代码之家 › 专栏 › 技术社区 › Venr

CastException尝试异步调用Action<KeyValuePair<>>委托

begininvoke c#

Venr · 技术社区 · 17 年前

var item = new KeyValuePair<string, string>("key", "value");

Action<KeyValuePair<string, string>> kvrAction = 
    kvr =>Console.WriteLine(kvr.Value);

var result = kvrAction.BeginInvoke(item, null, null);
kvrAction.EndInvoke(result);

Test method Utilities.Tests.IEnumerableExtensionTests.ProveDelegateAsyncInvokeFailsForKeyValuePair threw exception:  System.Runtime.Remoting.RemotingException: The argument type '[key, value]' cannot be converted into parameter type 'System.Collections.Generic.KeyValuePair`2[System.String,System.String]'.
--->  System.InvalidCastException: Object must implement IConvertible..

任何帮助都将不胜感激=)除了KeyValuePair之外,这段代码似乎适用于我扔给它的任何东西<>。

更新:似乎任何结构都存在这种情况。我没有注意到KeyValuePair<>是一个结构体,因此只对类进行测试。但我仍然不明白为什么会这样。

    public static IEnumerable<T> ForEachAsync<T>(this IEnumerable<T> input, Action<T> act)
    {
        foreach (var item in input)
        {
            act.BeginInvoke(item, new AsyncCallback(EndAsyncCall<T>), null);
        }

        return input;
    }

    private static void EndAsyncCall<T>(IAsyncResult result)
    {
        AsyncResult r = (AsyncResult)result;
        if (!r.EndInvokeCalled)
        {
            var d = (Action<T>)((r).AsyncDelegate);
            d.EndInvoke(result);
        }
    }

我宁愿不使用T约束来限制该方法,以确保只使用类,所以我按照如下方式重构了该方法以解决BeginInvoke的问题,但我以前没有直接使用过TreadPool,我想确保我没有遗漏任何重要的东西。

    public static IEnumerable<T> ForEachAsync<T>(this IEnumerable<T> input, Action<T> act)
    {
        foreach (var item in input)
            ThreadPool.QueueUserWorkItem(obj => act((T)obj), item);

        return input;
    }

1 回复 | 直到 17 年前

Simon Buchan 17 年前

奇怪,好像有什么毛病。NET(C#?),将参数编组到工作线程。

struct MyPair<TKey, TValue> : IConvertable
{
    public readonly TKey Key;
    public readonly TValue Value;

    public MyPair(TKey key, TValue value)
    {
        Key = key;
        Value = value;
    }

    // I just used the smart-tag on IConvertable to get all these...
    // public X ToX(IFormatProvider provider) { throw new InvalidCastException(); }

    ...

    public object ToType(Type conversionType, IFormatProvider provider)
    {
        if (typeof(MyPair<TKey, TValue>).GUID == conversionType.GUID)
            return this;
        throw new InvalidCastException();
    }
}

var data = new Dictionary<string, string> {
    { "Hello", "World" },
    { "How are", "You?" },
    { "Goodbye", "World!" }
};
foreach (var pair in data)
{
    var copy = pair; // define a different variable for each worker
    Action worker = () => Console.WriteLine("Item {0}, {1}", copy.Key, copy.Value);
    worker.BeginInvoke(null, null);
}

当然,如果你需要结果,你需要在另一个方向上存储IAsyncResults,它可能与参数有同样的问题。作为替代方案,您可以在它们完成时将它们添加到集合中,但锁定有点奇怪:

var data = new Dictionary<string, string> {
    { "Hello", "World" },
    { "How are", "You?" },
    { "Goodbye", "World!" }
};

var results = new List<KeyValuePair<string, string>>();
var pending = 0;
var done = new ManualResetEvent(false);

var workers = new List<Action>();
foreach (var pair in data)
{
    ++pending;
    var copy = pair; // define a different variable for each worker
    workers.Add(delegate()
    {
        Console.WriteLine("Item {0}, {1}", copy.Key, copy.Value);
        lock (results)
            results.Add(new KeyValuePair<string, string>("New " + copy.Key, "New " + copy.Value));
        if (0 == Interlocked.Decrement(ref pending))
            done.Set();
    });
}

foreach (var worker in workers)
    worker.BeginInvoke(null, null);

done.WaitOne();

foreach (var pair in results)
    Console.WriteLine("Result {0}, {1}", pair.Key, pair.Value);