解决多线程dic.Add报错数组越界问题

最新推荐文章于 2022-07-04 10:15:40 发布

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最新推荐文章于 2022-07-04 10:15:40 发布

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本文链接：https://blog.youkuaiyun.com/a1234012340a/article/details/109285308

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本文深入分析了.NET中Dictionary引发的IndexOutOfRangeException异常原因，并探讨了在多线程环境下使用Dictionary的潜在问题及解决方案。

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报错内容：

Unhandled Exception: System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at P2PSocket.Server.P2PServer.<>c__DisplayClass13_1.<AcceptSocket_ClientName>b__6() in E:\启梦\源码\P2PSocket\20201020\src\P2PSocket.Server\P2PServer.cs:line 213
   at P2PSocket.Core.Utils.EasyOp.Do(Action func, Action successHandle, Action`1 errorHandle) in E:\启梦\源码\P2PSocket\20201020\src\P2PSocekt.Core\Utils\EasyOp.cs:line 35
   at P2PSocket.Server.P2PServer.<>c__DisplayClass13_0.<AcceptSocket_ClientName>b__1() in E:\启梦\源码\P2PSocket\20201020\src\P2PSocket.Server\P2PServer.cs:line 203
   at P2PSocket.Core.Utils.EasyOp.Do(Action func, Action successHandle, Action`1 errorHandle) in E:\启梦\源码\P2PSocket\20201020\src\P2PSocekt.Core\Utils\EasyOp.cs:line 35
   at P2PSocket.Server.P2PServer.AcceptSocket_ClientName(IAsyncResult ar) in E:\启梦\源码\P2PSocket\20201020\src\P2PSocket.Server\P2PServer.cs:line 188
   at System.Net.LazyAsyncResult.Complete(IntPtr userToken)
   at System.Net.ContextAwareResult.CompleteCallback()
   at System.Net.ContextAwareResult.<>c.<Complete>b__15_0(Object s)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.ContextAwareResult.Complete(IntPtr userToken)
   at System.Net.LazyAsyncResult.ProtectedInvokeCallback(Object result, IntPtr userToken)
   at System.Net.Sockets.BaseOverlappedAsyncResult.CompletionCallback(Int32 numBytes, SocketError errorCode)
   at System.Net.Sockets.AcceptOverlappedAsyncResult.CompletionCallback(IntPtr acceptedFileDescriptor, Byte[] socketAddress, Int32 socketAddressLen, SocketError errorCode)
   at System.Net.Sockets.SocketAsyncContext.AcceptOperation.InvokeCallback(Boolean allowPooling)
   at System.Net.Sockets.SocketAsyncContext.O

其实我们平时使用Dictionary无非就用Add、Remove这样的方法，根本没有考虑过内部实现的机制。在Dictionary内部为了维护Dictionary的功能和高效的特性，有自己的一些计数器和状态维护机制。Dictionary.Add方法实际上里头只有一句话：this.Insert(key, value, true);也就是最终的实现都是在Insert方法里的。再用Reflector扒开Insert方法里的内容看看：

private void Insert(TKey key, TValue value, bool add)
{
    int freeList;
    if (key == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
    }
    if (this.buckets == null)
    {
        this.Initialize(0);
    }
    int num = this.comparer.GetHashCode(key) & 0x7fffffff;
    int index = num % this.buckets.Length;
    for (int i = this.buckets[index]; i >= 0; i = this.entries[i].next)
    {
        if ((this.entries[i].hashCode == num) && this.comparer.Equals(this.entries[i].key, key))
        {
            if (add)
            {
                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
            }
            this.entries[i].value = value;
            this.version++;
            return;
        }
    }
    if (this.freeCount > 0)
    {
        freeList = this.freeList;
        this.freeList = this.entries[freeList].next;
        this.freeCount--;
    }
    else
    {
        if (this.count == this.entries.Length)
        {
            this.Resize();
            index = num % this.buckets.Length;
        }
        freeList = this.count;
        this.count++;
    }
    this.entries[freeList].hashCode = num;
    this.entries[freeList].next = this.buckets[index];
    this.entries[freeList].key = key;
    this.entries[freeList].value = value;
    this.buckets[index] = freeList;
    this.version++;
}

在这里可以看到有大量的计数器存在，而我们再来倒回头看看最开始抛出的异常对象：IndexOutOfRangeException，如果计数器出错，相当有可能在使用计数器做下标时出现下标越界的情况。那么这是.Net的Bug么？

在上面引用MSDN的时候微软已经明确说了，在多线程访问的时候不要使用Dictionary而应该使用ConCurrentDictionay，利用ConCurrentDictionay里的TryAdd、TryUpdate方法来避免出现类似的错误。

其实多线程并发计算的时候，经常会出现计数错误的情况。