如何将异步与 ForEach 一起使用？

List<T>.ForEach 不能与 async 配合得特别好（LINQ-to-objects 也不能，原因相同）。

在这种情况下，我建议将每个元素投射到异步操作中，然后您可以（异步）等待它们全部完成。

using (DataContext db = new DataLayer.DataContext())
{
    var tasks = db.Groups.ToList().Select(i => GetAdminsFromGroupAsync(i.Gid));
    var results = await Task.WhenAll(tasks);
}

与将 async 委托给 ForEach 相比，这种方法的好处是：

错误处理更合适。来自 async void 的异常不能被 catch 捕获；这种方法将在 await Task.WhenAll 行传播异常，允许自然的异常处理。您知道任务在此方法结束时完成，因为它执行 await Task.WhenAll。如果使用 async void，则无法轻易判断操作何时完成。这种方法具有用于检索结果的自然语法。 GetAdminsFromGroupAsync 听起来像是一个产生结果的操作（管理员），如果这样的操作可以返回它们的结果而不是设置一个值作为副作用，那么这样的代码会更自然。

这个小扩展方法应该为您提供异常安全的异步迭代：

public static async Task ForEachAsync<T>(this List<T> list, Func<T, Task> func)
{
    foreach (var value in list)
    {
        await func(value);
    }
}

由于我们将 lambda 的返回类型从 void 更改为 Task，因此异常将正确传播。这将允许您在实践中编写如下内容：

await db.Groups.ToList().ForEachAsync(async i => {
    await GetAdminsFromGroup(i.Gid);
});

从 C# 8.0 开始，您可以异步创建和使用流。

    private async void button1_Click(object sender, EventArgs e)
    {
        IAsyncEnumerable<int> enumerable = GenerateSequence();

        await foreach (var i in enumerable)
        {
            Debug.WriteLine(i);
        }
    }

    public static async IAsyncEnumerable<int> GenerateSequence()
    {
        for (int i = 0; i < 20; i++)
        {
            await Task.Delay(100);
            yield return i;
        }
    }

More

简单的答案是使用 foreach 关键字而不是 List() 的 ForEach() 方法。

using (DataContext db = new DataLayer.DataContext())
{
    foreach(var i in db.Groups)
    {
        await GetAdminsFromGroup(i.Gid);
    }
}

这是上述异步 foreach 变体的实际工作版本，具有顺序处理：

public static async Task ForEachAsync<T>(this List<T> enumerable, Action<T> action)
{
    foreach (var item in enumerable)
        await Task.Run(() => { action(item); }).ConfigureAwait(false);
}

这是实现：

public async void SequentialAsync()
{
    var list = new List<Action>();

    Action action1 = () => {
        //do stuff 1
    };

    Action action2 = () => {
        //do stuff 2
    };

    list.Add(action1);
    list.Add(action2);

    await list.ForEachAsync();
}

关键区别是什么？ .ConfigureAwait(false); 在每个任务的异步顺序处理时保持主线程的上下文。

问题是 async 关键字需要出现在 lambda 之前，而不是在正文之前：

db.Groups.ToList().ForEach(async (i) => {
    await GetAdminsFromGroup(i.Gid);
});

添加此扩展方法

public static class ForEachAsyncExtension
{
    public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
    {
        return Task.WhenAll(from partition in Partitioner.Create(source).GetPartitions(dop) 
            select Task.Run(async delegate
            {
                using (partition)
                    while (partition.MoveNext())
                        await body(partition.Current).ConfigureAwait(false);
            }));
    }
}

然后像这样使用：

Task.Run(async () =>
{
    var s3 = new AmazonS3Client(Config.Instance.Aws.Credentials, Config.Instance.Aws.RegionEndpoint);
    var buckets = await s3.ListBucketsAsync();

    foreach (var s3Bucket in buckets.Buckets)
    {
        if (s3Bucket.BucketName.StartsWith("mybucket-"))
        {
            log.Information("Bucket => {BucketName}", s3Bucket.BucketName);

            ListObjectsResponse objects;
            try
            {
                objects = await s3.ListObjectsAsync(s3Bucket.BucketName);
            }
            catch
            {
                log.Error("Error getting objects. Bucket => {BucketName}", s3Bucket.BucketName);
                continue;
            }

