List<long> vs long[], memory usage

Question

Regarding the size in memory for the

List<long> ListOfLongs;
long[] ArrayOfLongs;

If each has N elements, how much memory they eat up?

I am asking that because as of my knowledge, .NET has not template (generics) specialization.

Answer 1

Practically the same amount of memory (technically, the List will probably consume some more because it has over-allocated so that it can grow more easily).

Generic collections in .NET do not need to box the items they hold, which would be a massive memory and performance sink.

Answer 2

The List<T> owns an array T[]. It uses an exponential growth strategy for this array, so a list with n elements usually has a backing array with size larger than n. Also the smaller arrays need to be garbage collected, which can be annoying if the are large enough to be on the LoH.

But you can avoid this by specifying a capacity manually, for example as a constructor parameter. Then a single array with the desired capacity will be allocated, so you avoid both of the above problems.

In addition List<T> has a small O(1) overhead for the list object itself.

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But there is no per element overhead when using generics. The runtime creates a specialized version for each value type you pass in. No boxing of the elements occurs.

But you can't use C++ style template specialization, where you effectively overload the implementation for certain type parameters. All generic instantiations share the same C# code.

i.e. there is no specialized IL code, but each value type gets a specialized machine code implementation based on the same source-code.