Why isn't List<A> a subclass of List<B> if A is a subclass of B ?
This fact causes a lot of design problems. Making the generic collections
loose their type safty strength in large designs.
Maybe there is a design pattern to overcome this problem ? Or maybe someone
knows other related facts ? 4 1976
uriye <ur***@discussions.microsoft.com> wrote: Why isn't List<A> a subclass of List<B> if A is a subclass of B ?
This fact causes a lot of design problems. Making the generic collections loose their type safty strength in large designs. Maybe there is a design pattern to overcome this problem ? Or maybe someone knows other related facts ?
Suppose you have List<FileStream> and try to use it as a List<Stream> -
you would be able to add a List<NetworkStream> to it, which wouldn't be
good. (This is how arrays work, with runtime exceptions, and I'm not at
all sure it's ideal.)
So in fact, currently we *do* have type safe collections - but with
your suggestion, they wouldn't be type safe any more.
--
Jon Skeet - <sk***@pobox.com> http://www.pobox.com/~skeet
If replying to the group, please do not mail me too
Hi Jon, Why isn't List<A> a subclass of List<B> if A is a subclass of B ?
This fact causes a lot of design problems. Making the generic collections loose their type safty strength in large designs. Maybe there is a design pattern to overcome this problem ? Or maybe someone knows other related facts ?
Suppose you have List<FileStream> and try to use it as a List<Stream> - you would be able to add a List<NetworkStream> to it, which wouldn't be good. (This is how arrays work, with runtime exceptions, and I'm not at all sure it's ideal.)
So in fact, currently we *do* have type safe collections - but with your suggestion, they wouldn't be type safe any more.
Hmm, I don't get that explanation. Supposing I have a method
void LoadFrom(Stream stm)
That method can be type-safely called with a XyzStream, as long
the XyzStream is a subclass of Stream.
Now extrapolated:
void LoadFrom(List<Stream> list)
I'd naively expect to be able to pass a List<XyzStream> object
to that method, too.
bye
Rob
Robert Jordan <ro*****@gmx.net> wrote: Suppose you have List<FileStream> and try to use it as a List<Stream> - you would be able to add a List<NetworkStream> to it, which wouldn't be good. (This is how arrays work, with runtime exceptions, and I'm not at all sure it's ideal.)
So in fact, currently we *do* have type safe collections - but with your suggestion, they wouldn't be type safe any more. Hmm, I don't get that explanation. Supposing I have a method
void LoadFrom(Stream stm)
That method can be type-safely called with a XyzStream, as long the XyzStream is a subclass of Stream.
Indeed - because any change to the value of stm is only visible within
the method. Note that if it were a pass-by-reference parameter, you
couldn't, for precisely the reason given below.
Now extrapolated:
void LoadFrom(List<Stream> list)
I'd naively expect to be able to pass a List<XyzStream> object to that method, too.
And what would you expect to happen if LoadFrom contained:
list.Add (new MemoryStream());
?
--
Jon Skeet - <sk***@pobox.com> http://www.pobox.com/~skeet
If replying to the group, please do not mail me too
Hi Jon, Now extrapolated:
void LoadFrom(List<Stream> list)
I'd naively expect to be able to pass a List<XyzStream> object to that method, too.
And what would you expect to happen if LoadFrom contained:
list.Add (new MemoryStream());
Ok. Got it.
Thanks
Rob This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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