const and readonly
Learn the difference between const and readonly.
const fields
A const (constant) field is an immutable value computed at compile time. Declaration: public const double Pi = 3.14;.
Const values are substituted at compile time; every reference to Pi is replaced with 3.14. So const fields can only take constant expressions; no method calls or runtime values. Const fields are implicitly static; accessed as ClassName.Pi.
readonly fields
A readonly field holds an immutable value set at runtime. Declaration: public readonly int Radius;. It can be assigned only where it is declared or inside a constructor; afterwards it cannot change.
Unlike const, readonly can take a runtime value like new DateTime(2024,1,1). It is also an instance member; each instance may have a different value. Use readonly when the value can't be computed at compile time or is per-instance.
Exercise Circle perimeter
Add public const double Pi = 3.14; and public readonly int Radius; to Circle. The constructor assigns this.Radius = radius;. The Perimeter method returns (int)(2 * Pi * Radius). In Main, create new Circle(5) and print the perimeter.
Show example solution
Add `public const double Pi = 3.14;` and `public readonly int Radius;` to Circle. In the ctor write `this.Radius = radius;`. In Perimeter write `return (int)(2 * Pi * Radius);`. In Main write `var c = new Circle(5); Console.WriteLine(c.Perimeter());`.
Example solution:
using System;
class Circle
{
public const double Pi = 3.14;
public readonly int Radius;
public Circle(int radius)
{
this.Radius = radius;
}
public int Perimeter()
{
return (int)(2 * Pi * Radius);
}
}
class Program
{
static void Main()
{
var c = new Circle(5);
Console.WriteLine(c.Perimeter());
}
}Test yourself
-
Which statement about const is correct?
- A const field can be assigned at runtime.
- Const fields are implicitly static.
- Const fields can be changed in a constructor.
- Const fields can only be strings.
Show answer
Const fields are implicitly static. — Correct. Const fields are implicitly static; accessed through the class name.
-
Fill in the blank: const declaration
class MathConst { public const double Pi = ____; }Show answer
class MathConst { public const double Pi = 3.14; } -
Match: const and readonly
- const
- compile-time constant
- readonly
- assigned in constructor
- const Pi
- implicitly static
- readonly Radius
- separate per instance