Problem in Interfaces (polymorphism) C#

hi.

i have two classes which have some common methods like funcA(), funcB()

and some methods are only related to its class...

what i did is made interface of TestInterface

public interface TestInterface
{
   void funcA()
   void funcB()

}


public class ClassA : TestInterface
{
  public  void funcA()
   {
     Console.WriteLine("This is ClassA FuncA()");
   }
  public  void funcB()
   {
     Console.WriteLine("This is ClassA FuncB()");
   }
  public void myFuncA()
   {
      Console.WriteLine("This is My Own Function A");
    }

}

public class ClassB : TestInterface
    {
      public  void funcA()
       {
         Console.WriteLine("This is ClassB FuncA()");
       }
      public  void funcB()
       {
         Console.WriteLine("This is ClassB FuncB()");
       }
      public void myFuncB()
       {
          Console.WriteLine("This is My Own Function B");
       }

    }



public static void main()
{
 TestInterface test = new ClassA();
 test.funcA();
}

as u see in above two classes. i have two functions myFuncA() and myFuncB() are not part of interface. they only belongs to their own class.

how can i call them from the main method. where i am creating object of TestInterface and initializing it with some child class.???