await foreach

A plain iterator cannot await. If each value needs a network call or a database round trip, the old options were both bad: block a thread, or give up laziness and return a whole Task<List<T>>.

C# 8 added the third option.

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

class Program
{
    static async IAsyncEnumerable<int> Ticks(int count)
    {
        for (int i = 1; i <= count; i++)
        {
            await Task.Delay(10);
            yield return i;
        }
    }

    static async Task Main()
    {
        await foreach (var tick in Ticks(3))
        {
            Console.WriteLine(tick);
        }
    }
}
   

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Three pieces, and they always travel together:

• async on the method, with return type IAsyncEnumerable<T>
• yield return in the body, alongside await
• await foreach at the consumer

The protocol underneath is the same shape as before, with tasks in it: GetAsyncEnumerator(), ValueTask<bool> MoveNextAsync(), Current, and DisposeAsync().