If you have worked with async methods in C#, you might have noticed that returning streams is not possible, or is hard to achieve with existing features. This would, however, be a helpful feature, which would make development tasks much simpler. This is why C# 8 has introduced a new interface called IAsyncEnumerable. With this new interface, asynchronous streams of data can be returned. Let me explain a little bit more about this.

Before async streams, in the C# programming language an async method was not able to return a stream of data—it could could only return a single value.

Let's take a look at an example of code that doesn't use an async stream:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ExploreVS
{
 class Program
 {
 public static void Main(string[] args)
 {
 var numbers = GetNumbersAsync();
 foreach(var n in GetSumOfNums(numbers))
 {
 Console.WriteLine(n);
 }
 Console.ReadKey();
 }
 public static IEnumerable<int> GetNumbersAsync()
 {
 List<int> a = new List<int>();
 a.Add(1);
 a.Add(2);
 a.Add(3);
 a.Add(4);
 return a;
 }
 public static IEnumerable<int> GetSumOfNums(IEnumerable<int> nums)
 {
 var sum = 0;
 foreach(var num in nums)
 {
 sum += num;
 yield return sum;
 }
 }

 }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ExploreVS
{
 class Program
 {
 public static async void Main(string[] args)
 {
 var numbers = GetNumbersAsync();
 await foreach(var n in GetSumOfNums(numbers))
 {
 Console.WriteLine(n);
 }
 Console.ReadKey();
 }
 public static IEnumerable<int> GetNumbersAsync()
 {
 List<int> a = new List<int>();
 a.Add(1);
 a.Add(2);
 a.Add(3);
 a.Add(4);
 return a;
 }
 public static async IAsyncEnumerable<int> GetSumOfNums(IAsyncEnumerable<int> nums)
 {
 var sum = 0;
 await foreach(var num in nums)
 {
 sum += num;
 yield return sum;
 }
 }
 
 }
}