We can have a different return type of method in C # polymorphism

What you are talking about is method overloading; not polymorphism. And you can; with one caveat:

Overloaded methods cannot differ only in return type.

With polymorphism, all parameters and return type must match.

Method overloading occurs when you give two methods the same name:

public void MyMethod(string arg) { }
public int MyMethod(string arg, int arg2) { return 0; }

      

        
        
        
      

    

Polymorphism occurs when you override a base class function, but you must keep the same signature:

public class A
{
    public void MyMethod(string arg)
    { }
}

public class B : A
{
    public override void MyMethod(string arg)
    { }
}