Steve Fenton

C# 9 record types

Update! Since this article was published, it is almost certain that the keyword for record types will actually be record not data as it was in this early preview. I’ve updated code examples to reflect this.

We have taken a quick look at C# 9 Initializers and Immutability and C# 9 Non-Destructive Mutation. Let’s now look at the full transformation from an old class to a super character-light record type.

Our original record type is very much just a class with two auto-properties. It looks different to a normal class because we have the data keyword in there.

public record class Book {
    public string Author { get; init; }
    public string Title { get; init; }

Most of the text in this code file is “not the stuff I wrote”. It’s the stuff that expands out when I tab out from a snippet.

Because this is such a common code-shape, the C# team has made it possible to not even type it out. If we assume on a record that we want the properties to be public and that we want them to have an auto get and init then we end up with just this…

public record class Book {
    string Author;
    string Title;

This is a simple case and we’ve gone from 102 characters to 60 characters. My keyboard will last almost twice as long.

You can perform a similar trick if you want to force the parameters with a constructor.

public record class Book (string Author, string Title) { }

Or you can mix and match…

public record class Book (string Title) {
    string Author;

And finally, in the spirit of putting less stress on your hardware keys, new-ing up an object can now be done with less typing and less repetition.

Book book = new ("The Monk");

Note that I didn’t say new Book. We all know I want a book because I literally typed the type at the start of the line. Nice.

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