C# Records - the rundown

Daily Knowledge Drop

Heard about the new feature of C# called records, but not entirely sure whats its all about? Here's the brief overview.

Records:

• Are reference types - just like normal classes
• Have equality based on value and not memory - unlike normal classes
• Are immutable (sometimes) - unlike normal classes
• Can be inherited - just like normal classes

Internally the compiler converts will convert a record declaration to a specialized class, so that it conforms to the above.

Declaring a record

Records can be declared similar to class declaration. Records created with nominal syntax are NOT immutable by default:

public record Song
{
    public string Artist { get; set; }
    public string Title { get; set; }
}

Records can also be declared using positional syntax. These are immutable by default:

    public record Song(string Artist, string Title);

Internally when using the positional syntax, a class is creates with init properties. This can also be done manually using nominal syntax to achieve the same result.

Record ToString

The ToString method of a record is overwritten (automatically) to output the record members, and not the record name (as with a class).

• Consider the class declaration and usage (using .NET 6 minimal startup):

var song1 = new Song("Foo Fighters", "Everlong");

// This will output the class name: Song
Console.WriteLine(song1.ToString());

public class Song
{  
    public string Artist { get; set; }
    public string Title { get; set; }
}

The above will output the class name: Song

• Now lets look at a record declaration and usage (using .NET 6 minimal startup):

var song1 = new Song("Foo Fighters", "Everlong");

// This will output the class name:
// Song { Artist = Foo Fighters, Title = Everlong }
Console.WriteLine(song1.ToString());

public record Song(string Artist, string Title);

The above will output the record values: Song { Artist = Foo Fighters, Title = Everlong }

Record equality

Classes are equal if their memory location is equal (ie. they are the exact same object), while records are equal if the value of the members are equal.

• Consider the class declaration and usage (using .NET 6 minimal startup):

var song1 = new Song("Foo Fighters", "Everlong");
var song2 = new Song("Foo Fighters", "Everlong");
var song3 = song2;

// This will output FALSE, as the objects don't
// point to the same memory
Console.WriteLine(song1 == song2);

// This will output TRUE, as the objects do
// point to the same memory
Console.WriteLine(song2 == song3);

public class Song
{  
    public string Artist { get; set; }
    public string Title { get; set; }
}

The output for the above:
False
True

• Now lets look at a record declaration and usage (using .NET 6 minimal startup):

var song1 = new Song("Foo Fighters", "Everlong");
var song2 = new Song("Foo Fighters", "Everlong");
var song3 = song2;

// This will output TRUE, as the objects have
// the same property values
Console.WriteLine(song1 == song2);

// This will output TRUE, as the objects have
// the same property values
Console.WriteLine(song2 == song3);

public record Song(string Artist, string Title);

The output for the above:
True
True

Immutability and cloning

As mentioned, records declared with positional syntax are immutable.

❌ This will not even compile:

    var song1 = new Song("Foo Fighters", "Everlong");
    song1.Title = "Learn to Fly";

✅ This will compile and work:

    var song1 = new Song("Foo Fighters", "Everlong");
    var song2 = song1 with { Title = "Learn to Fly"};

This is creating a copy of the song1 instance, and then changing the Title property value. The original object is not changed at all.

With nominal syntax declaration, both of the above are valid.

Notes

As part of C#10, record struct was introduced. If not specified, then the default is class.

    // This will declare a record class
    public record Song(string Artist, string Title);

    // This will declare a record class
    public record class Song(string Artist, string Title);

    // This will declare a record struct
    public record struct Song(string Artist, string Title);

There is a lot more detail to records (see the references below) but the above should give a rundown of the record highlights.

References

C# Records
Microsoft Docs - Records

Daily Drop 02: 02-02-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.