Generics, constrains and inheritance

[Joe]

Hi All!

Here's the problem I'm having. I want to create a generic class that
inherits from CollectionBase and implements IBindingList. As part of
IBindingList I have to implement AddNew(). Since this is a generic class the
new throws an error "Cannot create an instance of the variable type 'T'
because it does not have the new constraint".

How do I add a constraint to my class and be able to keep the CollectionBase
and IBindingList?

When I add the where <T>: it means that I want the class T to implement
CollectionBase and IBindingList.

Is it possible to do what I want?

Thanks,
Joe

[Joe]

Re: Generics, constrains and inheritance

I believe I have my answer. I need to declare my class like this:

class MyClass<T: CollectionBase, IBindingList where T: new ()
{
....
}

"Joe" <jbassking@noem ail.noemailwrot e in message
news:e0tqttrMJH A.1156@TK2MSFTN GP05.phx.gbl...

Hi All!
>
Here's the problem I'm having. I want to create a generic class that
inherits from CollectionBase and implements IBindingList. As part of
IBindingList I have to implement AddNew(). Since this is a generic class
the new throws an error "Cannot create an instance of the variable type
'T' because it does not have the new constraint".
>
How do I add a constraint to my class and be able to keep the
CollectionBase and IBindingList?
>
When I add the where <T>: it means that I want the class T to implement
CollectionBase and IBindingList.
>
Is it possible to do what I want?
>
Thanks,
Joe
>
>

[puzzlecracker]

Re: Generics, constrains and inheritance

On Oct 20, 10:10 am, "Joe" <jbassk...@noem ail.noemailwrot e:

I believe I have my answer. I need to declare my class like this:
>
class MyClass<T: CollectionBase, IBindingList where T: new ()
{
    ....
>
}

I don't think compiler knows what t where T: new () means...

[Peter Duniho]

Re: Generics, constrains and inheritance

On Mon, 20 Oct 2008 12:57:26 -0700, puzzlecracker <ironsel2000@gm ail.com>
wrote:

On Oct 20, 10:10 am, "Joe" <jbassk...@noem ail.noemailwrot e:

>I believe I have my answer. I need to declare my class like this:
>>
>class MyClass<T: CollectionBase, IBindingList where T: new ()
>{
>    ....
>>
>}

I don't think compiler knows what t where T: new () means...

Why don't you think that?

http://msdn.microsoft.com/en-us/library/sd2w2ew5.aspx

[puzzlecracker]

Re: Generics, constrains and inheritance

On Oct 20, 4:18 pm, "Peter Duniho" <NpOeStPe...@nn owslpianmk.com>
wrote:

On Mon, 20 Oct 2008 12:57:26 -0700, puzzlecracker <ironsel2...@gm ail.com> 
wrote:
>

On Oct 20, 10:10 am, "Joe" <jbassk...@noem ail.noemailwrot e:

I believe I have my answer. I need to declare my class like this:

>

class MyClass<T: CollectionBase, IBindingList where T: new ()
{
    ....

>

}

I don't think compiler knows what t where T: new () means...

>
Why don't you think that?
>
http://msdn.microsoft.com/en-us/library/sd2w2ew5.aspx

I should have rtfm'ed. I didn't know that you can constrain types
based on different constructor types... learn something new every
day...

[Ben Voigt [C++ MVP]]

Re: Generics, constrains and inheritance

puzzlecracker wrote:

On Oct 20, 4:18 pm, "Peter Duniho" <NpOeStPe...@nn owslpianmk.com>
wrote:

>On Mon, 20 Oct 2008 12:57:26 -0700, puzzlecracker
><ironsel2...@g mail.comwrote:
>>

>>On Oct 20, 10:10 am, "Joe" <jbassk...@noem ail.noemailwrot e:
>>>I believe I have my answer. I need to declare my class like this:

>>

>>>class MyClass<T: CollectionBase, IBindingList where T: new ()
>>>{
>>>....

>>

>>>}
>>I don't think compiler knows what t where T: new () means...

>>
>Why don't you think that?
>>
>http://msdn.microsoft.com/en-us/library/sd2w2ew5.aspx

>
I should have rtfm'ed. I didn't know that you can constrain types
based on different constructor types... learn something new every
day...

You can't. At least not the part about different constructor types. There
is a constraint for "public parameterless constructor" and that's it, sadly.

[Jon Skeet [C# MVP]]

Re: Generics, constrains and inheritance

On Oct 20, 10:22 pm, "Ben Voigt [C++ MVP]" <r...@nospam.no spamwrote:

I should have rtfm'ed. I didn't know that you can constrain types
based on different constructor types... learn something new every
day...

>
You can't.  At least not the part about different constructor types.  There
is a constraint for "public parameterless constructor" and that's it, sadly.

For a suggestion of how an alternative might work, see

http://msmvps.com/blogs/jon_skeet/archive/2008/08/29/lessons-learned-from-protocol-buffers-part-4-static-interfaces.aspx

Jon