C# 9: Records Revisited

We learn a few more things about records using the released C# 9 spec proposal.

Dave Brock
Dave Brock

We had a lot of fun playing with the new C# 9 features over the last few months. As with learning anything brand new, we had to decipher the info the best we could: we went off the announcement in May during Build, meeting minutes and GitHub issues from the team, and good old trial and error.

With .NET 5 fast approaching this fall, things have taken shape and Microsoft has released the set of C# 9 specification proposals. These proposals offer clarity on the new features and help to reinforce things I’ve heard or saw, and also allowed me to learn a thing or two.

I wanted to quickly share with you some new things I came across while scanning the records proposal.

What are records?

What are records?

Short-ish answer: a record is a construct that allows you to encapsulate property state by providing immutable behavior without the need for extensive boilerplate (like for readonly structs). Record types are reference types, like class declarations, but allow you to implement value-like behaviors.

Long answer: check out last month’s post for a deep dive on the topic.

Random facts that I assumed but am glad are in writing

  • Record parameters can’t use ref, out, or this, but can use in and params
  • Records can inherit from other records, but cannot inherit from classes (unless it’s object) and classes can’t inherit from records

It looks like, as long as the record derives from object, records do include a synthesized method that allows you to inspect all the members for that record:

bool PrintMembers(System.StringBuilder builder);

From the spec, they note that it’ll iterate through a records public field and property members and take the format of this.firstName = "Dave", this.lastName = "Brock" and so on. You can declare this method explicitly.

I think using PrintMembers will definitely come in handy when you are working with complex records/inheritance and don’t want to override ToString. Quick and easy—I like it.

(It looks like the synthesized ToString method that ships with records invokes the PrintMembers method.)

Wrap up

This was a quick post to show a few things I came across while reading the specification. Take a look yourself to check out details on how it works.

CSharpCSharp 9