required modifier (C# Reference)

The required modifier indicates that the field or property it's applied to must be initialized by an object initializer. Any expression that initializes a new instance of the type must initialize all required members. The required modifier is available beginning with C# 11. The required modifier enables developers to create types where properties or fields must be properly initialized, yet still allow initialization using object initializers. Several rules ensure this behavior:

• The required modifier can be applied to fields and properties declared in struct, and class types, including record and record struct types. The required modifier can't be applied to members of an interface.
• Explicit interface implementations can't be marked as required. They can't be set in object initializers.
• Required members must be initialized, but they may be initialized to null. If the type is a non-nullable reference type, the compiler issues a warning if you initialize the member to null. The compiler issues an error if the member isn't initialized at all.
• Required members must be at least as visible as their containing type. For example, a public class can't contain a required field that's protected. Furthermore, required properties must have setters (set or init accessors) that are at least as visible as their containing types. Members that aren't accessible can't be set by code that creates an instance.
• Derived classes can't hide a required member declared in the base class. Hiding a required member prevents callers from using object initializers for it. Furthermore, derived types that override a required property must include the required modifier. The derived type can't remove the required state. Derived types can add the required modifier when overriding a property.
• A type with any required members may not be used as a type argument when the type parameter includes the new() constraint. The compiler can't enforce that all required members are initialized in the generic code.
• The required modifier isn't allowed on the declaration for positional parameters on a record. You can add an explicit declaration for a positional property that does include the required modifier.

Some types, such as positional records, use a primary constructor to initialize positional properties. If any of those properties include the required modifier, the primary constructor adds the SetsRequiredMembers attribute. This indicates that the primary constructor initializes all required members. You can write your own constructor with the System.Diagnostics.CodeAnalysis.SetsRequiredMembersAttribute attribute. However, the compiler doesn't verify that these constructors do initialize all required members. Rather, the attribute asserts to the compiler that the constructor does initialize all required members. The SetsRequiredMembers attribute adds these rules to constructors:

• A constructor that chains to another constructor annotated with the SetsRequiredMembers attribute, either this(), or base(), must also include the SetsRequiredMembers attribute. That ensures that callers can correctly use all appropriate constructors.
• Copy constructors generated for record types have the SetsRequiredMembers attribute applied if any of the members are required.

The following code shows a class hierarchy that uses the required modifier for the FirstName and LastName properties:

public class Person
{
    public Person() { }

    [SetsRequiredMembers]
    public Person(string firstName, string lastName) =>
        (FirstName, LastName) = (firstName, lastName);

    public required string FirstName { get; init; }
    public required string LastName { get; init; }

    public int? Age { get; set; }
}

public class Student : Person
{
    public Student() : base()
    {
    }

    [SetsRequiredMembers]
    public Student(string firstName, string lastName) :
        base(firstName, lastName)
    {
    }

    public double GPA { get; set; }
}

For more information on required members, see the C#11 - Required members feature specification.