Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
Export (0) Print
Expand All
Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

readonly (C# Reference)

The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class. In this example, the value of the field year cannot be changed in the method ChangeYear, even though it is assigned a value in the class constructor:

class Age
    readonly int _year;
    Age(int year)
        _year = year;
    void ChangeYear()
        _year = 1967; // Will not compile.

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;
  • For an instance field, in the instance constructors of the class that contains the field declaration, or for a static field, in the static constructor of the class that contains the field declaration. These are also the only contexts in which it is 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 initialized either at the declaration or in a 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 runtime constants as in the following example:


public static readonly uint l1 = (uint)DateTime.Now.Ticks;

// cs_readonly_keyword.cs
// Readonly fields
using System;
public class ReadOnlyTest
   class SampleClass
      public int x;
      // Initialize a readonly field
      public readonly int y = 25;
      public readonly int z;

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

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

   static void Main()
      SampleClass p1 = new SampleClass(11, 21, 32);   // OK
      Console.WriteLine("p1: x={0}, y={1}, z={2}", p1.x, p1.y, p1.z);
      SampleClass p2 = new SampleClass();
      p2.x = 55;   // OK
      Console.WriteLine("p2: x={0}, y={1}, z={2}", p2.x, p2.y, 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 this:

p2.y = 66; // Error

you will get the compiler error message:

The left-hand side of an assignment must be an l-value

which is the same error you get when you attempt to assign a value to a constant.

For more information, see the following section in the C# Language Specification:

  • 10.4.2 Readonly fields

Community Additions

© 2015 Microsoft