Question

Why can’t I preallocate a hashset<T>?

There are times when i might be adding a lot of elements to it and i want to eliminate resizing.

Answer 1

Answer below was written in 2011. It’s now in .NET 4.7.2 and .NET Core 2.0; it will be in .NET Standard 2.1.

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There’s no technical reason why this shouldn’t be possible – Microsoft just hasn’t chosen to expose a constructor with an initial capacity.

If you can call a constructor which takes an IEnumerable<T> and use an implementation of ICollection<T>, I believe that will use the size of the collection as the initial minimum capacity. This is an implementation detail, mind you. The capacity only has to be large enough to store all the distinct elements…

EDIT: I believe that if the capacity turns out to be way larger than it needs to be, the constructor will trim the excess when it’s finished finding out how many distinct elements there really are.

Anyway, if you have the collection you’re going to add to the HashSet<T> and it implements ICollection<T>, then passing it to the constructor instead of adding the elements one by one is going to be a win, basically 🙂

EDIT: One workaround would be to use a Dictionary<TKey, TValue> instead of a HashSet<T>, and just not use the values. That won’t work in all cases though, as it won’t give you the same interface as HashSet<T>.