More About Classes - Illustrated C# 2005 - page 125

The readonly Modifier
A field can be declared with the readonly modifier. The effect is similar to declaring a field as
const , in that once the value is set, it cannot be changed.

￿Wh le a const field can only be initialized in the field's declaration statement, a readonly
field can have its value set in either of the following:
-

The field declaration statement—like a const .

Any of the class constructors. If it is a static field, then it must be done in the static
constructor.

-

While the value of a const field must be determinable at compile time, the value of a
readonly field can be determined at run time. This additional freedom allows you to set
different values in different constructors!

￿

Unlike a const , which always acts like a static, the following is true of a readonly field:
-

￿

It can be either an instance field or a static field.

- It has a storage location in memory.

For example, the following code declares a class called Shape , with two readonly fields.

￿Field PI is initialized in its declaration.

￿Field NumberOfSides is set to either 3 or 4, depending on which constructor is called.

class Shape
{ Keyword Initialized
↓ ↓
readonly double PI = 3.1416;
readonly int NumberOfSides;
↑ ↑
Keyword Not initialized

public Shape(double side1, double side2) // Constructor
{
// Shape is a rectangle
NumberOfSides = 4;
↑
... Set in constructor
}

public Shape(double side1, double side2, double side3) // Constructor
{
// Shape is a triangle
NumberOfSides = 3;
↑
... Set in constructor
}
}