I'm trying to figure out the "best" way to implement a strongly typed
ArrayList. Using inheritance is one approach. It has the advantage that I
only have to write overrides for the Add method and indexer. There are
problems with this however. First the indexer "override" has a different
return type. This doesn't cause a compile time error, but using the indexer
doesn't seem to produce a properly typed expression. The Add override must
have an arg of type object so the method doesn't reflect the strong
typing -- until run time when the my hand coded type check is run. I've
added an Add overload that uses a strongly typed arg, but the original
method still appears in the command completion popup.
An alternative approach would be to use composition, but this requires that
I implement wrapper methods for all of the ArrayList methods that I want to
make available in the strongly typed ArrayList class.
I think the latter (composition) approach provides the best solution in
terms of providing a truly strongly typed object, but am bothered by the
need for the "duplicate" methods.
Is there another alternatives, suggestions?
Thanks in advance,
Bill 4 5726
Hello,
If you want a strongly-typed collection, the easiest thing to do is inherit
from CollectionBase and verify your values in the OnVerify method - but you
knew that already. ;-) However, if you want type-safe member signatures,
you'll need to use composition. Try this:
class MyCollection : IList {
...
public int Add(MyType value) {
...
}
...
int IList:Add(objec t value) {
if (value is MyType) {
return Add(value as MyType);
} else {
throw new ArgumentExcepti on();
}
}
}
Using this notation, Add(object) is only there to satisfy the interface
implementation; it will only be visible if the collection is cast to IList.
Attempting to add an instance of any type other than MyType to the
collection will cause a compile-time error (unless the type of said instance
can be implicitly converted to MyType), as long as you're working with a
MyCollection reference. If the collection is cast to IList at some point,
adding an object of an inappropriate type will throw an ArgumentExcepti on at
runtime.
Ugh, my mistake - the second method's seiganture should read:
int IList.Add(objec t value)
Note the period in place of the colin.
Erm, signature. I am not having a good night. >.<
Omni
Thanks for the suggestion. This is pretty much what I've already tried
(via inheritance). I guess that using an interface keeps the IList.Add
method from showing up in the pop up though, right? The OnVerify approach
doesn't solve the compile-time problem.
Thanks again,
Bill
"Omni" <om*****@pacbel l.net> wrote in message
news:es******** ******@tk2msftn gp13.phx.gbl... Hello,
If you want a strongly-typed collection, the easiest thing to do is
inherit from CollectionBase and verify your values in the OnVerify method - but
you knew that already. ;-) However, if you want type-safe member signatures, you'll need to use composition. Try this:
class MyCollection : IList { ... public int Add(MyType value) { ... } ... int IList:Add(objec t value) { if (value is MyType) { return Add(value as MyType); } else { throw new ArgumentExcepti on(); } } }
Using this notation, Add(object) is only there to satisfy the interface implementation; it will only be visible if the collection is cast to
IList. Attempting to add an instance of any type other than MyType to the collection will cause a compile-time error (unless the type of said
instance can be implicitly converted to MyType), as long as you're working with a MyCollection reference. If the collection is cast to IList at some point, adding an object of an inappropriate type will throw an ArgumentExcepti on
at runtime.
This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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only have to write overrides for the Add method and indexer. There are
problems with this however. First the indexer "override" has a different
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doesn't seem to produce a properly typed expression. The Add override must...
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