MaylorTaylor
MaylorTaylor

Const Vs. Readonly

27 Aug 2018

What is the difference between constant and readonly in C#?

Constant (const)

Constant can be used with the const keyword in C# which is also known immutable values which are known at compile time and do not change their values at run time.

Const’s are implicitly static. You use a ClassName.ConstantName notation to access them. Constant fields and locals aren’t variables and may not be modified. Constants can be numbers, Boolean values, strings, or a null reference.

Don’t create a constant to represent information that you expect to change at any time

The static modifier is not allowed in a constant declaration.

public class ConstTest
{
    class SampleClass
    {
        public int x;
        public int y;
        public const int c1 = 5;
        public const int c2 = c1 + 5;

        public SampleClass(int p1, int p2)
        {
            x = p1;
            y = p2;
        }
    }

    static void Main()
    {
        SampleClass mC = new SampleClass(11, 22);
        Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y);
        Console.WriteLine("c1 = {0}, c2 = {1}",
                        SampleClass.c1, SampleClass.c2);
    }
}
/* Output
    x = 11, y = 22
    c1 = 5, c2 = 10
*/

Readonly (readonly)

Readonly can be used by the readonly keyword in C# which is also known immutable values and are known at compile and run time.

readonly indicates that assignment to the field 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:

When the variable is initialized in the declaration, for example:

  • When the variable is initialized in the declaration, for example:

    public readonly int y = 5;
  • In an instance constructor of the class that contains the instance field declaration.

  • In the static constructor of the class that contains the static field declaration.

    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($"p1: x={p1.x}, y={p1.y}, z={p1.z}");
       SampleClass p2 = new SampleClass();
       p2.x = 55;   // OK
       Console.WriteLine($"p2: x={p2.x}, y={p2.y}, z={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 the following example:

p2.y = 66; // Error

you will get the compiler error message:

A readonly field cannot be assigned to (except in a constructor or a variable initializer)

References