What are Async Streams in C# 8.0?

C# 8.0 introduces async streams, which model a streaming source of data. Data streams often retrieve or generate elements asynchronously.

The code that generates the sequence can now use yield return to return elements in a method that was declared with the async modifier.

We can consume an async stream using an await foreach loop.

This below Syntax

static IEnumerable<string> Message(){
   yield return "Hello!";
   yield return "Hello!";
}
Can be replaced by IAsyncEnumerable
static async IAsyncEnumerable<string> MessageAsync(){
   await Task.Delay(2000);
   yield return "Hello!";
   await Task.Delay(2000);
   yield return "Hello!";
}

Example

class Program{
   public static async Task Main(){
      await foreach (var item in MessageAsync()){
         System.Console.WriteLine(item);
      }
      Console.ReadLine();
   }
   static async IAsyncEnumerable<string> MessageAsync(){
      await Task.Delay(2000);
      yield return "Hello!";
      await Task.Delay(2000);
      yield return "Hello!";
   }
}

Output

Hello!
Hello!