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As can be seen here, one of String.Join's overloads works with raw pointers and uses something called UnSafeCharBuffer. Why is this? Is it a performance optimization?

NetherGranite
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    Yes; that's for performance. All of `string` is very heavily optimized. – SLaks Jan 17 '19 at 21:35
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    `StringBuilder` still has to copy to a string as the final step. Writing directly to the return string skips that overhead. – Ry- Jan 17 '19 at 21:37
  • Also `StringBuilder` has a default size that may not be entirely consumed, and continues to increase by that buffer every time you exceed. So three or four smaller strings being created is still a lot faster. – Greg Jan 17 '19 at 21:53
  • @Greg: Three or four smaller strings? Anyway, you can create a `StringBuilder` with an exact initial (and maximum) capacity. – Ry- Jan 17 '19 at 21:54
  • @Ry- But that still is converted into a `string`. So `StringBuilder` will consume a percentage of memory defined, just to consume more memory when it becomes a `string`. Boils down to this http://www.yoda.arachsys.com/csharp/stringbuilder.html – Greg Jan 17 '19 at 22:01
  • @Greg: Correct. What do you mean by “three or four smaller strings” though? – Ry- Jan 18 '19 at 00:25
  • That smaller immutable strings consume less memory than the overhead generated by StringBuilder often. – Greg Jan 18 '19 at 06:28

1 Answers1

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Is a performance optimization?

Yes.

In general you should expect that unsafe code is either for low-level unmanaged language interop or for performance optimization. In this case it is the latter.

This then suggests the question:

Why not use the same techniques for StringBuilder?

Different scenarios can be tuned using different optimization techniques; StringBuilders are optimized for their scenarios.

The scenarios are different in several ways. Join knows ahead of time exactly how many bytes will be returned; StringBuilder does not. Join knows that the resulting string will be generated exactly once, but a StringBuilder has to support the create, append, ToString, append, ToString, ... workflow efficiently. And so on.

Eric Lippert
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  • but there are three other overloads/implementations of `String.Join` that actually use `StringBuilder`... You're only talking about a specific one. – adjan Jan 17 '19 at 22:22
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    @Adrian: I made the reasonable assumption that the original poster was talking about the implementation that does not use a stringbuilder, because the question was about that implementation, and the question was "Why doesn't C#'s String.Join use StringBuilder?" Why would anyone assume that the OP was asking about an implementation that does use StringBuilder, given that question? – Eric Lippert Jan 17 '19 at 22:26
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    I agree one can infer this from the question, although i would say that it (especially the title) sounds like there was only *this one* implementation, and no other – adjan Jan 17 '19 at 22:33
  • @Adrian I agree that the title is a little misleading. I'll revise it. – NetherGranite Jan 18 '19 at 01:46