C# General inexplicit upcast of instance by type

Question

Could you explain to me. How i have to resolve simple task below?

class Base{}
class Derived1: Base { void Method(); }
class Derived2: Base { void Method();}

static void Main(string[] args)
{   
    Base object; //it is just a declaring

    if (some condition is true)
        object = new Derived1();
    else
        object = new Derived2();

    //now i want to call one of methods of one of my derived classes
    //object.MyMethod(); //of course wrong, object has no that method

    //ok, i have to downcast it but i don't know which class to
    //((object.GetType())object).Method(); //wrong

    //is there only one way is to repeat conditions 
    //and to downcast explicitly?

    if (some condition is true again)
        (object as Derived1).Method();
    else
        (object as Derived2).Method();
}

Base class is know nothing about Method() of course.

Answer 1

I suggest using abstract method in the Base class overriden in both derived ones:

abstract class Base { public abstract void Method();}
class Derived1: Base { public override void Method(); }
class Derived2: Base { public override void Method(); }

Then

static void Main(string[] args) {   
  Base instance; 

  if (some condition is true)
    instance = new Derived1();
  else
    instance = new Derived2();

  instance.Method();
}

Implementing interface is an alternative:

interface IBase {void Method();}

class Derived1: IBase { void Method(); }
class Derived2: IBase { void Method(); }

static void Main(string[] args) {   
  IBase instance; 

  if (some condition is true)
    instance = new Derived1();
  else
    instance = new Derived2();

  instance.Method();
}