Why is ImmutableList a class but ImmutableArray a struct?

Question

When you read the documentation for ImmutableArray and ImmutableList, you can see that:

ImmutableArray<T> has been implemented as a struct
ImmutableList<T> has been implemented as a class

Question:

Can you explain why such design decision?

Especially, why ImmutableArray<T> is a struct.

After all, System.Array is a class so, why it isn't the case for ImmutableArray<T>?

From this post it sounds like the reason was for performance: https://devblogs.microsoft.com/dotnet/please-welcome-immutablearrayt/ — devNull, Dec 16 '20 at 00:04

score 2 · Accepted Answer · edited Sep 13 '22 at 19:56

This article sheds a bit of light on the decision to make .Net ImmutableArray a "struct" instead of a class:

Choosing Between Class and Struct

✔️ CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

❌ AVOID defining a struct unless the type has all of the following characteristics:

It logically represents a single value, similar to primitive types (int, double, etc.).

It has an instance size under 16 bytes.

It is immutable.

It will not have to be boxed frequently.

In all other cases, you should define your types as classes.

See also:

Could the same argument be made for `ImmutableList`? – mjwills Dec 16 '20 at 00:53 — mjwills, Dec 16 '20 at 00:53

Why is ImmutableList a class but ImmutableArray a struct?

1 Answers1