c# - 使用 IAsyncEnumerable 读取文本文件

Question

我在测试 C# 8.0 功能时遇到了 IAsyncEnumerable。我从 Anthony Chu (https://anthonychu.ca/post/async-streams-dotnet-core-3-iasyncenumerable/) 中找到了很好的例子。它是异步流和 Task<IEnumerable<T>> 的替代品

// Data Access Layer.
public async IAsyncEnumerable<Product> GetAllProducts()
{
    Container container = cosmosClient.GetContainer(DatabaseId, ContainerId);
    var iterator = container.GetItemQueryIterator<Product>("SELECT * FROM c");
    while (iterator.HasMoreResults)
    {
        foreach (var product in await iterator.ReadNextAsync())
        {
            yield return product;
        }
    }
}

// Usage
await foreach (var product in productsRepository.GetAllProducts())
{
    Console.WriteLine(product);
}

我想知道这是否可以应用于读取文本文件，如下面逐行读取文件的用法。

foreach (var line in File.ReadLines("Filename"))
{
    // ...process line.
}

我真的很想知道如何使用 IAsyncEnumerable<string>() 应用异步到上面的 foreach 循环，以便它在读取时流式传输。

如何实现迭代器以便我可以使用 yield return 逐行读取？

Answer 1

完全一样，但是没有异步工作负载，所以让我们假装

public async IAsyncEnumerable<string> SomeSortOfAwesomeness()
{
   foreach (var line in File.ReadLines("Filename.txt"))
   {
       // simulates an async workload, 
       // otherwise why would be using IAsyncEnumerable?
       // -- added due to popular demand 
       await Task.Delay(100);
       yield return line;
   }
}

或

这只是一个包装的 APM 工作负载，请参阅 Stephen Clearys澄清意见

public static async IAsyncEnumerable<string> SomeSortOfAwesomeness()
{
   using StreamReader reader = File.OpenText("Filename.txt");
   while(!reader.EndOfStream)
      yield return await reader.ReadLineAsync();
}

用法

await foreach(var line in SomeSortOfAwesomeness())
{
   Console.WriteLine(line);
}

更新 来自 Stephen Cleary

File.OpenText sadly only allows synchronous I/O; the async APIs are implemented poorly in that scenario. To open a true asynchronous file, you'd need to use a FileStream constructor passing isAsync: true or FileOptions.Asynchronous.

ReadLineAsync 基本上是这段代码的结果，如您所见，它只是包装了 Stream APM Begin 和 End 方法

private Task<Int32> BeginEndReadAsync(Byte[] buffer, Int32 offset, Int32 count)
{            
     return TaskFactory<Int32>.FromAsyncTrim(
                    this, new ReadWriteParameters { Buffer = buffer, Offset = offset, Count = count },
                    (stream, args, callback, state) => stream.BeginRead(args.Buffer, args.Offset, args.Count, callback, state), // cached by compiler
                    (stream, asyncResult) => stream.EndRead(asyncResult)); // cached by compiler
}