The IFinalList<T> concept denotes a non-lazy, immutable list that is covariant in its item type T.

Grab the NuGet here.

The motivation for IFinalList was that there is no concept of a list in .NET that carries all of these properties.

• IReadOnlyList<T> is covariant in T and eagerly loaded, but is read-only, not immutable
• System.Collections.Immutable.IImmutableList<T> is immutable and carries eager loading semantics, but is not covariant in T
• IEnumerable<T> is covariant in T, but uses lazy-loading and also read-only semantics

System.Collections.Immutable.IImmutableList<T> would have been the perfect candidate for an immutable list concept. But unfortunately, someone decided to provide the immutable Add(), Remove() and similar methods not only on the implementation ImmutableList<T>, but also on the interface, effectively making the item type invariant.

To differentiate the immutable list of this library from System.Collections.Immutable.IImmutableList<T>, I call it a Final List.

For a discussion about immutable vs. read-only semantics see the BCL team blog.