Re: Complex Casting problem in Generics

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"Ajeet" <asgrewal@xxxxxxxxx> wrote in message
news:1169119502.422928.148160@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Jon Skeet [C# MVP] wrote:
Ajeet wrote:
I am having some difficulty in casting using generics. These are the
classes.

<snip>

What do I have to do to be able to cast it as desired? If you can give
me links that describe the solution, I will be grateful (I tried some
googling but to no luck so far).

You can't. As far as I can tell, your problem is the same one as often
bites people that you can't cast from List<string> to List<object>.
That's an easier example to work with, so I'll do so, if you don't mind
:)

Suppose C# generics supported covariance in types. I could do:
List<string> sl = new List<string>();
List<object> ol = sl;
ol.Add (new object());

At that point I've clearly broken the idea that the list should only
contain strings.


that makes sense. I did not think of that.

Can I overload the cast operator and do this?

Should be possible, although it may be better to make an explicit conversion
function.

Something like:

public interface IProvider<PROF>
where PROF : IProviderProfile
{
private readonly PROF internalProfile;
private IProfile(PROF prov) : internalProvider(prov) {}
//Public constructors
//Some properties/methods
public IProvider<BASEPROF> Covary<BASEPROF>() where PROF : BASEPROF
{ return new IProvider<BASEPROF>(internalProfile); } // because
internalProfile casts to BASEPROF

// if IProfile has any internal state, you need to find
a way to share that too,

// possibly via pointer-to-implementation paradigm

// now you also need a generic operator==, not sure if that is possible
}



Now, it could be that your interfaces wouldn't actually have that
problem (i.e. there wouldn't be any methods/properties exposed which
gave rise to the issue), but that's why it's disallowed.

Are you absolutely sure you need generics here in the first place?


We deliberated quite a bit before deciding on generics, so there are
quite a few reasons for doing this. Although I admit we did not foresee
this problem occurring. Do you think there are disadvantages to using
generics?

Jon



.

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