如何实现一个线程安全的 ConcurrentHashSet ?

Posted 2022-04-14 dotNET跨平台

咨询区

• Sebastian

在 .NET 框架中并没有线程安全的 ConcurrentHashSet 类，我想模仿 ConcurrentDictionary 来实现一个，目前写了一下桩代码。

public class ConcurrentHashSet<TElement> : ISet<TElement>

    private readonly ConcurrentDictionary<TElement, object> _internal;

    public ConcurrentHashSet(IEnumerable<TElement> elements = null)
    
        _internal = new ConcurrentDictionary<TElement, object>();
        if (elements != null)
            UnionWith(elements);
    

    public void UnionWith(IEnumerable<TElement> other)
    
        if (other == null) throw new ArgumentNullException("other");

        foreach (var otherElement in other)
            Add(otherElement);
    

    public bool Add(TElement item)
    
        return _internal.TryAdd(item, null);
    

    // I am not sure here if that fullfills contract correctly
    void ICollection<TElement>.Add(TElement item)
    
        Add(item);
    

    public bool Contains(TElement item)
    
        return _internal.ContainsKey(item);
    

    public bool Remove(TElement item)
    
        object ignore;
        return _internal.TryRemove(item, out ignore);
    

    public int Count
    
        get  return _internal.Count; 
    

    public IEnumerator<TElement> GetEnumerator()
    
        return _internal.Keys.GetEnumerator();
    

我不确定这代码能否在 foreach 中还能保证原子性，请问是否有更好的实现？

回答区

• Ben Mosher

我最近遇到了类似的场景，不过我更关注的更高性能的 Add,Contains,Remove 方法，下面是我的实现。

using System.Collections.Generic;
using System.Threading;

namespace BlahBlah.Utilities

    public class ConcurrentHashSet<T> : IDisposable
    
        private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);
        private readonly HashSet<T> _hashSet = new HashSet<T>();

        #region Implementation of ICollection<T> ...ish
        public bool Add(T item)
        
            try
            
                _lock.EnterWriteLock();
                return _hashSet.Add(item);
            
            finally
            
                if (_lock.IsWriteLockHeld) _lock.ExitWriteLock();
            
        

        public void Clear()
        
            try
            
                _lock.EnterWriteLock();
                _hashSet.Clear();
            
            finally
            
                if (_lock.IsWriteLockHeld) _lock.ExitWriteLock();
            
        

        public bool Contains(T item)
        
            try
            
                _lock.EnterReadLock();
                return _hashSet.Contains(item);
            
            finally
            
                if (_lock.IsReadLockHeld) _lock.ExitReadLock();
            
        

        public bool Remove(T item)
        
            try
            
                _lock.EnterWriteLock();
                return _hashSet.Remove(item);
            
            finally
            
                if (_lock.IsWriteLockHeld) _lock.ExitWriteLock();
            
        

        public int Count
        
            get
            
                try
                
                    _lock.EnterReadLock();
                    return _hashSet.Count;
                
                finally
                
                    if (_lock.IsReadLockHeld) _lock.ExitReadLock();
                
            
        
        #endregion

        #region Dispose
        public void Dispose()
        
            if (_lock != null) _lock.Dispose();
        
        #endregion
    

点评区

实现一个 ConcurrentSet<T> 其实蛮有必要的，你在 github 或者其他第三方网站上会找到很多类似的 ConcurrentSet<T> 的实现，比如：https://pastebin.com/8REHRFFL