无法在Main继承的类中调用方法

Question

I have created a basic template program helping me understand Interfaces, classes and the such.

What I am trying to achieve is simply calling a method that resides in an inherited class hierarchy.

I am getting this error:

'Book' does not contain a definition for 'BulkOrder' and no extension method for 'BulkOrder' accepting a first argument of type 'Book' could be found(are you missing a using directive or an assembly reference?)

This is the main program

Main
{
 BulkBook book2 = new BulkBook(FILLER);
 BulkOrder(book2);
}


public static void BulkOrder(Book book2)
{ 
Console.WriteLine(Filler text);
book2.BulkOrder(); <------- belongs in inherited class
}

This is the inherited class structure

 abstract class Publication
 various code
 class Book : Publication
 various code 
 class BulkBook : Book
 various code
 public void BulkOrder() <------ method
    {
        Copies = Copies + BATCH_SIZE;
    }

Answer 1

class BulkBook : Book
    ...
    public void BulkOrder()
    {
        Copies = Copies + BATCH_SIZE;
    }

The BulkOrder method only exists in the BulkBook class, not the Book class.

Answer 2

Since BulkOrder exists in BulkBook and not Book you need to cast book2 to BulkBook :

((BulkBook)book2).BulkOrder();

However you may wish to verify that book2 is in fact a BulkBook :

if (book2 is BulkBook)
{
  ((BulkBook)book2).BulkOrder();
}

An alternative solution would be to have the method accept the type BulkBook rather than Book :

public static void BulkOrder(BulkBook book2)
{ 
Console.WriteLine(Filler text);
book2.BulkOrder(); //This should now work
}