The document discusses asynchronous programming in C#, covering various patterns such as the Event Asynchronous Pattern and the Task Asynchronous Pattern. It highlights the use of async streams and async enumerables, providing code examples for reading lines from a CSV file asynchronously. Additionally, it emphasizes best practices like avoiding synchronous wrappers and using cancellation tokens for efficient async operations.

1. Async Streams in C#
Tamir Dresher
Twitter: @tamir_dresher

2. 2
System Architect @ @tamir_dresher
Tamir Dresher
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Software Engineering Lecturer
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3. Async work in C#
.NET 2.0
- Event Asynchronous
Pattern
(Async method + event)
.NET 1.1
Asynchronous Programming Model
(Begin/End Methods)
.NET 4.5
Task Asynchronous
Pattern
C# 7.0
Async Main
C# 5.0
Async Await
C# 8
- Async Streams
- Async Disposables
Single
Value
• IObservable<T> , IObserver<T>
• Reactive Extensions (Rx)

4. Async Stream
g(x)
f(x)

5. Demo
IEnumerable and Iterators
SharpLab Demo

6. Sync over Async
Avoid Exposing Synchronous Wrappers for Asynchronous Implementations
Sync over async will lead to ThreadPool starvation on high load
Thread Pool
Thread
Request
Async IO
Thread

7. IAsyncEnumerable
namespace System.Collections.Generic
{
public interface IAsyncEnumerable<out T>
{
IAsyncEnumerator<T> GetAsyncEnumerator(
CancellationToken cancellationToken = default);
}
public interface IAsyncEnumerator<out T> : IAsyncDisposable
{
ValueTask<bool> MoveNextAsync();
T Current { get; }
}
}
SharpLab
namespace System
{
public interface IAsyncDisposable
{
ValueTask DisposeAsync();
}
}
namespace System.Collections.Generic
{
public interface IEnumerable<out T> : IEnumerable
{
IEnumerator<T> GetEnumerator();
}
public interface IEnumerator<out T> : IDisposable,
IEnumerator
{
bool MoveNext ();
T Current { get; }
}
}

8. Async stream example
public static async Task Main()
{
var records = ReadLines("transactions.csv");
await foreach (var record in records)
{
Console.WriteLine($"Transaction from {record[0]} to {record[1]}");
}
}
public static async IAsyncEnumerable<string[]> ReadLines(string path)
{
var lines = await System.IO.File.ReadAllLinesAsync(path);
foreach (var line in lines)
{
yield return line.Split(',');
}
}

9. Summary
Single Value Multiple Value
Sync T IEnumerable<T>
Async Task<T> IAsyncEnumerable<T>
IObservable<T>
Avoid as much as possible Sync-Over-Async
Cancellation - Mark the CancelleationToken parameter with EnumeratorCancellation
attribute and use the WithCancellation() extension method propagate the token
Use the System.Linq.Async and System.Interactive.Async NuGet packages to add async
LINQ operators
@tamir_dresher