C# Language Specification

Private constructors

When a class T declares only private instance constructors, it is not possible for classes outside the program text of T to derive from T or to directly create instances of T. Thus, if a class contains only static members and isn’t intended to be instantiated, adding an empty private instance constructor will prevent instantiation. For example:

public class Trig

{
private Trig() {} // Prevent instantiation

public const double PI = 3.14159265358979323846;

public static double Sin(double x) {...}
public static double Cos(double x) {...}
public static double Tan(double x) {...}
}

The Trig class groups related methods and constants, but is not intended to be instantiated. Therefore it declares a single empty private instance constructor. At least one instance constructor must be declared to suppress the automatic generation of a default constructor.

6.