mono/netcore/System.Private.CoreLib/shared/System/Text/StringBuilderCache.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

namespace System.Text
{
    /// <summary>Provide a cached reusable instance of stringbuilder per thread.</summary>
    internal static class StringBuilderCache
    {
        // The value 360 was chosen in discussion with performance experts as a compromise between using
        // as litle memory per thread as possible and still covering a large part of short-lived
        // StringBuilder creations on the startup path of VS designers.
        private const int MaxBuilderSize = 360;
        private const int DefaultCapacity = 16; // == StringBuilder.DefaultCapacity

        // WARNING: We allow diagnostic tools to directly inspect this member (t_cachedInstance).
        // See https://github.com/dotnet/corert/blob/master/Documentation/design-docs/diagnostics/diagnostics-tools-contract.md for more details. 
        // Please do not change the type, the name, or the semantic usage of this member without understanding the implication for tools. 
        // Get in touch with the diagnostics team if you have questions.
        [ThreadStatic]
        private static StringBuilder t_cachedInstance;

        /// <summary>Get a StringBuilder for the specified capacity.</summary>
        /// <remarks>If a StringBuilder of an appropriate size is cached, it will be returned and the cache emptied.</remarks>
        public static StringBuilder Acquire(int capacity = DefaultCapacity)
        {
            if (capacity <= MaxBuilderSize)
            {
                StringBuilder sb = t_cachedInstance;
                if (sb != null)
                {
                    // Avoid stringbuilder block fragmentation by getting a new StringBuilder
                    // when the requested size is larger than the current capacity
                    if (capacity <= sb.Capacity)
                    {
                        t_cachedInstance = null;
                        sb.Clear();
                        return sb;
                    }
                }
            }
            return new StringBuilder(capacity);
        }

        /// <summary>Place the specified builder in the cache if it is not too big.</summary>
        public static void Release(StringBuilder sb)
        {
            if (sb.Capacity <= MaxBuilderSize)
            {
                t_cachedInstance = sb;
            }
        }

        /// <summary>ToString() the stringbuilder, Release it to the cache, and return the resulting string.</summary>
        public static string GetStringAndRelease(StringBuilder sb)
        {
            string result = sb.ToString();
            Release(sb);
            return result;
        }
    }
}