# How to: Define Constants in C#

Constants are fields whose values are set at compile time and can never be changed. Use constants to provide meaningful names instead of numeric literals ("magic numbers") for special values.

Note

In C# the #define preprocessor directive cannot be used to define constants in the way that is typically used in C and C++.

To define constant values of integral types (int, byte, and so on) use an enumerated type. For more information, see enum.

To define non-integral constants, one approach is to group them in a single static class named Constants. This will require that all references to the constants be prefaced with the class name, as shown in the following example.

## Example

static class Constants
{
public const double Pi = 3.14159;
public const int SpeedOfLight = 300000; // km per sec.
}
class Program
{
static void Main()
{
double radius = 5.3;
double area = Constants.Pi * (radius * radius);
int secsFromSun = 149476000 / Constants.SpeedOfLight; // in km
}
}


The use of the class name qualifier helps ensure that you and others who use the constant understand that it is constant and cannot be modified.