Streaming lines of text from file with IAsyncEnumerable

Question

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);
}

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.
}

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

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

Answer 1

I did some performance tests and it seems that a large bufferSize is helpful, together with the FileOptions.SequentialScan option.

public static async IAsyncEnumerable<string> ReadLinesAsync(string filePath)
{
    using var stream = new FileStream(filePath, FileMode.Open, FileAccess.Read,
        FileShare.Read, 32768, FileOptions.Asynchronous | FileOptions.SequentialScan);
    using var reader = new StreamReader(stream);
    while (true)
    {
        var line = await reader.ReadLineAsync().ConfigureAwait(false);
        if (line == null) break;
        yield return line;
    }
}

The enumeration in not trully asynchronous though. According to my experiments (.NET Core 3.1) the xxxAsync methods of the StreamReader class are blocking the current thread for a duration longer than the awaiting period of the Task they return. For example reading a 6 MB file with the method ReadToEndAsync in my PC blocks the current thread for 120 msec before returning the task, and then the task is completed in just 20 msec. So I am not sure that there is much value at using these methods. Faking asynchrony is much easier by using the synchronous APIs together with some Linq.Async:

IAsyncEnumerable<string> lines = File.ReadLines("SomeFile.txt").ToAsyncEnumerable();

---

.NET 6 update: The implementation of the asynchronous filesystem APIs has been improved on .NET 6. For experimental data with the File.ReadAllLinesAsync method, see here.

Answer 2

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;
   }
}

or

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();
}

Usage

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

Update from 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 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
}