C#: Interface as a subclass of Interface

Following code snippet shows how a Interface can have a Interface as a subclass.

public interface Class1
{
void Display(string str);
}

public interface Class2 : Class1
{
void DisplayInt(int i);
}

public class Class3 : Class2
{
// Must be implemented, even though
 // its base class interface doesn't declare this method
 public void Display(string str)
{
 Console.WriteLine(str);
}
public void DisplayInt(int i)
{
 Console.WriteLine(i.ToString());
}
}


public class MyClass
{
public static void Main()
{
 Class1 c = new Class3();
 //c.DisplayInt(2);  // Will work only when Class3 c = new Class3();
  c.Display("for tango");
}
}

//ouput
for tango