readonly.md

title	readonly (C# Reference)
ms.date	07/20/2015
ms.prod	.net
ms.technology	devlang-csharp
ms.topic	article
f1_keywords	readonly_CSharpKeyword readonly
helpviewer_keywords	readonly keyword [C#]
ms.assetid	2f8081f6-0de2-4903-898d-99696c48d2f4
caps.latest.revision	16
author	BillWagner
ms.author	wiwagn

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.

Readonly field example

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:

[!code-csharpcsrefKeywordsModifiers#14]

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.

Note

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 timeStamp = (uint)DateTime.Now.Ticks;  

Comparing readonly and non-readonly instance fields

[!code-csharpcsrefKeywordsModifiers#15]

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.

C# Language Specification

[!INCLUDECSharplangspec]

See Also

C# Reference
C# Programming Guide
C# Keywords
Modifiers
const
Fields