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init (C# Reference)

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The init keyword defines an accessor method in a property or indexer. An init-only setter assigns a value to the property or the indexer element only during object construction. An init enforces immutability, so that once the object is initialized, it can't be changed. An init accessor enables calling code to use an object initializer to set the initial value. As a contrast, an auto-implemented property with only a get setter must be initialized by calling a constructor. A property with a private set accessor can be modified after construction, but only in the class.

The following example defines both a get and an init accessor for a property named YearOfBirth. It uses a private field named _yearOfBirth to back the property value.

class Person_InitExample
{
     private int _yearOfBirth;

     public int YearOfBirth
     {
         get { return _yearOfBirth; }
         init { _yearOfBirth = value; }
     }
}

Often, the init accessor consists of a single statement that assigns a value, as it did in the previous example. Because of init, the following doesn't work:

var john = new Person_InitExample
{
    YearOfBirth = 1984
};

john.YearOfBirth = 1926; //Not allowed, as its value can only be set once in the constructor

An init accessor doesn't force callers to set the property. Instead, it allows callers to use an object initializer while prohibiting later modification. You can add the required modifier to force callers to set a property. The following example shows an init only property with a nullable value type as its backing field. If a caller doesn't initialize the YearOfBirth property, that property will have the default null value:

class Person_InitExampleNullability
{
    private int? _yearOfBirth;

    public int? YearOfBirth
    {
        get => _yearOfBirth;
        init => _yearOfBirth = value;
    }
}

To force callers to set an initial non-null value, you add the required modifier, as shown in the following example:

class Person_InitExampleNonNull
{
    private int _yearOfBirth;

    public required int YearOfBirth
    {
        get => _yearOfBirth;
        init => _yearOfBirth = value;
    }
}

The init accessor can be used as an expression-bodied member. Example:

class Person_InitExampleExpressionBodied
{
    private int _yearOfBirth;

    public int YearOfBirth
    {
        get => _yearOfBirth;
        init => _yearOfBirth = value;
    }
}

The init accessor can also be used in autoimplemented properties, as the following example code demonstrates:

class Person_InitExampleAutoProperty
{
    public int YearOfBirth { get; init; }
}

The following example shows the distinction between a private set, read only, and init property. Both the private set version and the read only version require callers to use the added constructor to set the name property. The private set version allows a person to change their name after the instance is constructed. The init version doesn't require a constructor. Callers can initialize the properties using an object initializer:

class PersonPrivateSet
{
    public string FirstName { get; private set; }
    public string LastName { get; private set; }
    public PersonPrivateSet(string first, string last) => (FirstName, LastName) = (first, last);

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

class PersonReadOnly
{
    public string FirstName { get; }
    public string LastName { get; }
    public PersonReadOnly(string first, string last) => (FirstName, LastName) = (first, last);
}

class PersonInit
{
    public string FirstName { get; init; }
    public string LastName { get; init; }
}

PersonPrivateSet personPrivateSet = new("Bill", "Gates");
PersonReadOnly personReadOnly = new("Bill", "Gates");
PersonInit personInit = new() { FirstName = "Bill", LastName = "Gates" };

C# language specification

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

See also