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; // Compile error if uncommented. } }
You can assign a value to a readonly field only in the following contexts:
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When the variable is initialized in the declaration, for example:
public readonly int y = 5;
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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.
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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;
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); } } /* Output: 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 C# Language Specification. The language specification is the definitive source for C# syntax and usage.