What are and how to use constants in C#

An constant is an identifier used to represent a predefined constant value that does not change during program execution.

Constants are used to avoid repeating literal values in the code and to improve its clarity and readability.

Furthermore, using constants makes updating and maintaining the code easier, as if you need to change a constant’s value, you only need to do it in one place.

Constant Syntax

The basic syntax for defining a constant in C# is as follows:

public const type name = value;

For example, let’s see how we can define a constant for the value of PI.

public const double PI = 3.14159;

Console.WriteLine(PI); // Output: 3.14159

PI = 3.5; // ❌ this would give an error, you cannot reassign a constant

Using Constants

Console.WriteLine(Constants.Pi);

double area = Constants.Pi * radius * radius;

Naming Convention

It is not mandatory, but it is relatively common to use uppercase names for constants to distinguish them from other variables.

public const double PI = 3.14159;

Readonly Variables

Readonly variables (readonly) are similar to constants, but their value can be assigned or changed in the class constructor. Once assigned, their value cannot change. This is useful for defining constants that need to be calculated at runtime or initialized in the constructor.

public class Example
{
    public readonly int number;

    public Example(int value)
    {
        number = value;
    }
}

Practical Examples

public class MathematicalConstants
{
    public const double Pi = 3.14159;
    public const double Euler = 2.71828;
}

public class Configuration
{
    public const int LoginAttemptLimit = 3;
    public const string DateFormat = "dd/MM/yyyy";
}