What is Use of Private Constructor in C# ? Scenarios Example

Answer to interview questions, what is use of private constructor in c# language includes definition of it and when or why to use private constructors in c sharp  application with c# program example.

Private Constructor in C#?

Private constructor is constructor that is preceded by private access specifier. For example, below class has a private constructor.

We know that if we don’t write constructor in the class then by default constructor gets called on object creation which is public. Or if we want to allow object creation of the class then we write public constructor or else we explicitly specify private access specifier on class constructor.

Can we create object of class with private constructor in C#?

No, object of a class having private constructor cannot be instantiated from outside of the class. However, we can create object of the class inside class methods itself.

class A
{
    //private constructor
    private A()
    {
    }
}

class Program
{
    static void Main(string[] args)
    {
        A obj = new A();//Error cannot access private constructor
    }
}

Use of private constructor in C# Sharp Programming:

Following are the scenarios when we can make use of private constructor in C# code.

1)Stop object creation of a class

If we want to stop object creation of a class, then we can make the class constructor private. So, if we try to create an object from the main program or other classes, then compiler will flash an error.

NOTE:

i)To stop object creation of a class completely, you need to make all the overloaded constructors private.

ii)There may be a chance that you need to stop an specific type of object creation. You can do so by making specific constructor private.

For example, you don’t want to allow anyone to create an object of a class with an empty constrictor but all other overloaded constructors. Then, you can make the empty constructor private and rest of the constructors public as shown in below code example.

class A {
    //private constructor
    private A() {
    }

    public A(int a)
    {       
    }
    public A(int a, int b)
    {
    }
}

class Program
{
    static void Main(string[] args)
    {
       // A obj1 = new A();//Error 

        A obj2 = new A(5);// OK

        A obj3 = new A(5, 10);//OK
    }
}

2)Use in Singleton class

We know that singleton class allows only single instance of it throughout the execution of the application. When we design a singleton class, we use private constructor, so, no user can create object of this class to stop multiple object creation. And singleton class itself provide the same object over and over on demand.

You may like reading what is singleton class and how to design a thread safe singleton class in C# software projects.

3)Stop a class to be inherited

If we don’t want a class to be inherited, then we make the class constructor private. So, if we try to derive another class from this class then compiler will flash an error. Why compiler will flash an error? We know the order of execution of constructor in inheritance that when we create an object of a derived class then first constructor of the base call will be called then constructor of derived class. Since, base class constructor is private, hence, derived class will fail to access base class constructor.

In below c sharp code example, compiler flashes error on creation of object of derived class or as soon as you write constructor of derived class.

//base
class B
{
    private B()
    {
    }
}
//Derived
class D : B
{
    public D()
    {
    }
    
}

class Program
{
    static void Main(string[] args)
    {
       D obj = new D();//error
    }
}

NOTE:

We can also use a sealed class to stop a class to be inherited. Also, recommend to read the real time example of sealed class in C# .