What are and how to use constants in C#

A constant is an identifier used to represent a constant and predefined value that does not change during the execution of the program.

Constants are used to avoid repetition of literal values in the code and to improve clarity and readability.

Additionally, using constants makes it easier to update and maintain the code, since if you need to change the value of a constant, you only need to do it in one place.

Syntax of constants

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 with 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

Accessing constants

Constants are accessed using the class name followed by the constant name.

Console.WriteLine(Constants.Pi);

Using in expressions

Constants can be used in expressions instead of literal values to improve the readability and clarity of the code.

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 must 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";
}