C# 9 Records

In C# 9.0, a new record type is introduced which is a reference type that provides synthesized methods to provide value semantics for equality.

• It is largely classified as reference types including classes and anonymous types and value types including structs and tuples.
• Records cannot inherit from classes unless the class is an object, and classes cannot inherit from records, records can inherit from other records.
• The record type immutable by default and make it easy to create immutable reference types in .NET.
• The immutable value types are recommended, mutable value types don't often introduce errors.

The record keyword makes an object immutable and behaves like a value type.

public record Customer
{
    public string LastName { get; }
    public string FirstName { get; }

    public Customer(string first, string last) => (FirstName, LastName) = (first, last);
}

• One of the key differences between regular classes and records is that records are intended to be immutable and their state never changes.
• Therefore, each record represents the state of an object at a specific point in time.
• Whenever the object's state changes, it will create a copy of the record, updating the members that have changed, rather than changing the original record directly.

You can also make the whole object immutable using the init keyword on each property if you are using an implicit parameterless constructor.

public record Customer
{
    public string LastName { get; init; }
    public string FirstName { get; init; }
}

Records support inheritance and you can declare a new record derived from the base class.

public record Person
{
    public string LastName { get; }
    public string FirstName { get; }

    public Person(string first, string last) => (FirstName, LastName) = (first, last);
}

public record Employee : Person
{
    public string Title { get; }

    public Employe(string first, string last, string title) : base(first, last) => Title = title;
}

Equality

By default, regular classes are considered equal when they share the same underlying reference.

• If you create two instances of the same class, holding the same values, they will not be considered equal.
• When you declare your class as a record and compare two instances of the record it will not compare the object's references, records are compared by value.
• It means that two different objects holding the same values will be considered equal.

Customer customer1 = new Customer
{
    FirstName = "Mark",
    LastName = "Upston"
};

Customer customer2 = new Customer
{
    FirstName = "Mark",
    LastName = "Upston"
};

Console.WriteLine(customer1.Equals(customer2)); // True