Copy in blocks in StringBuilder.CopyTo(int, Span<char>, int)

[SimonCropp]

The StringBuilder.CopyTo(int, Span<char>, int) polyfill (active on .NET Framework / netstandard2.0, where the BCL has no span overload) copied one char at a time through the StringBuilder indexer. On those runtimes the indexer walks the chunk list from the end for every call, so the copy costs O(count x chunks).

It now copies in blocks of up to 16,384 chars: the BCL's own CopyTo(int, char[], int, int) fills a buffer rented from ArrayPool<char>.Shared, and the buffer is copied to the destination span. Argument validation and the exceptions thrown are untouched, and nothing is rented when count is 0.

Measurements

net48, Release, x64, best of 3, a 1,000,000 char builder made of 100,000 appends, copied in full:

before	after
77.04 ms	0.17 ms

Tests

Added to PolyfillTests_StringBuilder.cs:

• StringBuilderCopyTo_MultiChunk: a 50,000 char builder assembled from many small appends (so it has many chunks), with 14 sourceIndex/count combinations either side of the 16,384 block size (16,383 / 16,384 / 16,385, two blocks, two blocks plus one), ranges that start mid-builder and end at the last char, and zero-length copies at the middle and the very end. Each case checks the copied text against ToString().Substring(...), that nothing past count is written to a larger destination, and that the builder is unchanged.
• StringBuilderCopyTo_ExactDestination, StringBuilderCopyTo_Empty
• the existing StringBuilderCopyTo_InvalidArgs still covers the exceptions