I would like to implement a simple in-memory LRU cache system and I was thinking about a solution based on an IDictionary implementation which could handle an hashed LRU mechanism.
Coming from java, I have experiences with LinkedHashMap, which works fine for what I need: I can't find anywhere a similar solution for .NET.
Has anyone developed it or has anyone had experiences like this?
There is nothing in the base class libraries that does this.
On the free side, maybe something like C5's HashedLinkedList would work.
If you're willing to pay, maybe check out this C# toolkit. It contains an implementation.
This a very simple an fast implementation we developed for a web site we own.
We try to improve the code as much as possible but keeping it thread safe. I think the code is very simple and clear, but if you need some explanation or a guide related to how to use it, don't hesitate to ask.
namespace LRUCache
{
public class LRUCache<K,V>
{
private int capacity;
private Dictionary<K, LinkedListNode<LRUCacheItem<K, V>>> cacheMap = new Dictionary<K, LinkedListNode<LRUCacheItem<K, V>>>();
private LinkedList<LRUCacheItem<K, V>> lruList = new LinkedList<LRUCacheItem<K, V>>();
public LRUCache(int capacity)
{
this.capacity = capacity;
}
[MethodImpl(MethodImplOptions.Synchronized)]
public V get(K key)
{
LinkedListNode<LRUCacheItem<K, V>> node;
if (cacheMap.TryGetValue(key, out node))
{
V value = node.Value.value;
lruList.Remove(node);
lruList.AddLast(node);
return value;
}
return default(V);
}
[MethodImpl(MethodImplOptions.Synchronized)]
public void add(K key, V val)
{
if (cacheMap.Count >= capacity)
{
RemoveFirst();
}
LRUCacheItem<K, V> cacheItem = new LRUCacheItem<K, V>(key, val);
LinkedListNode<LRUCacheItem<K, V>> node = new LinkedListNode<LRUCacheItem<K, V>>(cacheItem);
lruList.AddLast(node);
cacheMap.Add(key, node);
}
private void RemoveFirst()
{
// Remove from LRUPriority
LinkedListNode<LRUCacheItem<K,V>> node = lruList.First;
lruList.RemoveFirst();
// Remove from cache
cacheMap.Remove(node.Value.key);
}
}
class LRUCacheItem<K,V>
{
public LRUCacheItem(K k, V v)
{
key = k;
value = v;
}
public K key;
public V value;
}
}
Found you answer while googling, also found this:
http://code.google.com/p/csharp-lru-cache/
csharp-lru-cache: LRU cache collection class library
This is a collection class that functions as a least-recently-used cache. It implements
ICollection<T>, but also exposes three other members:
Capacity, the maximum number of items the cache can contain. Once the collection is at capacity, adding a new item to the cache will cause the least recently used item to be discarded. If the Capacity is set to 0 at construction, the cache will not automatically discard items.Oldest, the oldest (i.e. least recently used) item in the collection.DiscardingOldestItem, an event raised when the cache is about to discard its oldest item. This is an extremely simple implementation. While its Add and Remove methods are thread-safe, it shouldn't be used in heavy multithreading environments because the entire collection is locked during those methods.
I've recently released a class called LurchTable to address the need for a C# variant of the LinkedHashMap. A brief discussion of the LurchTable can be found here.
Basic features:
Source Code: http://csharptest.net/browse/src/Library/Collections/LurchTable.cs
GitHub: https://github.com/csharptest/CSharpTest.Net.Collections
HTML Help: http://help.csharptest.net/
PM> Install-Package CSharpTest.Net.Collections
This takes Martin's code with Mr T's suggestions and makes it Stylecop friendly. Oh, it also allows for disposal of values as they cycle out of the cache.
namespace LruCache
{
using System;
using System.Collections.Generic;
/// <summary>
/// A least-recently-used cache stored like a dictionary.
/// </summary>
/// <typeparam name="TKey">
/// The type of the key to the cached item
/// </typeparam>
/// <typeparam name="TValue">
/// The type of the cached item.
/// </typeparam>
/// <remarks>
/// Derived from https://stackoverflow.com/a/3719378/240845
/// </remarks>
public class LruCache<TKey, TValue>
{
private readonly Dictionary<TKey, LinkedListNode<LruCacheItem>> cacheMap =
new Dictionary<TKey, LinkedListNode<LruCacheItem>>();
private readonly LinkedList<LruCacheItem> lruList =
new LinkedList<LruCacheItem>();
private readonly Action<TValue> dispose;
/// <summary>
/// Initializes a new instance of the <see cref="LruCache{TKey, TValue}"/>
/// class.
/// </summary>
/// <param name="capacity">
/// Maximum number of elements to cache.
/// </param>
/// <param name="dispose">
/// When elements cycle out of the cache, disposes them. May be null.
