c# - 在 “addValueFactory”中将TPL与ConcurrentDictionary一起使用会带来什么风险？ MSDN暗示线程问题

关于多线程环境中的addValueFactory的MSDN says the following:



There is a closed connect issue on this，但是似乎什么也没做。当我阅读以上摘录时，我猜测可能正在发生以下情况之一:

连续调用Update()方法，直到成功为止(可能会变成慢速/争用，应在高负载下避免。)

更新被放弃，应用程序认为更新成功。 (腐败)

数据在变量的线程本地副本上更新，而不在其他线程上更新。 (这可能是一小段时间，但是由于未记录在案，所以可能正在发生任何事情)

谁能解释这句话的意思？

第2部分

@Douglas共享了我修改的代码。有趣的是看到每个“添加”相对于每个“更新”被调用了多少次。输出很有趣，因为只有一个返回值中包含“加”值。尽管“添加”被多次调用，但其他所有"file"都被“更新”。

我想以某种方式阻止更新方法被调用。

        ConcurrentDictionary<int, string> numbers = new ConcurrentDictionary<int, string>();
        Parallel.For(0, 30, x =>
        {
           var returned = numbers.AddOrUpdate(1,
                i =>
                {
                    Console.WriteLine("addValueFactory has been called");
                    return i.ToString() + "AAA" + x;
                },
                (i, s) =>
                {
                    Console.WriteLine("updateValueFactory has been called");
                    return i.ToString() + "u" + x;
                });

           Console.WriteLine(returned);
        });

输出

addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
updateValueFactory has been called
addValueFactory has been called
updateValueFactory has been called
addValueFactory has been called
1u15
updateValueFactory has been called
addValueFactory has been called
1u24
updateValueFactory has been called
1u25
updateValueFactory has been called
updateValueFactory has been called
1AAA0
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u26
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u18
updateValueFactory has been called
1u19
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u21
1u20
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u22
updateValueFactory has been called
1u16
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u1
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u3
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u4
1u2
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u6
1u27
updateValueFactory has been called
updateValueFactory has been called
1u23
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u9
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u10
updateValueFactory has been called
1u12
updateValueFactory has been called
updateValueFactory has been called
1u17
1u7
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u28
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u11
updateValueFactory has been called
updateValueFactory has been called
1u8
updateValueFactory has been called
updateValueFactory has been called
1u5
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
1u29
1u13
updateValueFactory has been called
1u14

最佳答案

实际上，这意味着您的addValueFactory方法应该是线程安全的和幂等的，这意味着如果对同一值多次(可能同时)调用它，它不会给出错误的结果。

考虑以下人为的示例:

ConcurrentDictionary<int, string> numbers = new ConcurrentDictionary<int, string>();
Parallel.For(0, 10, x =>
{
    numbers.AddOrUpdate(1,
        i =>
        {
            Console.WriteLine("addValueFactory has been called");
            return i.ToString();
        },
        (i, s) =>
        {
            Console.WriteLine("updateValueFactory has been called");
            return i.ToString();
        });
});

如果AddOrUpdate方法完全同步，则您希望输出记录一个addValueFactory调用，然后是九个updateValueFactory调用。但是，由于提到了该子句，您可能会得到多个addValueFactory调用。

addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
addValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called
updateValueFactory has been called

在大多数情况下，这是一个良性竞争条件，尤其是因为大多数addValueFactory委托(delegate)人都不需要更改任何状态。