Finding missing entries between two Dictionaries in C# : Suppose that I have two dictionaries as such:

    Dictionary<int, int> a = new Dictionary<int, int>(), b = new Dictionary<int, int>();
    a.Add(1, 1);
    a.Add(2, 2);
    a.Add(3, 3);
    b.Add(1, 1);
    b.Add(2, 2);

What's the best way to extract the difference here, returning a type `Dictionary<int, int>`?