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lang: ja-jp
breaks: true
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# C# スレッド単位 `[ThreadStatic]` で静的な `StringBuilder` のキャッシュ機構 2022-07-24

## Dapper

> DapperLib/Dapper
> https://github.com/DapperLib/Dapper
> https://github.com/DapperLib/Dapper/blob/ca00feeb5fafe5262166689c0bec2b80b53add4e/Dapper/SqlMapper.cs#L3796

// one per thread
[ThreadStatic]
private static StringBuilder perThreadStringBuilderCache;
private static StringBuilder GetStringBuilder()
{
    var tmp = perThreadStringBuilderCache;
    if (tmp != null)
    {
        perThreadStringBuilderCache = null;
        tmp.Length = 0;
        return tmp;
    }
    return new StringBuilder();
}

private static string ToStringRecycle(this StringBuilder obj)
{
    if (obj == null) return "";
    var s = obj.ToString();
    perThreadStringBuilderCache ??= obj;
    return s;
}

## .NET Core

> Reducing allocations by caching with StringBuilderCache
> https://andrewlock.net/a-deep-dive-on-stringbuilder-part-5-reducing-allocations-by-caching-stringbuilders-with-stringbuildercache/

> StringBuilderCache.cs
> https://github.com/dotnet/runtime/blob/main/src/libraries/Common/src/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.

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 little memory per thread as possible and still covering a large part of short-lived
        // StringBuilder creations on the startup path of VS designers.
        internal const int MaxBuilderSize = 360;
        private const int DefaultCapacity = 16; // == StringBuilder.DefaultCapacity

        [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;
        }
    }
}

###### tags: `C#` `ThreadStatic` `StringBuilder` `キャッシュ` `ThreadStatic`