C# 8.0 抢先看-- Async Stream

如果一个方法要回传IAsyncEnumerable ，而方法内部使用yield return 该怎么写呢？
我们一样就拿ReadLineAsync 来示范，首先建立一个类别实作IAsyncEnumerator ，当然这也包含了实作IAsyncDisposable：

internal class AsyncEnumerator : IAsyncEnumerator<string>
    {
        private readonly StreamReader _reader;

        private bool _disposed;

        public string Current { get; private set; }

        public AsyncEnumerator(string path)
        {
            _reader = File.OpenText(path);
            _disposed = false;
        }
        async public ValueTask<bool> MoveNextAsync()
        {
            var result = await _reader.ReadLineAsync();
            Current = result;
            return result != null;
        }
        async public ValueTask DisposeAsync()
        {
            await Task.Run(() => Dispose());
        }

        private void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        private void Dispose(bool disposing)
        {
            if (!this._disposed)
            {
                if (_reader != null)
                {
                    _reader.Dispose();
                }
                _disposed = true;
            }
        }
    }

接着建立另外一个类别， 这个类别很简单，只包含一个静态的方法async static public IAsyncEnumerable ReadLineAsync(string path)，实作内容如下：

async static public IAsyncEnumerable<string> ReadLineAsync(string path)
        {

            var enumerator = new AsyncEnumerator(path);
            try
            {
                while (await enumerator.MoveNextAsync())
                {
                    await Task.Delay(100);
                    yield return enumerator.Current;
                }
            }
            finally
            {
                await enumerator.DisposeAsync();
            }
        }
    }

程式码没有错，但编译过不了，观察一下错误讯息：

错误CS0656：缺少编译器所需成员'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.GetResult'
错误CS0656：缺少编译器所需成员'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.GetStatus'
错误CS0656：缺少编译器所需的成员'系统。 Threading.Tasks.ManualResetValueTaskSourceLogic`1.get_Version'
错误CS0656：缺少编译器所需成员'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.OnCompleted'
错误CS0656：缺少编译器所需成员'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.Reset'
错误CS0656：缺少编译器所需的成员'System.Threading.Tasks.ManualResetValueTaskSourceLogic`1.SetException'
错误CS0656：缺少编译器所需的成员'System.Threading.Tasks.ManualResetValueTaskLogic.cn
.SetResult ' 错误CS0656：缺少编译器所需的成员'System.Runtime.CompilerServices.IStrongBox`1.get_Value'
错误CS0656：缺少编译器所需的成员

很明显，编译器需要两个型别(1) System.Threading.Tasks.ManualResetValueTaskSourceLogic (2) System.Runtime.CompilerServices.IStrongBox才能完成编译。感谢open source与git hub，在微软的dotnet/corclr的专案中找到了这么一段讨论~~ ，有位 (应该是微软员工而且是这个专案的成员)提到『It's not missing exactly, but like  said things are just out-of-sync between the compiler and library in Preview 1. The compiler is looking for the old design (ManualResetValueTaskSourceLogic and IStrongBox) , while the libraries include the approved API surface area (ManualResetValueTaskSourceCore), and we didn't have time to get the compiler updated.』，简单说就是编译器和框架目前的更新进度不一致，导致少了点什么。既然如此，我们就遵照本草纲目的指示，补上这两个型别，请注意，这两个型别的命名空间必须正确：

using System;
using System.Runtime.CompilerServices;
using System.Threading.Tasks.Sources;


namespace System.Threading.Tasks{

    internal struct ManualResetValueTaskSourceLogic<TResult>
    {
        private ManualResetValueTaskSourceCore<TResult> _core;
        public ManualResetValueTaskSourceLogic(IStrongBox<ManualResetValueTaskSourceLogic<TResult>> parent) : this() { }
        public short Version => _core.Version;
        public TResult GetResult(short token) => _core.GetResult(token);
        public ValueTaskSourceStatus GetStatus(short token) => _core.GetStatus(token);
        public void OnCompleted(Action<object> continuation, object state, short token, ValueTaskSourceOnCompletedFlags flags) => _core.OnCompleted(continuation, state, token, flags);
        public void Reset() => _core.Reset();
        public void SetResult(TResult result) => _core.SetResult(result);
        public void SetException(Exception error) => _core.SetException(error);
    }
}

namespace System.Runtime.CompilerServices
{
    internal interface IStrongBox<T> { ref T Value { get; } }
}

补上去后就大功告成，可以快乐地非同步yielld return。