Remove item from dictionary where value is empty list

Question

What is the best way to remove item from 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, Jon's answer will do the trick.

Answer 2

Well, if you need to perform this in-place, you could 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're happy creating a new dictionary:

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

Note that if you get a chance to change the way the dictionary is constructed, you could consider creating an ILookup instead, via the ToLookup method. That's usually simpler than a dictionary where each value is a list, even though they're conceptually very similar. A lookup has the nice feature where if you ask for an absent key, you get an empty sequence instead of an exception or a null reference.