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// 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.
#nullable enable
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 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.
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;
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();
return result;
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