Muokkaa

Jaa

new operator - The new operator creates a new instance of a type

  • Artikkeli

The new operator creates a new instance of a type. You can also use the new keyword as a member declaration modifier or a generic type constraint.

Constructor invocation

To create a new instance of a type, you typically invoke one of the constructors of that type using the new operator:

var dict = new Dictionary<string, int>();
dict["first"] = 10;
dict["second"] = 20;
dict["third"] = 30;

Console.WriteLine(string.Join("; ", dict.Select(entry => $"{entry.Key}: {entry.Value}")));
// Output:
// first: 10; second: 20; third: 30

You can use an object or collection initializer with the new operator to instantiate and initialize an object in one statement, as the following example shows:

var dict = new Dictionary<string, int>
{
    ["first"] = 10,
    ["second"] = 20,
    ["third"] = 30
};

Console.WriteLine(string.Join("; ", dict.Select(entry => $"{entry.Key}: {entry.Value}")));
// Output:
// first: 10; second: 20; third: 30

Target-typed new

Constructor invocation expressions are target-typed. That is, if a target type of an expression is known, you can omit a type name, as the following example shows:

List<int> xs = new();
List<int> ys = new(capacity: 10_000);
List<int> zs = new() { Capacity = 20_000 };

Dictionary<int, List<int>> lookup = new()
{
    [1] = new() { 1, 2, 3 },
    [2] = new() { 5, 8, 3 },
    [5] = new() { 1, 0, 4 }
};

As the preceding example shows, you always use parentheses in a target-typed new expression.

If a target type of a new expression is unknown (for example, when you use the var keyword), you must specify a type name.

Array creation

You also use the new operator to create an array instance, as the following example shows:

var numbers = new int[3];
numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;
Console.WriteLine(string.Join(", ", numbers));
// Output:
// 10, 20, 30

Use array initialization syntax to create an array instance and populate it with elements in one statement. The following example shows various ways how you can do that:

var a = new int[3] { 10, 20, 30 };
var b = new int[] { 10, 20, 30 };
var c = new[] { 10, 20, 30 };
Console.WriteLine(c.GetType());  // output: System.Int32[]

For more information about arrays, see Arrays.

Instantiation of anonymous types

To create an instance of an anonymous type, use the new operator and object initializer syntax:

var example = new { Greeting = "Hello", Name = "World" };
Console.WriteLine($"{example.Greeting}, {example.Name}!");
// Output:
// Hello, World!

Destruction of type instances

You don't have to destroy earlier created type instances. Instances of both reference and value types are destroyed automatically. Instances of value types are destroyed as soon as the context that contains them is destroyed. Instances of reference types are destroyed by the garbage collector at some unspecified time after the last reference to them is removed.

For type instances that contain unmanaged resources, for example, a file handle, it's recommended to employ deterministic clean-up to ensure that the resources they contain are released as soon as possible. For more information, see the System.IDisposable API reference and the using statement article.

Operator overloadability

A user-defined type can't overload the new operator.

C# language specification

For more information, see The new operator section of the C# language specification.

For more information about a target-typed new expression, see the feature proposal note.

See also