A design question for experienced OO designers

[Zeng]

From the below situation, it's painful to keep declaring and implement the
similar methods just different return type for each list class;
alternatively, we can just force the client code use RecordBaseList's
methods directly and converting the return object to the correct type on its
own (from RecordBase to RecordTypeA or RecordTypeB)- which is also painful.
Any experienced OO designer out there that can suggest me a better design?
Thank you very much in advance....

abstract class RecordBase
{
abstract public int commonF1();
abstract public int commonF2();
}

public class RecordTypeA : RecordBase
{
override int commonF1() {...}
override int commonF2() {...}

public int FooA() {...}
}

public class RecordTypeB : RecordBase
{
override int commonF1() {...}
override int commonF2() {...}

public int FooB() {}
}


abstract class RecordBaseList
{
public RecordBase FindRecord( int id ) {...}
public RecordBase AddNewRecord() {...}
public RecordBase FirstRecord() {...}
...
...
}

public class RecordTypeAList : RecordBaseList
{
new public RecordTypeA FindRecord( int id ) // we are hiding the method
of base class here
{
return (RecordTypeA) base.FindRecord( id );
}

new public RecordTypeA AddNewRecord() // we are hiding the method of
base class here
{
return (RecordTypeA) base.AddNewRecord( id );
}

new public RecordTypeA FirstRecordRecord() // we are hiding the method
of base class here
{
return (RecordTypeA) base.AddNewRecord( id );
}
}

//// FOR RecordTypeBList, WE WOULD SIMILAR CODE AS RecordTypeAList, JUST
DIFFERENT RETURN TYPE AND TYPE CONVERSION AT RETURN STATEMENT.

 

[Jay B. Harlow [MVP - Outlook]]

Zeng,
Have you considered using Eiffel. As it fully supports covariant return
types (overriding a function with a derived return type).

http://www.eiffel.com

I normally avoid designing class hierarchies where I need to change the
return type on derived classes. Although a couple of places I thought it
would have been useful. I figured out I didn't really need it. As you say
the consumer can change the type if they really need to.

Hope this helps
Jay

 

[Frans Bouma]

From the below situation, it's painful to keep declaring and implement
the similar methods just different return type for each list class;
alternatively, we can just force the client code use RecordBaseList's
methods directly and converting the return object to the correct type on
its own (from RecordBase to RecordTypeA or RecordTypeB)- which is also
painful. Any experienced OO designer out there that can suggest me a
better design? Thank you very much in advance....

//// FOR RecordTypeBList, WE WOULD SIMILAR CODE AS RecordTypeAList, JUST
DIFFERENT RETURN TYPE AND TYPE CONVERSION AT RETURN STATEMENT.[/QUOTE]

Basicly you are trying to write a typed collection. This is not
possible in currenct C#, you have to wait for generics OR use a generator to
produce this code, since it is very repetitive.

FB