various C# / .net implementations of a custom substring function

MySubstrings.cs

using System;
using System.Linq;
using System.Text;

namespace MySubstring
{
    // ReSharper disable once ClassNeverInstantiated.Global
    public class Program
    {
        // ReSharper disable once UnusedParameter.Global
        public static void Main(string[] args)
        {
            const string hello = "Hello World!";

            Console.WriteLine($"original: {hello}");
            Console.WriteLine($"solution  1: {hello.MySubstring1(6)}; {hello.MySubstring1(8, 3)}");
            Console.WriteLine($"solution  2: {hello.MySubstring2(6)}; {hello.MySubstring2(8, 3)}");
            Console.WriteLine($"solution  3: {hello.MySubstring3(6)}; {hello.MySubstring3(8, 3)}");
            Console.WriteLine($"solution  4: {hello.MySubstring4(6)}; {hello.MySubstring4(8, 3)}");
            Console.WriteLine($"solution  5: {hello.MySubstring5(6)}; {hello.MySubstring5(8, 3)}");

            Console.ReadLine();
        }
    }

    public static class Implementations
    {
        // using built-in feature
        public static string MySubstring1(this string s, int a, int? b = null) => b.HasValue ? s.Substring(a, b.Value) : s.Substring(a);

        // new C# 8.0 indices / ranges with return type enforcing .ToString() on the ReadOnlySpan<char> which is special as it gives a string back
        public static string MySubstring2(this string s, int a, int? b = null) => b.HasValue ? s[a..(a+b.Value)] : s[a..];

        // same but condensed with null coalescing
        public static string MySubstring3(this string s, int a, int? b = null) => s[a..(a+b ?? ^0)]; // I ♥ this

        // linq magic
        public static string MySubstring4(this string s, int a, int? b = null) => string.Concat(b.HasValue ? s.Skip(a).Take(b.Value).ToArray() : s.Skip(a).ToArray());

        // ye olden for loop with StringBuilder
        public static string MySubstring5(this string s, int a, int? b = null)
        {
            var sb = new StringBuilder();
            for (var i = a; i < (a + b ?? s.Length); i++) sb.Append(s[i]);
            return sb.ToString();
        }
    }
}