StringBuilderPool.cs

using System;
using System.Collections.Concurrent;
using System.Text;

internal static class StringBuilderPool
{
    private static readonly ConcurrentQueue<PooledStringBuilder> Cache = new ConcurrentQueue<PooledStringBuilder>();

    internal static PooledStringBuilder Borrow()
    {
        if (Cache.TryDequeue(out var item))
        {
            return item;
        }

        return new PooledStringBuilder();
    }

    internal static string Return(this PooledStringBuilder stringBuilder)
    {
        var text = stringBuilder.GetTextAndClear();
        Cache.Enqueue(stringBuilder);
        return text;
    }

    internal class PooledStringBuilder
    {
        private readonly StringBuilder inner = new StringBuilder();

        public void Clear() => this.inner.Clear();

        public PooledStringBuilder AppendLine(string text)
        {
            this.inner.AppendLine(text);
            return this;
        }

        public PooledStringBuilder AppendLine()
        {
            this.inner.AppendLine();
            return this;
        }

        [Obsolete("Use StringBuilderPool.Return", true)]
#pragma warning disable CS0809 // Obsolete member overrides non-obsolete member
        public override string ToString() => throw new InvalidOperationException("Use StringBuilderPool.Return");
#pragma warning restore CS0809 // Obsolete member overrides non-obsolete member

        public string GetTextAndClear()
        {
            var text = this.inner.ToString();
            this.inner.Clear();
            return text;
        }
    }
}