Introducing C# 9: Extension GetEnumerator support for foreach loops

Introduction

As you may know, in order to be able to iterate on a collection in C# with the foreach loop the collection must expose a public method GetEnumerator() which doesn’t exist on IEnumerator<T> and IAsyncEnumerator<T> interfaces. C# 9 allows to create an extension method to allow foreach loops on those interfaces. Let’s see in this article how to proceed.

Implement a GetEnumerator extension

It’s quite easy you just have to create an extension method like this:

public static class Extensions
{
        public static IEnumerator<T> GetEnumerator<T>(this IEnumerator<T> enumerator) => enumerator;
}

Make sure that extension method is accessible everywhere in your program, and you’ll be able to apply it anywhere.

Usually when you try to iterate with a foreach loop on a collection typed IEnumerator<T> or IAsyncEnumerator<T> you’ll get this message:

Error CS1579 foreach statement cannot operate on variables of type ‘IEnumerator’ // because ‘IEnumerator’ does not contain a public instance or extension definition for ‘GetEnumerator’

With C# 9 and the previous extension (together only) shown above only we can make it work, example:

	using System;
	using System.Collections.Generic;
	namespace CSharp9Demo
	{
	public static class Extensions
	{
	public static IEnumerator<T> GetEnumerator<T>(this IEnumerator<T> enumerator) => enumerator;
	}
	class Program
	{
	static void Main()
	{
	IEnumerator<string> countryEnumerator = (IEnumerator<string>)new List<string> { "France", "Canada", "Japan" };
	// Before C# 9: Error CS1579 foreach statement cannot operate on variables of type 'IEnumerator<string>'
	// because 'IEnumerator<string>' does not contain a public instance or extension definition for 'GetEnumerator'
	foreach (var country in countryEnumerator)
	{
	Console.WriteLine($"{country} is a beautiful country");
	}
	}
	}
	}

view raw Program.cs hosted with by GitHub

This is possible because countryEnumerator exposes a public GetEnumerator() method (which is an extension) that is recognized by the foreach statement, example:

Practical isn’t it ?