> I have a xml file with data from various sources. For example:
>
> <root>
>   <Account>
>     <Id>1</Id>
>     <Name>Somename</Name>
>     <City>Somecity</City>
>     <ContactPersons>
>       <ContactPerson>
>         <Id>1</Id>
>         <Name>Somename</Name>
>       </ContactPerson>
>       <ContactPerson>
>         <Id>2</Id>
>         <Name>Somename2</Name>
>       </ContactPerson>
>     </ContactPersons>
>   </Account>
> </root>
>
> I created a generic businesscollection (named BusinessCollection) and
> a business class (named BusinessObject). Then I created a specific
> business class named Account en another one called ContactPerson. They
> both derive from the BusinessObject class.
>
> I created a instance of a businesscollection with accounts using:
> BusinessCollection <Account> collection = new BusinessCollection
> <Account>();
>
> I also made a parser that 'translates' the xml into the correct
> business classes. So in case of the data above I provide a instance of
> BusinessCollection <Account>. Then I create Account objects and add
> them to the collection. This is all done using reflection and some
> XPath classes.
>
> The parser has the folling method signature:
> public static void ParseToBusinessObjects <T>(BusinessCollection <T>
> collection, XPathDocument document) where T : BusinessObject
>
> So far so good. But now I also want subcollections to parse. In the
> xml data ContactPersons will be my subcollection (property is named
> the same). If my parser detects a subcollection it will get the
> current instance (using reflection). I then want to call my entry
> method again for this collection (recursive)
>
> However when I do this:
> BusinessCollection <BusinessObject> subCollection =
> (BusinessCollection <BusinessObject>) property.GetValue(instance,
> null);
>
> I get a InvalidCastException saying I can't convert between a
> BusinessCollection<ContactPerson> and a
> BusinessCollection<BusinessObject>.
>
> Its the same when I change it to :
> BusinessCollection <T> subCollection = (BusinessCollection <T>)
> property.GetValue(instance, null);

> To bad thats not possible yet :(. I hope it will be in the feature.
>
> Since using interfaces isn't possibe because of various reasons (like
> internal methods that need to be called on the object etc). So I
> 'solved' it by creating an internal method that expects a plain object.
> I then use reflection to call the Add method on that collection (which
> is passed as an object)
>
> MethodInfo info = collection.GetType().GetMethod("Add");
> object[] paramaters = new object[1];
> paramaters[0] = businessObject;
>
> info.Invoke(collection, paramaters);
>
> Not a very clean way but it works for now.

> To bad thats not possible yet :(. I hope it will be in the feature.
>
> Since using interfaces isn't possibe because of various reasons (like
> internal methods that need to be called on the object etc). So I
> 'solved' it by creating an internal method that expects a plain
> object.  I then use reflection to call the Add method on that
> collection (which is passed as an object)
>
> MethodInfo info = collection.GetType().GetMethod("Add");
> object[] paramaters = new object[1];
> paramaters[0] = businessObject;
>
> info.Invoke(collection, paramaters);
>
> Not a very clean way but it works for now.

> Frans Bouma [C# MVP] <perseus.usenetNOSPAM@xs4all.nl> wrote:
> > > Its the same when I change it to :
> > > BusinessCollection <T> subCollection = (BusinessCollection <T>)
> > > property.GetValue(instance, null);
> >
> >     This isn't supported. It's called covariance, and that's not
> > supported in .NET generics. BusinessCollection<BusinessObject>
> > isn't a supertype of BusinessCollection<Contact>, even though it
> > might look like it.  That's why casts fail.
>
> Just a tiny correction - it's not supported in C# generics.
> Apparently .NET itself supports both covariance and contravariance,
> but C# doesn't expose it. See the following link for details - it's
> fascinating stuff...
>
> http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx