, although they are conceptually very similar. The search has a nice feature, where if you request a missing key, you get an empty sequence instead of an exception or a null reference.

Answer 3

_{An alternative is provided for completeness only.}

As an alternative (and completely depends on your use), do this at the time you make changes to the contents of the list on the fly, and not the party at a certain point in time. At such a time, you will probably know the key without having to repeat:

 var list = dictionary[0]; // Do stuff with the list. if (list.Count == 0) { dictionary.Remove(0); } 

Other answers relate to the need to do this ad-hoc throughout the dictionary.