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Value types (C# reference)

  • บทความ

Value types and reference types are the two main categories of C# types. A variable of a value type contains an instance of the type. This differs from a variable of a reference type, which contains a reference to an instance of the type. By default, on assignment, passing an argument to a method, and returning a method result, variable values are copied. In the case of value-type variables, the corresponding type instances are copied. The following example demonstrates that behavior:

using System;

public struct MutablePoint
{
    public int X;
    public int Y;

    public MutablePoint(int x, int y) => (X, Y) = (x, y);

    public override string ToString() => $"({X}, {Y})";
}

public class Program
{
    public static void Main()
    {
        var p1 = new MutablePoint(1, 2);
        var p2 = p1;
        p2.Y = 200;
        Console.WriteLine($"{nameof(p1)} after {nameof(p2)} is modified: {p1}");
        Console.WriteLine($"{nameof(p2)}: {p2}");

        MutateAndDisplay(p2);
        Console.WriteLine($"{nameof(p2)} after passing to a method: {p2}");
    }

    private static void MutateAndDisplay(MutablePoint p)
    {
        p.X = 100;
        Console.WriteLine($"Point mutated in a method: {p}");
    }
}
// Expected output:
// p1 after p2 is modified: (1, 2)
// p2: (1, 200)
// Point mutated in a method: (100, 200)
// p2 after passing to a method: (1, 200)

As the preceding example shows, operations on a value-type variable affect only that instance of the value type, stored in the variable.

If a value type contains a data member of a reference type, only the reference to the instance of the reference type is copied when a value-type instance is copied. Both the copy and original value-type instance have access to the same reference-type instance. The following example demonstrates that behavior:

using System;
using System.Collections.Generic;

public struct TaggedInteger
{
    public int Number;
    private List<string> tags;

    public TaggedInteger(int n)
    {
        Number = n;
        tags = new List<string>();
    }

    public void AddTag(string tag) => tags.Add(tag);

    public override string ToString() => $"{Number} [{string.Join(", ", tags)}]";
}

public class Program
{
    public static void Main()
    {
        var n1 = new TaggedInteger(0);
        n1.AddTag("A");
        Console.WriteLine(n1);  // output: 0 [A]

        var n2 = n1;
        n2.Number = 7;
        n2.AddTag("B");

        Console.WriteLine(n1);  // output: 0 [A, B]
        Console.WriteLine(n2);  // output: 7 [A, B]
    }
}

Note

To make your code less error-prone and more robust, define and use immutable value types. This article uses mutable value types only for demonstration purposes.

Kinds of value types and type constraints

A value type can be one of the two following kinds:

  • a structure type, which encapsulates data and related functionality
  • an enumeration type, which is defined by a set of named constants and represents a choice or a combination of choices

A nullable value type T? represents all values of its underlying value type T and an additional null value. You cannot assign null to a variable of a value type, unless it's a nullable value type.

You can use the struct constraint to specify that a type parameter is a non-nullable value type. Both structure and enumeration types satisfy the struct constraint. You can use System.Enum in a base class constraint (that is known as the enum constraint) to specify that a type parameter is an enumeration type.

Built-in value types

C# provides the following built-in value types, also known as simple types:

All simple types are structure types and differ from other structure types in that they permit certain additional operations:

  • You can use literals to provide a value of a simple type.
    For example, 'A' is a literal of the type char, 2001 is a literal of the type int and 12.34m is a literal of the type decimal.

  • You can declare constants of the simple types with the const keyword.
    For example, you can define const decimal = 12.34m.
    It's not possible to have constants of other structure types.

  • Constant expressions, whose operands are all constants of the simple types, are evaluated at compile time.

A value tuple is a value type, but not a simple type.

C# language specification

For more information, see the following sections of the C# language specification:

See also