C#
IAsyncEnumerable
Learn async iteration with IAsyncEnumerable<T>.
What is IAsyncEnumerable?
IAsyncEnumerable<T> is a collection that produces values asynchronously as a stream. Each element is produced with an await; this lets you process large data in pieces. Instead of awaiting a Task<List<T>> for the whole list, you can stream each element one by one.
A method returns async IAsyncEnumerable<T>. Its body uses yield return to produce values. The consumer uses await foreach to await each element. This is especially suited for database and API streams.
Defining a stream
An IAsyncEnumerable<T> method is marked async and produces values with yield return. Example: static async IAsyncEnumerable<int> Numbers() { await Task.CompletedTask; yield return 1; yield return 2; }.
Each yield return produces an element and pauses the stream; the consumer resumes when ready. You can use await at each step; this is used to wait for input/output in real async scenarios.
Exercise Define and consume an IAsyncEnumerable
Define static async IAsyncEnumerable<int> Numbers(). In its body write await Task.CompletedTask;, yield return 1;, yield return 2;, yield return 3;. Make Main a static async Task Main. Sum with int sum = 0; await foreach (int n in Numbers()) { sum += n; } and print with Console.WriteLine(sum);.
Show example solution
In Numbers body write await Task.CompletedTask; yield return 1; yield return 2; yield return 3;. Make Main static async Task Main() and write int sum = 0; await foreach (int n in Numbers()) { sum += n; } Console.WriteLine(sum);
Example solution:
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
class Program
{
static async IAsyncEnumerable<int> Numbers()
{
await Task.CompletedTask;
yield return 1;
yield return 2;
yield return 3;
}
static async Task Main()
{
int sum = 0;
await foreach (int n in Numbers())
{
sum += n;
}
Console.WriteLine(sum);
}
}
Test yourself
-
What does IAsyncEnumerable do?
- It awaits a Task
- It produces a stream of values asynchronously
- It returns a List<T>
- It starts a thread
Show answer
It produces a stream of values asynchronously — Correct. IAsyncEnumerable produces a stream of values asynchronously.
-
Complete the IAsyncEnumerable declaration
static ____ IAsyncEnumerable<int> Numbers() { await Task.CompletedTask; yield return 1; }
Show answer
static async IAsyncEnumerable<int> Numbers() { await Task.CompletedTask; yield return 1; }
-
Why use IAsyncEnumerable?
- To await the whole list at once
- To write synchronous code
- To process large data in pieces asynchronously
- To manage the thread pool
Show answer
To process large data in pieces asynchronously — Correct. IAsyncEnumerable is for processing large data in pieces asynchronously.
Next lesson: await foreach
