using System;

using System.Runtime.CompilerServices;

​

#nullable enable

public static class NullExtensions

{

    public static T ThrowIfNull<T>(

        this T? argument, 

        string? message = default, 

        [CallerArgumentExpression("argument")] string? paramName = default

    )

    {

        if (argument is null)

        {

            throw new ArgumentNullException(paramName, message);

        }

        else

        {

            return argument;

        }

    }

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        string? foo = "foo";

        string nonNullableFoo = foo.ThrowIfNull();  // success from "string?" to "string"

        Console.WriteLine(nonNullableFoo);

        bool? baz = true;

        bool nonNullableBaz = baz.ThrowIfNull();    // success from "string?" to "string"

        Console.WriteLine(nonNullableFoo);

        int? bar = 2;

        int nonNullableBar = bar.ThrowIfNull(); // error: Cannot implicitly convert type 'int?' to 'int'

        Console.WriteLine(nonNullableBar);

    }

}