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Record in C#

 

Record in C#

Introduction

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.

 The release of C# 10 brings records structs. They’ll carry a lot of the advantages of C# 9 records (which are reference types, like classes), but 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.

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 Naija {}

The C# 9 syntax remains valid:

public record Naija {}

Declaring a struct as a record looks like this:

public record struct Naija {}

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:

               public record struct Product

               {

                   public string Name { get; init; }

                  

                   public int CategoryId { get; init; }

               }

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

               using System;

               public class Program

               {

                        public static void Main()

                   {   

                       var product = new Product

                       {

                           Name = "VideoGame",

                           CategoryId = 1

                       };

                       product.Name = ""; // Error CS8852  Init-only property or indexer 'Product.Name' can only be assigned in an object initializer, or on 'this' or 'base' in an instance constructor or an 'init' accessor.

                   }

               }

 

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:

               public readonly record struct Product(string Name, int CategoryId);

 With-expressions

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

               using System;

               public class Program

               {

                   public static void Main()

                   {

                       var product = new Product

                       {

                           Name = "VideoGame",

                           CategoryId = 1

                       };

                        var newProduct = product with { CategoryId = 2 };

                   }

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:

               using System;

               public class Program

               {

                   public static void Main()

                   {

                       var product1 = new Product

                       {

                           Name = "VideoGame",

                           CategoryId = 1

                       };

                       var product2 = new Product

                       {

                           Name = "VideoGame",

                           CategoryId = 1

                       };

                       Console.WriteLine(product1.Equals(product2)); // Returns true

                       Console.WriteLine(product1 == product2); // Returns true. Only allowed on record structs, not allowed on regular structs

                   }

               }

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:

               public class Program

               {

                   public static void Main()

                   {

                       var product1 = new Product

                       {

                           Name = "VideoGame",

                           CategoryId = 1

                       };

                       Console.WriteLine(product); // Will output: Product { Name= VideoGame, CategoryId = 1 }

                   }

                }

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