How can I convert a C# list into something that's hashable?

Question

I want something along the lines of Python's tuples (or, for sets, frozensets), which are hashable. I have a List<String> which is most certainly not hashing correctly (i.e. by value).

Answer 1

You will have to define your own container, possibly wrapping the List, to get useful semantics for equality-hash-equals (GetHashCode and Equals). You could even make the wrapper conform to IList if you like.

To avoid mutability issues and a changing GetHashCode/Equals results (which would make use of your new object in a hashing Dictionary problematic!) you should also provide some kind of guard (perhaps make a copy of the input upon creation of your type) and/or document the constraints.

You can use SequenceEqual to implement Equals rather trivially, but you'll need to implement a GetHashCode in a relevant way -- a simple method is a shifting XOR of the GetHashCode of each element.

Alternatively, if this is just used in a single Dictionary you can supply a custom IEqualityComparer and avoid creating a wrapped type: Dictionary constructor overload.

It depends what your final goals are and there very well already be such wrapping containers :-)

Note: In .NET4 there is a set of Tuple<...> classes which override GetHashCode and Equals. See cadenza as the 3rd party alternative for prior .NET versions.