Multiple Key Maps
Next time you find yourself writing "Dictionary<..., Dictionary<..."
stop and ask yourself why are you not using Maps!
Use case
I want to store a collection of entities in a 3D space and be able to look them up very fast by x, y, and z coordinates. For clarity, let's assume the coordinate variable types are XCoor, YCoor, and ZCoor for x, y, and z respectively. I could use a Dictionary:
Dictionary<XCoor, Dictionary<YCoor, Dictionary<ZCoor, Entity>>> _entities;
That looks so ugly. Seriously. Or I could use a Map:
Map<XCoor, YCoor, ZCoor, Entity> _entities;