/// </param>
public LruCache(int capacity, Action<TValue> dispose = null)
{
this.Capacity = capacity;
this.dispose = dispose;
}
/// <summary>
/// Gets the capacity of the cache.
/// </summary>
public int Capacity { get; }
/// <summary>Gets the value associated with the specified key.</summary>
/// <param name="key">
/// The key of the value to get.
/// </param>
/// <param name="value">
/// When this method returns, contains the value associated with the specified
/// key, if the key is found; otherwise, the default value for the type of the
/// <paramref name="value" /> parameter. This parameter is passed
/// uninitialized.
/// </param>
/// <returns>
/// true if the <see cref="T:System.Collections.Generic.Dictionary`2" />
/// contains an element with the specified key; otherwise, false.
/// </returns>
public bool TryGetValue(TKey key, out TValue value)
{
lock (this.cacheMap)
{
LinkedListNode<LruCacheItem> node;
if (this.cacheMap.TryGetValue(key, out node))
{
value = node.Value.Value;
this.lruList.Remove(node);
this.lruList.AddLast(node);
return true;
}
value = default(TValue);
return false;
}
}
/// <summary>
/// Looks for a value for the matching <paramref name="key"/>. If not found,
/// calls <paramref name="valueGenerator"/> to retrieve the value and add it to
/// the cache.
/// </summary>
/// <param name="key">
/// The key of the value to look up.
/// </param>
/// <param name="valueGenerator">
/// Generates a value if one isn't found.
/// </param>
/// <returns>
/// The requested value.
/// </returns>
public TValue Get(TKey key, Func<TValue> valueGenerator)
{
lock (this.cacheMap)
{
LinkedListNode<LruCacheItem> node;
TValue value;
if (this.cacheMap.TryGetValue(key, out node))
{
value = node.Value.Value;
this.lruList.Remove(node);
this.lruList.AddLast(node);
}
else
{
value = valueGenerator();
if (this.cacheMap.Count >= this.Capacity)
{
this.RemoveFirst();
}
LruCacheItem cacheItem = new LruCacheItem(key, value);
node = new LinkedListNode<LruCacheItem>(cacheItem);
this.lruList.AddLast(node);
this.cacheMap.Add(key, node);
}
return value;
}
}
/// <summary>
/// Adds the specified key and value to the dictionary.
/// </summary>
/// <param name="key">
/// The key of the element to add.
/// </param>
/// <param name="value">
/// The value of the element to add. The value can be null for reference types.
/// </param>
public void Add(TKey key, TValue value)
{
lock (this.cacheMap)
{
if (this.cacheMap.Count >= this.Capacity)
{
this.RemoveFirst();
}
LruCacheItem cacheItem = new LruCacheItem(key, value);
LinkedListNode<LruCacheItem> node =
new LinkedListNode<LruCacheItem>(cacheItem);
this.lruList.AddLast(node);
this.cacheMap.Add(key, node);
}
}
private void RemoveFirst()
{
// Remove from LRUPriority
LinkedListNode<LruCacheItem> node = this.lruList.First;
this.lruList.RemoveFirst();
// Remove from cache
this.cacheMap.Remove(node.Value.Key);
// dispose
this.dispose?.Invoke(node.Value.Value);
}
private class LruCacheItem
{
public LruCacheItem(TKey k, TValue v)
{
this.Key = k;
this.Value = v;
}
public TKey Key { get; }
public TValue Value { get; }
}
}
}
The Caching Application Block of EntLib has an LRU scavenging option out of the box and can be in memory. It might be a bit heavyweight for what you want tho.
I don't believe so. I've certainly seen hand-rolled ones implemented several times in various unrelated projects (which more or less confirms this. If there was one, surely at least one of the projects would have used it).
It's pretty simple to implement, and usually gets done by creating a class which contains both a Dictionary and a List.
The keys go in the list (in-order) and the items go in the dictionary.
When you Add a new item to the collection, the function checks the length of the list, pulls out the last Key (if it's too long) and then evicts the key and value from the dictionary to match. Not much more to it really
I like Lawrence's implementation. Hashtable + LinkedList is a good solution. Regarding threading I would not lock this [MethodImpl(MethodImplOptions.Synchronized)], but rather using ReaderWriterLockSlim or spin lock (since contention usually fast) instead. In get function I would check if it's already the 1st item first, rather than always removing and adding. This gives you possibility to keep within a reader lock that not blocking other readers.
I've created an implementation of the LRU cache
https://github.com/mohsenShakiba/LRUCache
If it's an asp.net app you can use the cache class[1] but you'll be competing for space with other cached stuff, which may be what you want or may not be.
[1] http://msdn.microsoft.com/en-us/library/system.web.caching.cache.aspx
