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Best Use of C# Private Constructors [How To?]

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What do we need of private constructor in C#?
1.A private constructor means that the class can't be instantiated from outside the class.
2.Private constructors can be useful when using a factory pattern.
3.You cannot add public constructor without parameters. i.e.

4.If the class ONLY has private constructors, it cannot be instantiated from outside.

5.When you want to prevent the users of your class from instantiating the class directly.
6.A static method can call the private constructor to create a new instance of that class.
7.In this case, you have a static method defined inside the class that internally calls the private constructor. i.e.
class A
{
private A() { }
private static A theA = new A();
public static A getAInstance() { return theA; }
}

8.If you want to create object of class(ClassA) even if we have private constructor(ClassA) then we need to add a public parameterise constructor along with private constructor. i.e.

             namespace ConsoleNamespace
             {
                class ClassA
                {
                    private ClassA()
                    {
                        Console.WriteLine("ClassA::Constructor()");
                    }

                    public ClassA(int a)
                    {
                        Console.WriteLine("ClassA with params::Constructor()");
                    }
                 }
                 class Test
                 {
                    static void Main(string[] args)
                    {
                        ClassA a = new ClassA(10);
                        Console.ReadLine();
                    }
                 }
              }

 Constructor Interview Questions and Answers

1.We have two classes, one is ClassA and other is ClassB, the ClassB inherit base class ClassA. If we make the object of child class ClassB then which class constructor called first?
2.We know that Base class constructor called first. But if we creating object with parameters, and base class have both constructor default and parameter,then which constructor of base class called first.
3.How to call base class parameterized constructor first in c#?
4.What is the difference between constant, readonly and static keywords in C#?
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