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.
You can assign a value to a readonly field only in the following contexts:
public readonly int y = 5;
readonly
field as an out or ref parameter.// Readonly fields using System; public class ReadOnlyTest { class MyClass { public int x; public readonly int y = 25; // Initialize a readonly field public readonly int z; public MyClass() { z = 24; // Initialize a readonly instance field } public MyClass(int p1, int p2, int p3) { x = p1; y = p2; z = p3; } } public static void Main() { MyClass p1= new MyClass(11, 21, 32); // OK Console.WriteLine("p1: x={0}, y={1}, z={2}" , p1.x, p1.y, p1.z); MyClass p2 = new MyClass(); 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.