readonly (C# Reference)

The readonly keyword is a modifier that can be used in four contexts:

  • In a field declaration, readonly indicates that assignment to the field can only occur as part of the declaration or in a constructor in the same class. A readonly field can be assigned and reassigned multiple times within the field declaration and constructor.

    A readonly field can't be assigned after the constructor exits. This rule has different implications for value types and reference types:

    • Because value types directly contain their data, a field that is a readonly value type is immutable.
    • Because reference types contain a reference to their data, a field that is a readonly reference type must always refer to the same object. That object isn't immutable. The readonly modifier prevents the field from being replaced by a different instance of the reference type. However, the modifier doesn't prevent the instance data of the field from being modified through the read-only field.


    An externally visible type that contains an externally visible read-only field that is a mutable reference type may be a security vulnerability and may trigger warning CA2104 : "Do not declare read only mutable reference types."

  • In a readonly struct type definition, readonly indicates that the structure type is immutable. For more information, see the readonly struct section of the Structure types article.

  • In an instance member declaration within a structure type, readonly indicates that an instance member doesn't modify the state of the structure. For more information, see the readonly instance members section of the Structure types article.

  • In a ref readonly method return, the readonly modifier indicates that method returns a reference and writes aren't allowed to that reference.

Readonly field example

In this example, the value of the field year can't be changed in the method ChangeYear, even though it's assigned a value in the class constructor:

class Age
    private readonly int _year;
    Age(int year)
        _year = year;
    void ChangeYear()
        //_year = 1967; // Compile error if uncommented.

You can assign a value to a readonly field only in the following contexts:

  • When the variable is initialized in the declaration, for example:

    public readonly int y = 5;
  • In an instance constructor of the class that contains the instance field declaration.

  • In the static constructor of the class that contains the static field declaration.

These constructor contexts are also the only contexts in which it's valid to pass a readonly field as an out or ref parameter.


The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be assigned multiple times in the field declaration and in any constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for run-time constants as in the following example:

public static readonly uint timeStamp = (uint)DateTime.Now.Ticks;
public class SamplePoint
    public int x;
    // Initialize a readonly field
    public readonly int y = 25;
    public readonly int z;

    public SamplePoint()
        // Initialize a readonly instance field
        z = 24;

    public SamplePoint(int p1, int p2, int p3)
        x = p1;
        y = p2;
        z = p3;

    public static void Main()
        SamplePoint p1 = new SamplePoint(11, 21, 32);   // OK
        Console.WriteLine($"p1: x={p1.x}, y={p1.y}, z={p1.z}");
        SamplePoint p2 = new SamplePoint();
        p2.x = 55;   // OK
        Console.WriteLine($"p2: x={p2.x}, y={p2.y}, z={p2.z}");
        p1: x=11, y=21, z=32
        p2: x=55, y=25, z=24

In the preceding example, if you use a statement like the following example:

p2.y = 66;        // Error

you'll get the compiler error message:

A readonly field cannot be assigned to (except in a constructor or a variable initializer)

Ref readonly return example

The readonly modifier on a ref return indicates that the returned reference can't be modified. The following example returns a reference to the origin. It uses the readonly modifier to indicate that callers can't modify the origin:

private static readonly SamplePoint s_origin = new SamplePoint(0, 0, 0);
public static ref readonly SamplePoint Origin => ref s_origin;

The type returned doesn't need to be a readonly struct. Any type that can be returned by ref can be returned by ref readonly.

C# language specification

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

You can also see the language specification proposals:

See also