Software Technologies

A private constructor is a special instance constructor. It is commonly used in classes that contain static members only. If a class has one or more private constructors and no public constructors, then other classes (except nested classes) are not allowed to create instances of this class. For example:

class NLog
{
   // Private Constructor:
   private NLog() { }

   public static double e = System.Math.E;  //2.71828...
}

The declaration of the empty constructor prevents the automatic generation of a default constructor. Note that if you don't use an access modifier with the constructor it will still be private by default. However, the private modifier is usually used explicitly to make it clear that the class cannot be instantiated.

Private constructors are used to prevent the creation of instances of a class when there are no instance fields or methods, such as the Math class, or when a method is called to obtain an instance of a class. If all the methods in the class are static, consider making the entire class static. For more information see Static Classes and Static Class Members.

Example

The following is an example of a class using a private constructor.

C#

public class Counter
{
   private Counter() { }
   public static int currentCount;
   public static int IncrementCount()
   {
       return ++currentCount;
   }
}

class TestCounter
{
   static void Main()
   {
       // If you uncomment the following statement, it will generate
       // an error because the constructor is inaccessible:
       // Counter aCounter = new Counter();   // Error

       Counter.currentCount = 100;
       Counter.IncrementCount();
       System.Console.WriteLine("New count: {0}", Counter.currentCount);
   }
}

Output

New count: 101

Notice that if you uncomment the following statement from the example, it will generate an error because the constructor is inaccessible due to its protection level:

C#

// Counter aCounter = new Counter();   // Error