IfTrue/IfFalse interpolated string handler

ConditionalNestedInterpolation.cs

using System.Globalization;
using System.Runtime.CompilerServices;

Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;

static string m(bool condition) => string.Create(
    CultureInfo.GetCultureInfo("fr-fr"), 
    $"direct: {1.5} or {1:c}, nested: {If(condition, $"{1.5}", $"{1:C}")}");

Console.WriteLine(m(true));
Console.WriteLine(m(false));

static string If(bool condition,
    [InterpolatedStringHandlerArgument(nameof(condition))] IfTrueInterpolatedStringHandler thenHandler,
    [InterpolatedStringHandlerArgument(nameof(condition))] IfFalseInterpolatedStringHandler elseHandler)
    => condition ? thenHandler.ToStringAndClear() : elseHandler.ToStringAndClear();

[InterpolatedStringHandler]
public ref struct IfTrueInterpolatedStringHandler
{
    private DefaultInterpolatedStringHandler _inner;

    public IfTrueInterpolatedStringHandler(int literalLength, int formattedCount, bool condition, out bool shouldAppend)
    {
        shouldAppend = condition;
        if (!condition) return;
        _inner = new DefaultInterpolatedStringHandler(literalLength, formattedCount);
    }

    public IfTrueInterpolatedStringHandler(int literalLength, int formattedCount, bool condition, out bool shouldAppend, IFormatProvider? provider)
    {
        shouldAppend = condition;
        if (!condition) return;
        _inner = new DefaultInterpolatedStringHandler(literalLength, formattedCount, provider);
    }

    public string ToStringAndClear() => _inner.ToStringAndClear();

    public void AppendLiteral(string value) => _inner.AppendLiteral(value);
    public void AppendFormatted(string? value) => _inner.AppendFormatted(value);
    public void AppendFormatted(scoped ReadOnlySpan<char> value) => _inner.AppendFormatted(value);
    public void AppendFormatted(object? value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted(string? value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted(scoped ReadOnlySpan<char> value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted<T>(T value) => _inner.AppendFormatted(value);
    public void AppendFormatted<T>(T value, string? format) => _inner.AppendFormatted(value, format);
    public void AppendFormatted<T>(T value, int align) => _inner.AppendFormatted(value, align);
    public void AppendFormatted<T>(T value, int align, string? format) => _inner.AppendFormatted(value, align, format);
}

[InterpolatedStringHandler]
public ref struct IfFalseInterpolatedStringHandler
{
    private DefaultInterpolatedStringHandler _inner;

    public IfFalseInterpolatedStringHandler(int literalLength, int formattedCount, bool condition, out bool shouldAppend)
    {
        shouldAppend = !condition;
        if (condition) return;
        _inner = new DefaultInterpolatedStringHandler(literalLength, formattedCount);
    }

    public IfFalseInterpolatedStringHandler(int literalLength, int formattedCount, bool condition, out bool shouldAppend, IFormatProvider? provider)
    {
        shouldAppend = !condition;
        if (condition) return;
        _inner = new DefaultInterpolatedStringHandler(literalLength, formattedCount, provider);
    }

    public string ToStringAndClear() => _inner.ToStringAndClear();

    public void AppendLiteral(string value) => _inner.AppendLiteral(value);
    public void AppendFormatted(string? value) => _inner.AppendFormatted(value);
    public void AppendFormatted(scoped ReadOnlySpan<char> value) => _inner.AppendFormatted(value);
    public void AppendFormatted(object? value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted(string? value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted(scoped ReadOnlySpan<char> value, int align = 0, string? format = null) => _inner.AppendFormatted(value, align, format);
    public void AppendFormatted<T>(T value) => _inner.AppendFormatted(value);
    public void AppendFormatted<T>(T value, string? format) => _inner.AppendFormatted(value, format);
    public void AppendFormatted<T>(T value, int align) => _inner.AppendFormatted(value, align);
    public void AppendFormatted<T>(T value, int align, string? format) => _inner.AppendFormatted(value, align, format);
}