Generics HowTo

[Jacek Jurkowski]

How to ensure that generic Collection
accept only String as a typeparam?
This causes an compiler error...

public class StringCollection<T> : List<T> where T : String

{

}

[Jon Skeet [C# MVP]]

Jacek Jurkowski <pc*****@priv1.onet.pl> wrote:

How to ensure that generic Collection
accept only String as a typeparam?
This causes an compiler error...

public class StringCollection<T> : List<T> where T : String
{

}

If you only want to be able to accept a single type as the type
parameter, it's not really generic any more. Just use:

public class StringCollection : List<string>
{
}

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too

[Jacek Jurkowski]

Dou You think I better derive from CollectionBase instead
of using generics? Why a cant use "where T : String"?

If you only want to be able to accept a single type as the type
parameter, it's not really generic any more. Just use:

public class StringCollection : List<string>
{
}

Its still possible: StringCollection<int> = new StringCollection<int>();

[Abubakar]

If you read the error message it makes pretty clear that
"A constraint must be an interface, a non-sealed class or a type parameter".
String is a sealed class.

Ab.
http://joehacker.blogspot.com

"Jacek Jurkowski" <pc*****@priv1.onet.pl> wrote in message
news:%2****************@TK2MSFTNGP09.phx.gbl...

Dou You think I better derive from CollectionBase instead
of using generics? Why a cant use "where T : String"?

If you only want to be able to accept a single type as the type
parameter, it's not really generic any more. Just use:

public class StringCollection : List<string>
{
}

Its still possible: StringCollection<int> = new StringCollection<int>();

[Daniel O'Connell [C# MVP]]

"Jacek Jurkowski" <pc*****@priv1.onet.pl> wrote in message
news:%2****************@TK2MSFTNGP09.phx.gbl...

Dou You think I better derive from CollectionBase instead
of using generics? Why a cant use "where T : String"?

If you only want to be able to accept a single type as the type
parameter, it's not really generic any more. Just use:

public class StringCollection : List<string>
{
}

Its still possible: StringCollection<int> = new StringCollection<int>();

It shouldn't be, since StringCollection has no generic type. You don't want

public class StringCollection<T> : List<string> { };

you want
public class StringCollection : List<string> { };

You might want to look into the Collection<T> class as well if you need more
flexibility than List<T> offers.

[Jacek Jurkowski]

Thanks for Your help :-)