CSharpOption.cs

using System;

public abstract class Option<T>
{
    public static Option<T> some(T value) => new Some(value); //we need these to make the actual Some/None constructors internal
    public static Option<T> none() => new None();

    public sealed class Some : Option<T>
    {
        public readonly T value;

        internal Some(T value) //internal so nobody can create a standalone Some or None instance
        {
            this.value = value ?? throw new ArgumentNullException(nameof(value));
        }
    }

    public sealed class None : Option<T>
    {
        internal None() { }
    }
}

public class HelloWorld
{
    public static void Main(string[] args)
    {
        Option<int> someValue = Option<int>.some(42);
        Option<int> noneValue = Option<int>.none();
        
        var valueOrDefault = 
            someValue switch {
		Option<int>.Some { value: var num } => num,
		_ => 0
		//Option<int>.None => 0, //You could also match none but it's not exhaustive so you'd need _ anyway
	    };
		
	Console.WriteLine(valueOrDefault); //prints 42
    }
}