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Immutable collections exit, they were just added later. See System.Collections.Immutable:

https://learn.microsoft.com/en-us/dotnet/api/system.collecti...

I do a lot of .NET programming, and I've never seen them getting used. :O

Roslyn, the C#/VB compiler, uses them extensively, but in other code they're indeed quite rare.

Those collections are more like copy-on-write than actual immutable. System.Collections.Frozen is the real thing.

Isn't Frozen something you do to a set or dictionary to say, I'm not going to add any more values, please give me a version of this which is optimized for lookup only?

Adding to the sibling comment, there are also Frozen{Dictionary,Set}: https://learn.microsoft.com/en-us/dotnet/api/system.collecti...

One man's constant is another man's variable.