Re: Returning an Abstract class from a property

On Sun, 16 Mar 2008 17:24:01 -0700, Ant <Ant@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Consider a Bank account & a loan. The loan cannot exist without a bank
account, therefore you can create a bank account but you cannot create a loan
on its own.
To create a loan, you create a bank account which issues the loan.

That describes a sort of parent/child relationship between the account and a loan, but it has very little to do with the way the classes are defined. On the other hand, "abstract" is all about how the classes are defined.

Hence, the loan (I think) should be an Abstract class & the Bank account can
be instantiated. It has a method (or property)which will pass out a loan
based on information held by the Bank account.

Either class could be an abstract class, and either could be concrete. Regardless of how you define the classes, they would presumably have the same parent/child relationship.

Therefore, you instanitate the Bank account, add information to its
properties; these properties are used to create a loan object which is then
passed out to an object of type Loan.

Here's where you seem to misunderstand what an abstract class is. "Abstract" means that the class cannot itself be instantiated. If you want to be able to "create a loan object", then the "loan object" _cannot_ be abstract. You are asking how to do an impossible act.

You can define an abstract loan object class if you like, but then you would still need to define a sub-class of that class that is not abstract, and thus can be instantiated. Your account object then could either create a brand new instance of the non-abstract sub-class, or perhaps in some situations you might already have created an instance of the non-abstract sub-class previously and are just returning it.

Either way, even if the type of the class that you're returning is the abstract loan object, you _must_ have a non-abstract concrete class that is actually implementing that abstract loan class. Otherwise, there's no way to get an instance of it.

Now, all that said, I suspect that what's really going on here is that you have misunderstood the word "abstract" as it applies to C# classes (and OOP classes generally). You haven't written anything that suggests that the loan class does in fact need to be abstract.

As far as this specific question goes:

[...]
but how can I create a Loan object from within the Bank Account Class?

You would create a Loan object the same way you'd create any other object. Use "new" with your desired constructor. Just because that code happens to be in some other class, that doesn't change how it works. If you know how to create a "Bank Account Class" instance, then you also know how to create a "Loan object [class]" instance.

I have the impression from your original post that you may simply want to prevent any other code except your account class from creating an instance of the loan class. I'm not convinced that's really a necessary restriction -- after all, why not allow a loan class instance to be created as long as the instantiator has a reference to an account instance to provide when creating the loan instance? -- but if you want that restriction, you need to provide a way for any loan class constructors to be visible only to the account class.

I know of at least two ways to do that: you can make the account class a nested class within the loan class, making the loan class constructors "private"; or you can put both classes into the same assembly, without any other classes, and make the loan class constructors "internal". The latter solution seems more sensible to me.

Pete
.

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