How to invoke or instantiate obj using private constructor?

Question

Hello, i would like to know how to instantiate an object using private constructor? and what are the main benefits to use private constructor?

 

namespace Practice

{

public class Test

{

int a;

public Test()

{

}

private Test(int a)

{

this.a = a;

}

}

public class Test2

{

void method()

{

Test obj1 = new Test();

}

}

}