Doesn't await when using ForEachAsync with await inside Action

问题

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;

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

回答1:

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

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

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

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?

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));
        }
    }
}

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

来源：https://stackoverflow.com/questions/29787098/doesnt-await-when-using-foreachasync-with-await-inside-action