What is the need of private constructor in C#?

Question

What is the need of private constructor in C#? I got it as a question for a C# test.

Answer 1

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.

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

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

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

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);
    }
}