csharp9Records.cs

namespace cs9test
{
    // @jbogard "it looks like the "immutable-by-default" behavior of C# 9 records is really only with the compact positional syntax, not with just the "record" keyword"
    // https://twitter.com/jbogard/status/1321120266676850688


    /// <summary>
    /// immutable with compact positional syntax
    /// </summary>
    public record Person(string FirstName, string LastName);

    /// <summary>
    /// immutable with nominal syntax
    /// </summary>
    public record Person2
    {
        public string FirstName { get; init; }
        public string LastName { get; init; }
    }

    /// <summary>
    /// mutable with nominal syntax
    /// </summary>
    public record Person3
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var p1 = new Person("James", "Bond");

            // var p1a = new Person { FirstName = "James", LastName = "Bond" }; error CS7036: There is no argument given that corresponds to the required formal parameter 'FirstName' of 'Person.Person(string, string)'
            var p1b = new Person(LastName: "Bond", FirstName:"James");
            var p2 = new Person2{FirstName="James", LastName="Bond"};
            var p3 = new Person3 { FirstName = "James", LastName = "Bond" };

            // immuatable / mutable
            // p1.FirstName = "Blah"; // Error	CS8852	Init-only property or indexer 'Person.FirstName' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.	
            // p2.FirstName = "Blah"; // error CS8852: Init-only property or indexer 'Person2.FirstName' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.
            p3.FirstName = "Blah"; // ok


            Console.WriteLine($"Hello World! {p1} {p2} {p3} {p1b}");
        }
    }
}