Implement ISpanFormattable to improve string interpolation performance

[sonnemaf]

Are you planning to implement the ISpanFormattable interface on the structs? This can improve the performance of string interpolation. It only works in .NET 6 so it requires some conditional compilation.

[Corniel]

I thought of it. The big downside I see, is that it will result in the formatting code being duplicated. I'm not sure yet if that is worth the small gain in performance. Do you have experience with it yourself?

[sonnemaf]

I did some comparisons and saw a 50% performance improvement. But it is a lot of work to write the TryFormat method correctly.

readonly record struct Point {

    public readonly int X, Y;

    public Point(int x, int y) => (X, Y) = (x, y);

    public override string ToString() => $"({X},{Y})";
}

readonly record struct Point : ISpanFormattable {
    public readonly int X, Y;
    public Point(int x, int y) => (X, Y) = (x, y);

    public bool TryFormat(Span<char> destination, out int charsWritten, ReadOnlySpan<char> format, IFormatProvider? provider) {
        destination[0] = '(';
        charsWritten = 1;
        destination = destination.Slice(1);
        if (!X.TryFormat(destination, out int tmpCharsWritten, format, provider)) {
            return false;
        }
        charsWritten += tmpCharsWritten;
        destination = destination.Slice(tmpCharsWritten);
        if (destination.Length < 3) { // not enough room to write the , Y and )
            return false;
        }
        destination[0] = ',';
        destination = destination.Slice(1);
        charsWritten++;
        if (!Y.TryFormat(destination, out tmpCharsWritten, format, provider)) {
            return false;
        }
        if (destination.Length < tmpCharsWritten + 1) { // not enough room to write the )
            return false;
        }
        destination[tmpCharsWritten] = ')';
        charsWritten += tmpCharsWritten + 1;
        return true;
    }

    public override string ToString() => $"({X},{Y})";

    public string ToString(string? format, IFormatProvider? formatProvider) => $"({X.ToString(format, formatProvider)},{Y.ToString(format, formatProvider)})";
}

You can use a simple approach using the MemoryExtensions.TryWrite(). This will give you a performance improvement of around 30%. This TryWrite uses the string interpolation syntax.

var p = new Point(3, 4);
Console.WriteLine($"Point {p}");

readonly record struct Point : ISpanFormattable {

    public readonly int X, Y;

    public Point(int x, int y) => (X, Y) = (x, y);

    public bool TryFormat(Span<char> destination, out int charsWritten,
                          ReadOnlySpan<char> format, IFormatProvider? provider) =>
        destination.TryWrite($"({X},{Y})", out charsWritten);

    public override string ToString() => $"({X},{Y})";

    public string ToString(string? format, IFormatProvider? formatProvider) =>
        $"({X.ToString(format, formatProvider)},{Y.ToString(format, formatProvider)})";

}

[Corniel]

We need 26 of them, and most of them are way more complex than this Point implementation. :(

[sonnemaf]

Maybe you can put it on the backlog