Records

Records

A record is a type intended primarily for storing data. The compiler automatically generates value equality, GetHashCode, ToString, a copy method, and deconstruction for it. The shortest, positional form of the declaration is:

record Book(string Title, string Author, int Year);

For each parameter, a read-only property with an init accessor is created, along with a primary constructor and a Deconstruct method. The record declaration is equivalent to record class: a record is a reference type. Compared with a class:

• value equality: == and Equals compare the values of all properties and the record type, not references (in Topic 9, you had to override Equals and GetHashCode manually for this);
• ToString prints the type name and property values: Book { Title = Kobzar, … };
• the properties of a positional record are immutable: book.Year = 2025 causes error CS8852.

A record can have a body with additional properties, methods, and parameter validation. Fig. 11.3 shows how a record looks in the debugger.

Figure 11.3. Automatic ToString of a record in the debugger

with expressions, deconstruction, inheritance

An immutable record is not changed; instead, a modified copy is created with a with expression: all properties are copied, and those listed in the braces get new values. Deconstruction breaks a record into variables in the order of the positional parameters; unneeded values are discarded with the _ symbol:

Book book = new("Kobzar", "Taras Shevchenko", 1840);
Book reprint = book with { Year = 2024 };
var (title, _, year) = reprint;     // "Kobzar", 2024

A record can inherit only from another record. A derived record passes parameters to the base one, and equality also checks the type: a Book and an EBook with the same title, author, and year are not equal. with expressions also work for structures and anonymous types.

A positional record shortens a data declaration compared with a class with a primary constructor and properties (Fig. 11.4); in addition, it gets value equality and the other generated record members.

Figure 11.4. A class with properties and a positional record

record struct and choosing a type

The record struct declaration creates a record structure: a value type with generated Equals, ==, GetHashCode, ToString, and deconstruction. Unlike record class, the properties of a positional record struct are mutable (get; set;). An immutable record structure is declared as readonly record struct:

record struct Size(int Width, int Height);          // mutable
readonly record struct Point(double X, double Y);  // immutable

Table 11.1 summarizes the options.

Table 11.1. Comparison of classes, structures, and records

Feature	class	struct	record	record struct
kind of type	reference	value	reference	value
equality	of references	of fields	of values	of values
== generated	no	no	yes	yes
ToString with data	no	no	yes	yes
positional properties immutable	–	–	yes	with readonly
with expression	no	yes	yes	yes
inheritance	yes	no	from records	no

Practical recommendations: entities with behavior and identity (an account, a user) are classes; immutable data compared by value (a book in a catalog, a message, a calculation result) are records; small values (a point, an amount of money, a color) are readonly struct or readonly record struct.