How do I append values when copying a "record" (C#) to a different "record" using the "with" expression?

Question

I was reading about the " with " expression on MSDN, and decided to tinker a bit with the sample code provided there. How do I append the list of tags when creating var copy , without writing out the three tags explicitly, like I did here ( new List<string> { "A", "B", "C"} )? I tried Tags = original.Tags + "C" , but that doesn't work, of course.

using System;
using System.Globalization;
using System.Threading.Tasks;

namespace AsyncBreakfast
{
    public class ExampleWithReferenceType
    {
        public record TaggedNumber(int Number, List<string> Tags)
        {
            public string PrintTags() => string.Join(", ", Tags);
        }

        public static void Main()
        {
            var original = new TaggedNumber(1, new List<string> { "A", "B" });

            var copy = original with { Tags = new List<string> { "A", "B", "C"} };
            Console.WriteLine($"Tags of {nameof(copy)}: {copy.PrintTags()}");
            // output: Tags of copy: A, B, C

            original.Tags.Add("C");
            Console.WriteLine($"Tags of {nameof(copy)}: {copy.PrintTags()}");
            // output: Tags of copy: A, B, C

        }
    }
}

Answer 1

You can create a copy of a list, with an extra element, by doing something like:

with { Tags = original.Tags.Append("C").ToList() }

However, if you want your records to be immutable (and you probably should, if you're following the approach of doing modifications by creating a copy), then you probably don't want to be using a List<T> at all, since List<T> is mutable.

Instead, you can use an ImmutableList<T> . This has an Add method which returns a new ImmutableList<T> containing the specified element, meaning you can write:

with { Tags = original.Tags.Add("C") }