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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.
/*============================================================
**
**
** Purpose: provide a cached reusable instance of stringbuilder
** per thread it's an optimisation that reduces the
** number of instances constructed and collected.
**
** Acquire - is used to get a string builder to use of a
** particular size. It can be called any number of
** times, if a stringbuilder is in the cache then
** it will be returned and the cache emptied.
** subsequent calls will return a new stringbuilder.
**
** A StringBuilder instance is cached in
** Thread Local Storage and so there is one per thread
**
** Release - Place the specified builder in the cache if it is
** not too big.
** The stringbuilder should not be used after it has
** been released.
** Unbalanced Releases are perfectly acceptable. It
** will merely cause the runtime to create a new
** stringbuilder next time Acquire is called.
**
** GetStringAndRelease
** - ToString() the stringbuilder, Release it to the
** cache and return the resulting string
**
===========================================================*/
using System.Threading;
namespace System.Text
{
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 MAX_BUILDER_SIZE = 360;
[ThreadStatic]
private static StringBuilder CachedInstance;
public static StringBuilder Acquire(int capacity = StringBuilder.DefaultCapacity)
{
if(capacity <= MAX_BUILDER_SIZE)
{
StringBuilder sb = StringBuilderCache.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)
{
StringBuilderCache.CachedInstance = null;
sb.Clear();
return sb;
}
}
}
return new StringBuilder(capacity);
}
public static void Release(StringBuilder sb)
{
if (sb.Capacity <= MAX_BUILDER_SIZE)
{
StringBuilderCache.CachedInstance = sb;
}
}
public static string GetStringAndRelease(StringBuilder sb)
{
string result = sb.ToString();
Release(sb);
return result;
}
}
}
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