3 ms·

No. The Select-statement is not evaluated until necessary, i.e. until ToArray() is called.

0xffff2 8y ago
While your comment is true, I'm not sure how it's relevant. `ToArray` is still operating on the `IEnumerable` produced by Select, which doesn't tell you that the Select is itself operating on an array. The CLR does do some type checking to try to propagate the size if it can, but that's by no means guaranteed. Edit: In skimming the source available on sourceof.net, it looks to me like the array doesn't actually get propagated far enough, so the construct in GP will in fact incur several unnecessary array copies.
- munchbunny 8y ago
  I'm curious what you saw. My understanding was that it's mostly generators, i.e. it's done through yields, not extra allocated arrays.
  - 0xffff2 8y ago
    It's `ToArray` itself that's the problem. Since it doesn't know how big the array will be in the first place, it has to allocate a small array, start iterating, copy everything into a bigger array each time it runs out of space, and finally allocate an array of exactly the right size and copy everything into that. You can see the `ToArray` implementation at [0], which defers the implementation to [1]. The implementation checks for ICollection to get the exact size, but the type [2] that `Select` returns doesn't implement ICollection, so `ToArray` has to fall back to the less efficient algorithm. [0] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,783a052330e7d48d https://referencesource.microsoft.com/#System.Core/System/Li... [1] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,ed118118b642d9d4 https://referencesource.microsoft.com/#System.Core/System/Li... [2] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,cffd4efbf86815cb https://referencesource.microsoft.com/#System.Core/System/Li...
    - cm2187 8y ago
      That's more or less what I had in mind without being certain of the details. Was trying to open your links but I am served an expired tls certificate from Microsoft!
      - 0xffff2 8y ago
        Oddly enough it seems that the certificate just happened to expire today in between the posting of my first and second comment. I didn't mention it only because I'm at work and couldn't be 100% certain that it's not my organizations security service messing with things.

While your comment is true, I'm not sure how it's relevant. `ToArray` is still operating on the `IEnumerable` produced by Select, which doesn't tell you that the Select is itself operating on an array. The CLR does do some type checking to try to propagate the size if it can, but that's by no means guaranteed. Edit: In skimming the source available on sourceof.net, it looks to me like the array doesn't actually get propagated far enough, so the construct in GP will in fact incur several unnecessary array copies.

munchbunny 8y ago
I'm curious what you saw. My understanding was that it's mostly generators, i.e. it's done through yields, not extra allocated arrays.
- 0xffff2 8y ago
  It's `ToArray` itself that's the problem. Since it doesn't know how big the array will be in the first place, it has to allocate a small array, start iterating, copy everything into a bigger array each time it runs out of space, and finally allocate an array of exactly the right size and copy everything into that. You can see the `ToArray` implementation at [0], which defers the implementation to [1]. The implementation checks for ICollection to get the exact size, but the type [2] that `Select` returns doesn't implement ICollection, so `ToArray` has to fall back to the less efficient algorithm. [0] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,783a052330e7d48d https://referencesource.microsoft.com/#System.Core/System/Li... [1] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,ed118118b642d9d4 https://referencesource.microsoft.com/#System.Core/System/Li... [2] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,cffd4efbf86815cb https://referencesource.microsoft.com/#System.Core/System/Li...
  - cm2187 8y ago
    That's more or less what I had in mind without being certain of the details. Was trying to open your links but I am served an expired tls certificate from Microsoft!
    - 0xffff2 8y ago
      Oddly enough it seems that the certificate just happened to expire today in between the posting of my first and second comment. I didn't mention it only because I'm at work and couldn't be 100% certain that it's not my organizations security service messing with things.