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// Copyright (c) Microsoft Corporation. All rights reserved. See License.txt in the project root for license information.
using System.Collections.Generic;
using System.Diagnostics.CodeAnalysis;
using System.Threading;
namespace System.Web.Mvc
{
[SuppressMessage("Microsoft.Design", "CA1001:TypesThatOwnDisposableFieldsShouldBeDisposable", Justification = "Instances of this type are meant to be singletons.")]
internal abstract class ReaderWriterCache<TKey, TValue>
{
private readonly Dictionary<TKey, TValue> _cache;
private readonly ReaderWriterLockSlim _readerWriterLock = new ReaderWriterLockSlim();
protected ReaderWriterCache()
: this(null)
{
}
protected ReaderWriterCache(IEqualityComparer<TKey> comparer)
{
_cache = new Dictionary<TKey, TValue>(comparer);
}
protected Dictionary<TKey, TValue> Cache
{
get { return _cache; }
}
protected TValue FetchOrCreateItem(TKey key, Func<TValue> creator)
{
// first, see if the item already exists in the cache
_readerWriterLock.EnterReadLock();
try
{
TValue existingEntry;
if (_cache.TryGetValue(key, out existingEntry))
{
return existingEntry;
}
}
finally
{
_readerWriterLock.ExitReadLock();
}
// insert the new item into the cache
TValue newEntry = creator();
_readerWriterLock.EnterWriteLock();
try
{
TValue existingEntry;
if (_cache.TryGetValue(key, out existingEntry))
{
// another thread already inserted an item, so use that one
return existingEntry;
}
_cache[key] = newEntry;
return newEntry;
}
finally
{
_readerWriterLock.ExitWriteLock();
}
}
}
}
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