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 instruct
, andclass
types, includingrecord
andrecord struct
types. Therequired
modifier can't be applied to members of aninterface
. - 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 tonull
. 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 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. - 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 therequired
modifier. The derived type can't remove therequired
state. Derived types can add therequired
modifier when overriding a property. - A type with any
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. - 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 therequired
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, eitherthis()
, orbase()
, must also include theSetsRequiredMembers
attribute. That ensures that callers can correctly use all appropriate constructors. - Copy constructors generated for
record
types have theSetsRequiredMembers
attribute applied if any of the members arerequired
.
Warning
The SetsRequiredMembers
disables the compiler's checks that all required
members are initialized when an object is created. Use it with caution.
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.
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