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Introducing C# 10: Record struct

Introduction

Last year with the release of C# 9, Microsoft introduced records. The record keyword gives a reference type new superpowers like immutability declared with positional records (or by using init-only properties), equality comparisons that mimic value types, and with-expressions that allows you to create a new record instance with the same property values, the properties you need to change. This drastically simplifies the process to copy objects.
This year, C# 10 brings records structs. They’ll carry a lot of the advantages of C# 9 records (which are reference types, like classes), but don’t get fooled: there are differences with structs because…. structs are different from classes!
In this article, we will see what a record struct is, and why a record class doesn’t behave like a record struct.
If you need a reminder on record class, you read this post:
https://anthonygiretti.com/2020/06/17/introducing-c-9-records/

This article has been made with my friend Dave Brock and I would like to take the opportunity to thanks him. I you wanna follow him you can find his post and his social medias infos: https://www.daveabrock.com/.

Syntax

First of all, Microsoft has made an improvement to record classes. With C# 9, to declare a record you replaced the “class” keyword with “record.” To avoid confusion when declaring structs as records, C# 10 allows a new syntax to declare a class as a record by mixing record and class keywords:

public record class Product {}

The C# 9 syntax remains valid:

public record Product {}

Declaring a struct as a record looks like this:

public record struct Product {}

It’s a more convenient approach to avoid confusion between a record class and a record struct. A record struct is a struct with all its struct properties and a record class is a class with all its class properties.

Immutability

Init-only properties are allowed on record structs:

If you try to reassign a property that has the init keyword set after its initialization you’ll get a compilation error:

Using positional records is quite different for record structs. Positional records on struct doesn’t make the record immutable as a record class. Because it’s a struct you have to set the readonly keyword to make the record struct immutable. The following code is equivalent to the previous declaration above:

With-expressions

Like a record class, a record struct allows the usage of with-expressions and works similarly to a record class:

Equality comparison

Because a record struct is a struct, comparing (with Equals method) two structs that have the same values will always return true. A struct is a value type, unlike a class. A regular struct doesn’t implement == and != operators, so it’s impossible to compare two structs with these operators. However, the comparison with these operators is allowed on a record struct:

Printing members

Record structs implement a new override of the ToString() method that allows printing a structured string of the struct record. With the same signature of Product struct, let’s compare the output:

If Product is a regular struct:

Id Product is a record struct:

Performance

In terms of performance, structs offer performance benefits. In some benchmarks, using record structs is 20 times faster than a regular struct. A future post will show off detailed benchmarks.

Conclusion

This article aimed to introduce record struct with C# 10. Because it’s a big feature, probably the most in C# 10, I probably missed some aspects of record structs. At this time of writing C# 10 is still in preview, and this article may evolve once C# 10 is released.

Hope you’ll enjoy record structs, personnaly I love it! 🙂

Written by

anthonygiretti

Anthony is a specialist in Web technologies (14 years of experience), in particular Microsoft .NET and learns the Cloud Azure platform. He has received twice the Microsoft MVP award and he is also certified Microsoft MCSD and Azure Fundamentals.
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