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Therequired
modifier indicates that thefield orproperty it's applied to must be initialized by anobject initializer. Any expression that initializes a new instance of the type must initialize allrequired members. Therequired
modifier is available beginning with C# 11. Therequired
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:
required
modifier can be applied tofields andproperties declared instruct
, andclass
types, includingrecord
andrecord struct
types. Therequired
modifier can't be applied to members of aninterface
.required
. They can't be set in object initializers.null
. If the type is a non-nullable reference type, the compiler issues a warning if you initialize the member tonull
. The compiler issues an error if the member isn't initialized at all.public
class can't contain arequired
field that'sprotected
. Furthermore, required properties must have setters (set
orinit
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.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 therequired
modifier. The derived type can't remove therequired
state. Derived types can add therequired
modifier when overriding a property.required
members may not be used as a type argument when the type parameter includes thenew()
constraint. The compiler can't enforce that all required members are initialized in the generic code.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 therequired
modifier.Some types, such aspositional records, use a primary constructor to initialize positional properties. If any of those properties include therequired
modifier, the primary constructor adds theSetsRequiredMembers
attribute. This indicates that the primary constructor initializes all required members. You can write your own constructor with theSystem.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. TheSetsRequiredMembers
attribute adds these rules to constructors:
SetsRequiredMembers
attribute, eitherthis()
, orbase()
, must also include theSetsRequiredMembers
attribute. That ensures that callers can correctly use all appropriate constructors.record
types have theSetsRequiredMembers
attribute applied if any of the members arerequired
.Warning
TheSetsRequiredMembers
disables the compiler's checks that allrequired
members are initialized when an object is created. Use it with caution.
The following code shows a class hierarchy that uses therequired
modifier for theFirstName
andLastName
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 theC#11 - Required members feature specification.
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