Kelly,
Thanks for the great response. I agree with your comments about inheritance
(it is usually ugly). I love the idea about making the interface non-generic
and moving the generic parameter to the method. One key detail that I forgot
to mention is that this is a platform, and not an application, hence my
inability to know what T is.
I completely agree about keeping very little code in the ViewModel (except
for UI specific code). My ultimate goal is to have a 100 level programmer be
able to create a new ViewModel with very little knowledge of how it works,
and do most of the presentation customization via XAML. The base VM contains
all the plumbing to actually make things happen, and the derived VM is the
one used by the dev. They would be expected to write something like the
following.
class MyDataObjectViewModel : FooViewModel {
public override IQueryable<DataObject> Query(IQueryable<DataObject>
basequery) {
var query = from d in basequery
select d; // or some other LINQ query)
return base.Query(query);
}
}
The base ViewModel actually handles the population and management of the
items in the resulting collection (which the UI/Control binds to in a
non-generic way). If I'm not mistaken, having the generic parameter on the
method should work for this. I'll have to try it out in the morning.
Aside from that, do you see a better way to do this? I generally don't like
inheritance, but it seems like a good way to keep the messy details away
from the end user/dev.
Also, as an aside, which project of Glenn's are you referring to? It sounds
interesting...
Thanks,
Erick
On Wed, Mar 3, 2010 at 8:24 PM, Kelly Leahy <[email protected]> wrote:
> If we take a step back, and think about what you're saying, I think
> you'll realize it doesn't make sense.
>
> It sounds like you're saying you want to have some generic object that
> you hold onto in your class but you don't want to know what the type
> of that object is. However, you want to be able to write code that
> uses that object. There's not any sensible way that the compiler
> could type-check your code without knowing what T is, which is why you
> either need to specify a value of T in your field (i.e. bind the
> parameter to a type), or you need to make T a generic parameter of
> your type (i.e. leave it as an 'open' generic and 'provide' the T from
> the use site of your class).
>
> The thing I'm not really getting is why the control author cares what
> T is. It seems to me that if you're doing VM / PM pattern well, you
> should be striving to have very little (if any) code in your controls,
> and all of the interaction should be through bindings to the VM (which
> are - by definition - untyped, though if you use something like the
> binding frameworks that Glenn has been trying to build, you'll see
> that you can use stronger typing in your bindings).
>
> What you can do, and I've done this in the past, is not make the
> repository generic, but rather make its methods generic (i.e.
> Query<T>(...) instead of IRepository<T>::Query(...)) but this won't
> allow you to resolve different repositories by type from the IoC
> container.
>
> The other thing I'd ask yourself is whether inheritance is the right
> solution to your problem. In nearly every case in my past that I
> thought it was, I turned out to be wrong. You might also think about
> whether the type of the repository is important to be in a base class,
> or whether you can get the same effect by having the base class be
> abstract, providing a few methods to 'hook' into the derived class
> implementations, and then you can have the type parameter on the base
> class and have the deriving class provide the "T" when it derives. In
> other words, you could make:
>
> public class FooContainerControl: ContainerControlBase<Foo> { ... }
>
> We do this a lot in our UI code (though we do it with the PMs, not the
> controls, I personally hate base classes for controls - it never works
> out well, we use interfaces exclusively for them). For instance, we
> have a:
>
> public class EditorGridPresentationModelBase<T> where T: IItemCore, class
> { ... }
>
> and then for Table and Variable (both IItemCore implementers), we have
>
> public class EditorGridPresentationModel:
> EditorGridPresentationModelBase<Table>
> { ... }
> (in the Tables namespace), and
>
> public class EditorGridPresentationModel:
> EditorGridPresentationModelBase<Variable>
> { ... }
> (in the Input namespace).
>
> Kelly
>
> On Wed, Mar 3, 2010 at 4:10 PM, Erick Thompson <[email protected]>
> wrote:
> > I have run into a situation where I can't see any good solutions, and I'm
> > hoping someone has run into a similar situation.
> >
> > I have a generic IRepository<T> interface which is used for loading data
> > from a store. I have the generic parameter as I want to allow typed
> queries
> > (the queries will end up hitting a ANDS service). I want to use classes
> > implementing interface this in custom controls and ModelViews in a
> > Silverlight 4 application.
> >
> > The problem is that you can't have a generic interface in a class without
> > having a generic type on the class.
> >
> > public class ControlBase {
> > protected IRepository<T> _repository;
> > }
> >
> > will not compile. As this is a base class, I can't specify the T unless
> > I use a inherited base DataClass. I would like to use POCOs if possible.
> >
> > In the fantasy world in my head, I would simply make the base class
> generic,
> > and Bob's your uncle. However, I want these classes to be easily consumed
> in
> > XAML, which doesn't have a good way to instantiate a generic type.
> >
> > What I ultimately want is for the user to be able to create the
> control/form
> > (bound to ModelView) without a generic type, but that the data access
> occurs
> > in a typed way. For example:
> >
> > public class CoolControl : ControlBase {
> > public CoolControl() {
> > _repository = IoC.Resolve<IRepository<SomeConcreteDataType>>();
> > }
> >
> > public LoadData () {
> > // do some typed IQueryable stuff on repository
> > }
> > }
> >
> > With this, the user can specify the control/ViewModel in XAML, and the
> > control/ViewModel author gets the benefit of typing.
> >
> > Is there any good way to do this or I am stuck?
> >
> > Thanks,
> > Erick
> >
> >
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