Suggestions? Returning subclass instances from parent class methods

[Niels Ull]

Hi!

I have a generic abstract base class MyCollection<Tw hich represents a collection
of T's with some common utility methods.
I then have a number of non-generic subclasses, e.g.

class FooCollection : MyCollection<Fo o>
{ /* utility methods only for foo collections */ }
class BarCollection : MyCollection<Ba r>
{ /* utility methods only for Bar collections */ }
Now, I'd like to provide a subset method in MyCollection - e.g. for getting
the first 10 items
(In real life selecting the subset is more complex and based on the structure
of MyCollection as well as properties of the T base type)

FooCollection allFoos = ...
FooCollection tenFoos = allFoos.GetSubs et(10);

But I'd like to return a FooCollection - not a MyCollection<Fo o>. The best
I have been able to come up is this method in MyCollection:
DERIVEDCOLLECTI ON GetSubset<DERIV EDCOLLECTION>(i nt count) where DERIVEDCOLLECTI ON:
MyCollection<T> , new()
{
DERIVEDCOLLECTI ON result = new DERIVEDCOLLECTI ON();
for(int i = 0; i < count; i++) // example logic only!
result.Add(this[i]);
return result;
}

This is not very elegant, since it requires the user to specify the target
class:
FooCollection tenFoos = allFoos.GetSubs et<FooCollectio n>(10);
An alternative would be to return an IEnumerable<T>, but that would require
the user to write

FooCollection allFoos = ...
FooCollection tenFoos = new FooCollection() ;
tenFoos.AddAll( allFoos.GetSubs et(10))

Or, finally, I could implement new GetSubset in all derived collections -
but since there are a number of such functions, that's not too useful either.

Does anybody have any suggestions?

[Adam Clauss]

One possibility, add a second type to the generic parameters:

class FooCollection : MyCollection<Fo o, FooCollection>
{}

where MyCollection is now something like:

class MyCollection<T, Swhere S : new()
{
public S GetSubset(int count)
{
S newCollection = new S();
for (int i = 0; i < count; i++)
{
newCollection.A dd(this[i]);
}
return newCollection;
}
}

This would then simplify the callers syntax to simply:

FooCollection tenFoos = allFoos.GetSubs et(10);

Which was what you wanted correct?

This comes with an assumption: that each of your subclasses has a default
constructor (presumably to create an empty collection).

--
Adam Clauss
"Niels Ull" <ni******@hotma il.comwrote in message
news:97******** *************** ***@msnews.micr osoft.com...

Hi!

I have a generic abstract base class MyCollection<Tw hich represents a
collection of T's with some common utility methods. I then have a number
of non-generic subclasses, e.g.

class FooCollection : MyCollection<Fo o>
{ /* utility methods only for foo collections */ }
class BarCollection : MyCollection<Ba r>
{ /* utility methods only for Bar collections */ }
Now, I'd like to provide a subset method in MyCollection - e.g. for
getting the first 10 items
(In real life selecting the subset is more complex and based on the
structure of MyCollection as well as properties of the T base type)

FooCollection allFoos = ...
FooCollection tenFoos = allFoos.GetSubs et(10);

But I'd like to return a FooCollection - not a MyCollection<Fo o>. The best
I have been able to come up is this method in MyCollection:
DERIVEDCOLLECTI ON GetSubset<DERIV EDCOLLECTION>(i nt count) where
DERIVEDCOLLECTI ON: MyCollection<T> , new()
{
DERIVEDCOLLECTI ON result = new DERIVEDCOLLECTI ON();
for(int i = 0; i < count; i++) // example logic only!
result.Add(this[i]);
return result;
}

This is not very elegant, since it requires the user to specify the target
class: FooCollection tenFoos = allFoos.GetSubs et<FooCollectio n>(10);
An alternative would be to return an IEnumerable<T>, but that would
require the user to write

FooCollection allFoos = ...
FooCollection tenFoos = new FooCollection() ;
tenFoos.AddAll( allFoos.GetSubs et(10))

Or, finally, I could implement new GetSubset in all derived collections -
but since there are a number of such functions, that's not too useful
either.

Does anybody have any suggestions?