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：

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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：

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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：

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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~~ ManualResetValueTaskSourceLogic`1 missing in System.Private.CoreLib #21379，有位stephentoub (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 @benaadams 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：

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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。

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C# 8.0 抢先看-- Async Stream