I thought about creating a small library that would expose a static IComparer<T> ToComparer<T>(this Func<T, T, int>) extension method (and a similar one for IEqualityComparer<T>). But I so far just was too lazy. If somebody from the C# API team reads this tho.... :P

And while you're at it dear C# team, add .Distinct<TSource, TDest>(Func<TSource, TDest>, IEqualityComparer<TDest>) similar to OrderBy and friends, and various constructors to the generic collection types taking Func<T, T, int> instead of IComparer<T>. Yeah, API bloat, but good one in this case IMHO.

C# and the .NET runtime and the compiler(s) are still evolving indeed, and I would guess a lot of issues will be addressed and fixed in the future.

PS: the most annoying bit to me about the immutable collection library is that they are dead slow whenever I tried them.