I've struggled to find a use case for the possibility to write .GetEnumerator() as extension methods in C# 9, and I think I found one. Ranges in C# is only for producing slices of arrays and not for defining a collection as in many other languages. But with a simple extension method, it's possible.

public static class RangeExtensions
{
    public static IEnumerator<int> GetEnumerator(this Range range) =>
        Enumerable.Range(range.Start.Value, range.End.Value - range.Start.Value + 1)
        .GetEnumerator();
}

Now it's possible to iterate over a Range with a foreach like this.

foreach(var indx in 1..11)
{
    Console.WriteLine(indx);
}

That will produce a result like this.

1
2
3
4
5
6
7
8
9
10
11

I hope you'll find some use for it. :)