Does the .NET BCL have an immutable Set type? I'm programming in a functional dialect of C# and would like to do something like

new Set.UnionWith(A).UnionWith(B).UnionWith(C)

But the best I can find is HashSet.UnionWith, which would require the following sequence of calls:

HashSet composite = new HashSet();
composite.UnionWith(A);
composite.UnionWith(B);
composite.UnionWith(C);

This use is highly referentially opaque, making it hard to optimize and understand. Is there a better way to do this without writing a custom functional set type?

c# set immutability functional-programming
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