.NET 1.1+

C# Constants and Enumerations

The forty-eighth part of the C# Fundamentals tutorial examines the use of constants and enumerations. These provide two methods of describing non-changing values using descriptive words rather than "magic numbers" to improve code readability.

Why Use Constants?

A constant is similar to a variable as it defines a name for a value. However, a constant differs from a variable because once defined, the value assigned to the constant cannot be changed. The benefit of constants is their assistance in creating self-documenting code. They also allow you to declare key values in a single place, permitting easy maintenance should the value need to be updated and the software recompiled.

To demonstrate the self-documenting value of constants, consider the following line of code:

You could probably work out what the developer was trying to do here. A price is being increased by 17.5% and then a further £4.99 is being added. However, if the code is rewritten using constants instead of magic numbers, the purpose becomes obvious.

Not only is this second code sample easier to read, it allows the definitions of tax rules and delivery charges to be changed in a single location. The constants can be modified once and the new values automatically be updated throughout your code. NB: The scope of a constant is defined in a similar manner to variable scopes.

Declaring a Constant

Constants are declared using similar syntax to other variables but with the const prefix . Here is how the previous example code looks with the constant definitions included:

double price = 100;
const double ValueAddedTax = 0.175;
const double DeliveryCharge = 4.99;

price += (price * ValueAddedTax) + DeliveryCharge;

Constants must be possible to evaluate at compile-time. This limits the use of constants to values that can be expressed directly in the source code, such as numeric values and strings. Values that cannot be initialised without a constructor are not permitted, meaning reference type constants may only be null.