使用IAsyncEnumerable读取文本文件 [英] Read text file with IAsyncEnumerable

问题描述

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/).它是异步流，并替代了 Task< IEnumerable< T>>

I came across IAsyncEnumerable while I am testing C# 8.0 features. I found remarkable examples from Anthony Chu (https://anthonychu.ca/post/async-streams-dotnet-core-3-iasyncenumerable/). It is async stream and replacement for Task<IEnumerable<T>>

// 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，如下面的用法那样逐行读取文件.

I am wondering if this can be applied to read text files like below usage that read file line by line.

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

我真的很想知道如何将具有 IAsyncEnumerable< string>()的异步应用于上述foreach循环，以便在阅读时流式传输.

I really want to know how to apply async with IAsyncEnumerable<string>() to the above foreach loop so that it streams while reading.

如何实现迭代器，以便可以使用yield return逐行读取?

How do I implement iterator so that I can use yield return to read line by line?

推荐答案

完全一样，但是没有异步工作负载，所以我们假装

Exactly the same, however there is no async workload, so let's pretend

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

这只是打包的APM工作负载，请参阅 Stephen Clearys 注释以进行澄清

This is just an wrapped APM workload, see Stephen Clearys comments for clarification

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 可悲的是只允许同步I/O ；异步API 是在这种情况下实施效果不佳.要打开真正的异步文件，您需要使用传递 isAsync 的 FileStream 构造函数:true或 FileOptions.Asynchronous .

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.

如您所见，

ReadLineAsync 基本上会生成此代码，就像只有Stream APM的 Begin 和 End 方法包装

ReadLineAsync basically results in this code, as you can see, it's only the Stream APM Begin and End methods wrapped

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
}

这篇关于使用IAsyncEnumerable读取文本文件的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！