Derived Interfaces

synack · May 19th, 2009, 01:52 PM

Hi All

I'm currently working through Professional C# 2008 book and I'm busy with the Derived interfaces part. I'm a programer, but I think my OO skills are a little lacking.

Anyway, working through the example in the book. I declared the following interface

Code:

public interface IBankAccount
{
 void PayIn(decimal amout);
 bool Withdraw(decimal amount);
 decimal Balance
 {
   get;
 }
}

Then I declared a derived interface from the previous interface

Code:

public interface ITransferBankAccount : IBankAccount
{
bool TransferTo(IBankAccount destination, decimal amount);
}

I then wrote two classes that uses these interfaces

Code:

public class SaverAccount : IBankAccount
{
// code here
}
 
public class CurrentAccount : ITransferBankAccount
{
// code here
}

Then in my main, it looks like this

Code:

class MainEntryPoint
{
 static void Main()
 {
   IBankAccount venusAccount = new SaverAccount();
   IBankAccount jupiterAccount = new CurrentAccount();
   venusAccount.PayIn(200);
   jupiterAccount.PayIn(500);
   Console.WriteLine(venusAccount.ToString());
   Console.WriteLine(jupiterAccount.ToString());
   Console.ReadLine();
 }
}

Now some of you will probably already have noticed that I've declared jupiterAccount as a IBankAccount type, rather than a ITransferBankAccount type, yet I've instanced it as a CurrentAccount object.

This isn't how the code in the book goes, but it was a mistake made by me, yet it compiles and works. When I explored this, it looks like I had access to all the methods/types defined in the IBankAccount interface, but even though the TransferTo() function is declared in the CurrentAccount class, I don't have access to it and calling it gives a compile error.

Is this intended?

Syn

C@uark · October 31st, 2009, 05:36 AM

Yep thats would be correct, Your CurrentAccount has an "is a" relationship with IBankAccount meaning; a CurrentAccount is an IBankAcount. Since CurrentAccount is an IBankAccount at its lowest level of inheritance, an IBankAcount can contain a CurrentAccount but it will only expose the method signitures that it knows. However, an IBankAccount is not a CurrentAccount therefore it is invalid to assigment of an IBankAccount to a CurrentAccount is not valid either i.e.

IBankAccount saverAccount = new SaverAccount();
CurrentAccount myCurAccount = SaverAccount; // not valid becuase IBankAcount is not a CurrentAccount
// this is becuase the CurrentAccount designation would expose
// variables and method signitures than an IBankAccount does not have to offer.