AsyncCallback通参数

Question

我想把List<items>传递给函数LoadHistoryAsync(List<int> Items)中的AsyncCallback，但是CallForNewData(IAsyncResult result)中的result.AsyncState是空的，为什么？

namespace ConsoleApplication3
{
class Program
{
    static void Main(string[] args)
    {
        List<int> Items = new List<int>();
        Data D = new Data();
        D.LoadHistoryAsync(Items);
        //D.LoadNewPointsAsync(Items);
        Console.ReadKey();
    }
}
public class Data
{
    public void LoadHistoryAsync(List<int> Items)
    {
        Action<List<int>> GetHistoryInformation = new Action<List<int>>(GetHistory);
        //IAsyncResult History = GetHistoryInformation.BeginInvoke(Items, null, null);
        IAsyncResult History = GetHistoryInformation.BeginInvoke(Items, new AsyncCallback(CallForNewData), null);
    }

    public void GetHistory(List<int> Items)
    {
        Random rnd = new Random();
        System.Threading.Thread.Sleep(rnd.Next(1,5000));
        Items.Add(1);
        Console.WriteLine("HistoryLoaded");
    }
    public void CallForNewData(IAsyncResult result)
    {
       Console.WriteLine("Result: {0}",result.AsyncState);
    }

    public void LoadNewPointsAsync(List<int> Items)
    {
        //while(!History.IsCompleted)
        //{
            System.Threading.Thread.Sleep(100);
        //}
        Action<List<int>> GetPointsInformation = new Action<List<int>>(GetPoints);
        IAsyncResult NewPoints = GetPointsInformation.BeginInvoke(Items, null, null);
    }

    public void GetPoints(List<int> Items)
    {
        Random rnd = new Random();
        System.Threading.Thread.Sleep(rnd.Next(1, 5000));
        Items.Add(2);
        Console.WriteLine("New data loaded");
    }
}
}

编辑：

IAsyncResult History = GetHistoryInformation.BeginInvoke(Items, new AsyncCallback(CallForNewData), Items);

问题解决了。

Answer 1

来自文档

BeginInvoke方法启动异步调用。它具有与要异步执行的方法相同的参数，外加两个额外的可选参数。第一个参数是一个AsyncCallback委托，它引用异步调用完成时要调用的方法。第二个参数是用户定义的对象，它将信息传递给回调方法。

你通过一个null，你得到一个null。