using System;

using System.Collections.Generic;

​

public class Program

{

    public void Main()

    {

        GetChars(null);

        Console.WriteLine("done!");

    }

​

    static IEnumerable<char> GetChars(string input)

    {

        if (input == null)

            throw new ArgumentNullException(input);

        foreach (char c in input)

            yield return c;

    }

}