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

Whenever you want to prevent direct instantiation of a class from outside of it, you'll use a private constructor. For example, prior to C# 2.0 which introduced static classes, you used a private constructor to accomplish roughly the same thing:

sealed class StaticClass {
     private StaticClass() {
     }
     public static void DoSomething() {
     }
}