Why do C# 4.5 tuples use classes not structs?

Question

Looking at the official Microsoft class vs struct design guidelines (from http://msdn.microsoft.com/en-us/library/ms229017(v=vs.110).aspx)

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.

Tuples should fit all of the above, except possibly the size. Smaller tuples will be less than 16 bytes, but larger tuples will not. Larger tuples are chepaer to pass by reference than pass by value (which means value copying).

Tuples *don't* represent a single value (except `Tuple`). The whole *point* of a `Tuple` is that it *represents multiple values*. They will very rarely be under 16 bytes, they are immutable, but not all immutable types should be structs, and you can't really assert that they *won't* be boxed; they're designed to be so general purpose that they will undoubtedly be used in contexts with and without boxing. One out of four is pretty bad. — Servy, May 06 '14 at 19:26
@Servy: "they will very rarely be under 16 bytes" - how come? The most commonly used `Tuple` is probably `Tuple`, and on an x86 it will hold two references in mere 8 bytes total. — vgru, Oct 15 '14 at 08:43
@Servy: there is would take 16 bytes, that's just according to guidelines. It's 16 bytes total to pass around, no additional allocations. In case of a reference type (`Tuple` in my case) you do pass only 8B around, but the heap-allocated instance has an additional 16 bytes of overhead, and you cannot do anything with these 8 bytes you are passing before you dereference the address to get the heap instance. And there is the GC burden. So, unless the struct is heavily boxed, it will perform better and put less stress on the GC. From my experience, you can avoid boxing in 99% of cases. — vgru, Oct 15 '14 at 15:22
@Groo You're still forgetting *the single most important decision* when deciding if you should use a struct, "does the type conceptually represent a single *value*". The answer here is clearly "no". Will there be certain contexts that you could use a value type and have it perform better, sure, and there will be situations in which it will be worse. If you are actually in a situation in which it's vital to your program to use a `struct` then you can write the code to do that yourself. `Tuple` is there to be widely applicable, to which a class serves it best. — Servy, Oct 15 '14 at 18:38
Why is the answer "clearly no"? Why wouldn't a tuple conceptually represent a single value? What's the difference between a point or a key-value pair, and a tuple? Also, I don't find the guideline *that* important; if a complex number or a 2D point is conceptually a single value, then an N-dimensional point is too, but it surely doesn't seem like a compelling use case for a struct. What I am trying to say is (just my opinion) is that they are sometimes suitable for cases where there exist allocation hotspots (found using a profiler), as a cheap opportunity to reduce CPU and memory pressure. — vgru, Oct 21 '14 at 08:40

Why do C# 4.5 tuples use classes not structs?

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