Only materialize the requested range in StringBuilder.Append(StringBuilder, int, int)

[SimonCropp]

The StringBuilder.Append(StringBuilder, int, int) polyfill (active on .NET Framework / netstandard2.0) ended with target.Append(value.ToString(), startIndex, count), materializing the entire source builder as a string to append a slice of it. Appending small ranges out of a large builder allocated and copied the whole source each time.

It now appends value.ToString(startIndex, count), which only copies the requested range. string's Append(string, int, int) used to do the range check, so that check is made explicitly first (startIndex > value.Length - count throws ArgumentOutOfRangeException for startIndex, the same exception and parameter name as before). The earlier checks, the null handling and the count == 0 early return are untouched.

Measurements

net48, Release, x64, best of 3, 1,000 appends of a 16 char range out of a 1,000,000 char source builder:

before	after
1,054.65 ms	0.64 ms

(Before also allocated a 2 MB string per call.)

Tests

Added to PolyfillTests_StringBuilder.cs:

• StringBuilder_Append_StringBuilder_Ranges: start, middle, end, whole and empty ranges, checking the returned instance and that the source is unchanged
• StringBuilder_Append_StringBuilder_MultiChunkSource: ranges of a 30,000 char source built from many small appends, including ones spanning chunk boundaries
• StringBuilder_Append_StringBuilder_Self (appending a builder to itself)
• StringBuilder_Append_StringBuilder_OutOfRange: ranges past the end, including int.MaxValue for either argument
• StringBuilder_Append_StringBuilder_Null