How to cast ImmutableArray<DerivedClass> to ImmutablyArray<BaseClass> and vice versa in C#?

Question

I know it's not possible to cast derived lists (why not is nicely explained here), but what about ImmutableArrays as they cannot be changed casting should be valid, right?

I would expect this to work:

class Animal {} class Dog : Animal {} ImmutableArray<Dog> dogs = ImmutableArray.Create<Dog>(new Dog()); ImmutableArray<Animal> animals = (ImmutableArray<Animal>) dogs; ImmutableArray<Dog> dogsBack = (ImmutableArray<Dog>) animals;

Why would you want to? Can't you access dogs as if it contained Animal instances or add Dog instances to animals on creation? — ChrisBD
– ChrisBD, Commented Aug 16, 2023 at 8:24
@ChrisBD I need this for a caching scenario, I made an example here — DomenPigeon
– DomenPigeon, Commented Aug 16, 2023 at 8:53

Theodor Zoulias · Accepted Answer · 2023-08-16 08:40:36Z

3

Yes this is possible:

ImmutableArray<Dog> dogs = ImmutableArray.Create<Dog>(new Dog()); ImmutableArray<Animal> animals = ImmutableArray<Animal>.CastUp(dogs); ImmutableArray<Dog> dogsBack = animals.CastArray<Dog>();

Note that you could also use CastArray to cast from dogs to animals, but due to covariance, CastUp is a significantly faster than the CastArray method.

edited Aug 16, 2023 at 8:40

Theodor Zoulias

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answered Aug 16, 2023 at 8:06

DomenPigeon

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2 Comments

Peter Csala Over a year ago

Why do you need ImmutableArray's CastUp? dogs.CastArray<Animal>() should be enough?

Marc Gravell Over a year ago

@PeterCsala both operations simply cast the underlying array with zero copy; "enough" suggests a difference - if anything, CastUp is preferable since it exploits a known covariance operation, rather than an unknown blind cast with type-test; source, for the curious: github.com/tunnelvisionlabs/dotnet-collections/blob/master/…

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