使用ForEachAsync和Action内部的await时不等待 [英] Doesn't await when using ForEachAsync with await inside Action

问题描述

以下内容应返回 C，但它返回 B

The following should return "C", but it returns "B"

using System.Data.Entity;
//...
var state = "A";
var qry = (from f in db.myTable select f);
await qry.ForEachAsync(async (myRecord) => {
   await DoStuffAsync(myRecord);
   state = "B";
});
state = "C";
return state;

它不等待DoStuffAsync完成， state = C 运行，然后执行 state = B （因为它内部仍在等待）。

It doesn't wait for DoStuffAsync to complete, state="C" runs through and then later state="B" executes (because inside it is still awaiting).

推荐答案

这是因为ForEachAsync的实现不会等待委托的操作

That's because the implementation of ForEachAsync doesn't await the delegated action

moveNextTask = enumerator.MoveNextAsync(cancellationToken);
action(current);

请参见 https://github.com/mono/entityframework/blob/master/src/EntityFramework/Infrastructure/IDbAsyncEnumerableExtensions.cs#L19

但这是因为，您不能等待操作，委托必须是一个返回任务的Func，请参见如何实现异步操作委托方法？

But that is because, you can't await an action, the delegate needs to be a Func which returns a Task - see How do you implement an async action delegate method?

因此，在Microsoft提供包括Func委托的签名并在等待之前调用它之前，您必须滚动自己的扩展方法。我现在正在使用以下内容。

Therefore, until Microsoft provides a signature which includes a Func delegate and calls it with await, you'll have to roll your own extension method. I'm using the following at the moment.

public static async Task ForEachAsync<T>(
    this IQueryable<T> enumerable, Func<T, Task> action, CancellationToken cancellationToken) //Now with Func returning Task
{
    var asyncEnumerable = (IDbAsyncEnumerable<T>)enumerable;
    using (var enumerator = asyncEnumerable.GetAsyncEnumerator())
    {

        if (await enumerator.MoveNextAsync(cancellationToken).ConfigureAwait(continueOnCapturedContext: false))
        {
            Task<bool> moveNextTask;
            do
            {
                var current = enumerator.Current;
                moveNextTask = enumerator.MoveNextAsync(cancellationToken);
                await action(current); //now with await
            }
            while (await moveNextTask.ConfigureAwait(continueOnCapturedContext: false));
        }
    }
}

有了这个，原始测试您的OP中的代码将按预期工作。

With this, the original test code in your OP will work as expected.

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