            // ForEachAsync (4 is how many tasks you want to run in parallel)
            await objects.S3Objects.ForEachAsync(4, async s3Object =>
            {
                try
                {
                    log.Information("Bucket => {BucketName} => {Key}", s3Bucket.BucketName, s3Object.Key);
                    await s3.DeleteObjectAsync(s3Bucket.BucketName, s3Object.Key);
                }
                catch
                {
                    log.Error("Error deleting bucket {BucketName} object {Key}", s3Bucket.BucketName, s3Object.Key);
                }
            });

            try
            {
                await s3.DeleteBucketAsync(s3Bucket.BucketName);
            }
            catch
            {
                log.Error("Error deleting bucket {BucketName}", s3Bucket.BucketName);
            }
        }
    }
}).Wait();

如果您使用的是 EntityFramework.Core，则有 an extension method ForEachAsync。

示例用法如下所示：

using Microsoft.EntityFrameworkCore;
using System.Threading.Tasks;

public class Example
{
    private readonly DbContext _dbContext;
    public Example(DbContext dbContext)
    {
        _dbContext = dbContext;
    }
    public async void LogicMethod()
    {
        
        await _dbContext.Set<dbTable>().ForEachAsync(async x =>
        {
            //logic
            await AsyncTask(x);
        });
    }

    public async Task<bool> AsyncTask(object x)
    {
        //other logic
        return await Task.FromResult<bool>(true);
    }
}

这是我为使用 ForEach 处理异步场景而创建的方法。

如果其中一项任务失败，则其他任务将继续执行。

您可以添加将在每个异常上执行的函数。

异常在最后被收集为 aggregateException 并且可供您使用。

可以处理 CancellationToken

 public static class ParallelExecutor
    {
        /// <summary>
        /// Executes asynchronously given function on all elements of given enumerable with task count restriction.
        /// Executor will continue starting new tasks even if one of the tasks throws. If at least one of the tasks throwed exception then <see cref="AggregateException"/> is throwed at the end of the method run.
        /// </summary>
        /// <typeparam name="T">Type of elements in enumerable</typeparam>
        /// <param name="maxTaskCount">The maximum task count.</param>
        /// <param name="enumerable">The enumerable.</param>
        /// <param name="asyncFunc">asynchronous function that will be executed on every element of the enumerable. MUST be thread safe.</param>
        /// <param name="onException">Acton that will be executed on every exception that would be thrown by asyncFunc. CAN be thread unsafe.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        public static async Task ForEachAsync<T>(int maxTaskCount, IEnumerable<T> enumerable, Func<T, Task> asyncFunc, Action<Exception> onException = null, CancellationToken cancellationToken = default)
        {
            using var semaphore = new SemaphoreSlim(initialCount: maxTaskCount, maxCount: maxTaskCount);

            // This `lockObject` is used only in `catch { }` block.
            object lockObject = new object();
            var exceptions = new List<Exception>();
            var tasks = new Task[enumerable.Count()];
            int i = 0;

            try
            {
                foreach (var t in enumerable)
                {
                    await semaphore.WaitAsync(cancellationToken);
                    tasks[i++] = Task.Run(
                        async () =>
                        {
                            try
                            {
                                await asyncFunc(t);
                            }
                            catch (Exception e)
                            {
                                if (onException != null)
                                {
                                    lock (lockObject)
                                    {
                                        onException.Invoke(e);
                                    }
                                }

                                // This exception will be swallowed here but it will be collected at the end of ForEachAsync method in order to generate AggregateException.
                                throw;
                            }
                            finally
                            {
                                semaphore.Release();
                            }
                        }, cancellationToken);

                    if (cancellationToken.IsCancellationRequested)
                    {
                        break;
                    }
                }
            }
            catch (OperationCanceledException e)
            {
                exceptions.Add(e);
            }

            foreach (var t in tasks)
            {
                if (cancellationToken.IsCancellationRequested)
                {
                    break;
                }

                // Exception handling in this case is actually pretty fast.
                // https://gist.github.com/shoter/d943500eda37c7d99461ce3dace42141
                try
                {
                    await t;
                }
#pragma warning disable CA1031 // Do not catch general exception types - we want to throw that exception later as aggregate exception. Nothing wrong here.
                catch (Exception e)
#pragma warning restore CA1031 // Do not catch general exception types
                {
                    exceptions.Add(e);
                }
            }

            if (exceptions.Any())
            {
                throw new AggregateException(exceptions);
            }
        }
    }

我想补充一点，有一个内置 ForEach 函数的 Parallel class 可用于此目的。