异步/等待续集

Question

异步/等待续集

7

在下面的代码中，我需要并行执行三个 Get... 方法。当一个 Get... 方法完成后，我需要立即调用 Save... 方法。请注意，Save... 以 thing 作为参数。所有 Get 和 Save 方法必须在 DoStuffAsync 返回之前完成。

我的猜测是，我需要在 Get... 方法上进行继续操作，但我不知道如何构造它。

protected async void DoStuffAsync()
{
    SomeThing thing = new SomeThing { ID = 5 };
    SomeRepository rep = new SomeRepository();

    // We need to run all three Get... methods in parallel
    // As soon as a Get... method completes we need to save the result to the correct property on thing and call the Save... method .

    var getRed = rep.GetRedAsync().ContinueWith<Task<string>>(async x => { thing.Color1 = x.Result; await rep.SaveRedAsync(thing); return x; }); // does not compile
    var getBlue = rep.GetBlueAsync();
    var getGreen = rep.GetGreenAsync();

    string red = await getRed.Result; // this is not good because getBlue may finish before getRed.  We want dont want to wait on getRed before calling SaveBlue
    await rep.SaveRedAsync(thing);
    var b = await getBlue;  
    var c = await getGreen;

    // thing must be fully initialized before DoStuffAsync returns

    }


public class SomeThing
{
    public int ID { get; set; }
    public string Color1 { get; set; }
    public string Color2 { get; set; }
    public string Color3 { get; set; }
}

public class SomeRepository
{
    public async Task<string> GetRedAsync()
    {
        return await Task.Run(() => "red");
    }

    public async Task<string> GetBlueAsync()
    {
        return await Task.Run(() => "blue");
    }

    public async Task<string> GetGreenAsync()
    {
        return await Task.Run(() => "green");
    }

    public async Task SaveRedAsync(SomeThing thing)
    {
        // We need thing.ID here as well as other properties
        await Task.Delay(1);
    }

    public async Task SaveBlueAsync(SomeThing thing)
    {
        await Task.Delay(1);
    }

    public async Task SaveGreenAsync(SomeThing thing)
    {
        await Task.Delay(1);
    }
}

- user579342

5

当您编写的代码认为可以工作但无法编译时，请包含编译错误信息。 - Jon Skeet

3个回答

7

我不会混合使用ContinueWith和await，而是直接使用async lambda表达式：

protected async Task  DoStuffAsync()
{
    SomeThing thing = new SomeThing { ID = 5 };
    SomeRepository rep = new SomeRepository();

    // We need to run all three Get... methods in parallel
    // As soon as a Get... method completes we need to save the result to the correct property on thing and call the Save... method .

    Func<Task<X>> getRedFunc = async() =>
    {
        var result = await rep.GetRedAsync();
        thing.Color1 = result;
        await rep.SaveRedAsync(thing);
        return result;
    };

    var getRed = getRedFunc();

    var getBlue = rep.GetBlueAsync();
    var getGreen = rep.GetGreenAsync();

    await Task.WhenAll(getRed, getBlue, getGreen);
}

此外，请不要将async void方法用于事件处理程序以外的任何事情。您将无法观察到像这样的方法的完成情况，也可能无法处理其中可能引发的异常。

- noseratio

你是否有理由支持“我不会将ContinueWith和await混用”的观点？ - NStuke

@NStuke，不一定按照这个顺序，但是：1）代码结构和可读性；2）当前的SynchronizationContext和TaskScheduler被考虑的差异，3）异常处理和传播的差异；4）继续运行lambda的差异（人们往往忘记了ExecuteSynchronously）；5）我确定我还漏掉了其他东西，但应该足够了 :) - noseratio - open to work

-1

你可以尝试并行框架：

using System.Threading;

using System.Threading.Tasks;

Func<Task<string>>[] functions = { rep.GetRedAsync, rep.GetBlueAsync, rep.GetGreenAsync }; 
Var[] GetArray = new Var[functions.Length]
int i=0;
Parallel.ForEach (var function in functions)  
{  
             GetArray[i++]=function();

}

注意：需要 .Net 4

- Dexters

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Jon Skeet · Accepted Answer

你可以明确地使用ContinueWith - 或者你可以将每个“获取和保存”分别分解为单独的异步方法或异步lambda表达式。例如：

async Task GetAndSaveRedAsync(SomeThing thing, SomeRepository rep)
{
    var red = await rep.GetRedAsync();
    thing.Red = red;
    await SaveRedAsync(red);
    // Return red if you want (change the return type to Task<string>)
}

// Ditto for the others

然后：

protected async void DoStuffAsync()
{
    SomeThing thing = new SomeThing { ID = 5 };
    SomeRepository rep = new SomeRepository();

    var handleRed = GetAndSaveRedAsync(thing, rep);
    var handleBlue = GetAndSaveBlueAsync(thing, rep);
    var handleYellow = GetAndSaveYellowAsync(thing, rep);

    // Or use Task.WhenAll
    await handleRed;
    await handleBlue;
    await handleYellow;
}