Proposal: Add Rune overload to StringBuilder.Append

[Joy-less]

An overload to StringBuilder.Append should be added that accepts Rune (see current overloads).

Here's an extension method that can be used as a workaround, and is how it could be implemented:

public static void AppendRune(this StringBuilder StringBuilder, Rune Rune) {
    Span<char> Chars = stackalloc char[2];
    int CharsWritten = Rune.EncodeToUtf16(Chars);
    StringBuilder.Append(Chars[..CharsWritten]);
}

The benefit of this is that, currently, using StringBuilder.Append with Rune causes the Rune to be cast to an object and converted to a string, causing an unnecessary string allocation, as you can see in the below benchmarks.

Benchmarks:

See code

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Text;

namespace RuneStringBuilderBenchmarks;

public class Program {
    public static void Main() {
        BenchmarkRunner.Run<Benchmarks>();
    }
}

[MemoryDiagnoser]
public class Benchmarks {
    private const int IterationCount = 10_000;

    [Benchmark]
    public void AppendRune() {
        StringBuilder StringBuilder = new();
        Rune Rune = (Rune)'a';
        for (int Counter = 0; Counter < IterationCount; Counter++) {
            StringBuilder.AppendRune(Rune);
        }
    }
    [Benchmark]
    public void AppendObject() {
        StringBuilder StringBuilder = new();
        Rune Rune = (Rune)'a';
        for (int Counter = 0; Counter < IterationCount; Counter++) {
            StringBuilder.Append(Rune);
        }
    }
    [Benchmark]
    public void AppendRuneSurrogatePair() {
        StringBuilder StringBuilder = new();
        Rune Rune = new('\uD83D', '\uDE00'); // 😀 (grinning face emoji)
        for (int Counter = 0; Counter < IterationCount; Counter++) {
            StringBuilder.AppendRune(Rune);
        }
    }
    [Benchmark]
    public void AppendObjectSurrogatePair() {
        StringBuilder StringBuilder = new();
        Rune Rune = new('\uD83D', '\uDE00'); // 😀 (grinning face emoji)
        for (int Counter = 0; Counter < IterationCount; Counter++) {
            StringBuilder.Append(Rune);
        }
    }
}

public static class Extensions {
    public static void AppendRune(this StringBuilder StringBuilder, Rune Rune) {
        Span<char> Chars = stackalloc char[2];
        int CharsWritten = Rune.EncodeToUtf16(Chars);
        StringBuilder.Append(Chars[..CharsWritten]);
    }
}

Specs:

BenchmarkDotNet v0.14.0, Windows 11 (10.0.26100.1742)
Intel Core i3-9100F CPU 3.60GHz, 1 CPU, 4 logical and 4 physical cores
.NET SDK 9.0.101
  [Host]     : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2 [AttachedDebugger]
  DefaultJob : .NET 9.0.0 (9.0.24.52809), X64 RyuJIT AVX2

Results:

Method	Mean	Error	StdDev	Gen0	Allocated
AppendRune	45.74 us	0.088 us	0.078 us	10.4980	32.4 KB
AppendObject	79.23 us	0.516 us	0.483 us	163.4521	501.15 KB
AppendRuneSurrogatePair	47.08 us	0.160 us	0.142 us	15.6250	48.09 KB
AppendObjectSurrogatePair	94.88 us	0.994 us	0.929 us	194.0918	594.97 KB

[MihaZupan]

public partial sealed class StringBuilder
{
    public StringBuilder Append(Rune value);
}

This API has already been approved in #27912, just not yet implemented.