public abstract class ReadFreeCache<TKey, TValue>
{
    
protected ReadFreeCache()
    
protected ReadFreeCache(IEqualityComparer<TKey> comparer)
        
this.m_storage = new Dictionary<TKey, TValue>(comparer);
    
public abstract TValue Create(TKey key);
    
private Dictionary<TKey, TValue> m_storage;
    
private readonly object m_writeLock = new object();
    
public TValue Get(TKey key)
        
if (this.m_storage.TryGetValue(key, out value))
            
if (this.m_storage.TryGetValue(key, out value))
            
value = this.Create(key);
            
var newStorage = this.m_storage.ToDictionary(
                
this.m_storage.Comparer);
            
newStorage.Add(key, value);
            
this.m_storage = newStorage;
}

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public abstract class ReadWriteCache<TKey, TValue>
{
    
protected ReadWriteCache()
    
protected ReadWriteCache(IEqualityComparer<TKey> comparer)
        
this.m_storage = new Dictionary<TKey, TValue>(comparer);
    
private readonly Dictionary<TKey, TValue> m_storage;
    
private readonly ReaderWriterLockSlim m_rwLock = new ReaderWriterLockSlim();
    
protected abstract TValue Create(TKey key);
    
public TValue Get(TKey key)
        
this.m_rwLock.EnterReadLock();
            
if (this.m_storage.TryGetValue(key, out value))
            
this.m_rwLock.ExitReadLock();
        
this.m_rwLock.EnterWriteLock();
            
if (this.m_storage.TryGetValue(key, out value))
            
value = this.Create(key);
            
this.m_storage.Add(key, value);
            
this.m_rwLock.ExitWriteLock();
}