Streaming Asynchronous Sequences with `IAsyncEnumerable<T>`

1Concept

`IAsyncEnumerable<T>` combines async/await with `yield return`, allowing servers to stream records (from SQL/gRPC) item-by-item over HTTP WebSockets or SSE without buffering entire tables in memory.

2Architecture Diagram

Database Stream
       │
       ▼ (Reads row 1)
yield return Row 1 ──> Sent over HTTP chunked response immediately to client!
       │
       ▼ (Reads row 2)
yield return Row 2 ──> Sent over HTTP chunked response immediately to client!

3Code Example

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;

public class AsyncEnumerableDemo
{
    public static async IAsyncEnumerable<int> FetchDataStreamAsync([EnumeratorCancellation] CancellationToken ct = default)
    {
        for (int i = 1; i <= 3; i++)
        {
            await Task.Delay(10, ct);
            yield return i * 100;
        }
    }

    public static async Task Main()
    {
        await foreach (int item in FetchDataStreamAsync())
        {
            Console.WriteLine($"Streamed Element: {item}");
        }
    }
}

4Expected Output

Streamed Element: 100
Streamed Element: 200
Streamed Element: 300

5Key Takeaways

• ✓Use `[EnumeratorCancellation]` attribute on CancellationToken parameters in `IAsyncEnumerable`.
• ✓Supported natively in ASP.NET Core Minimal APIs for streaming JSON.
• ✓Eliminates client-side latency by streaming early results immediately.