Generics and Type casting.

[Ashish]

Hi all,

I have interface declared like

public IBaseInterface
{
}

then a generic collection like

public GCollection<T> Where T:IBaseInterface
{
}


then i have a class like

public class Address:IBaseInterface
{}


so i declare a collection of Address type

GCollection<Address> mcol = new GCollection<Address>();

then i try to type cast it to

GCollection<IBaseInterface> mysecondcol = (GCollection<IBaseInterface>)
mcol; // this throws type cast exception !

can someone explain why this is not allowed and what can be a way around ?

TIA

 

[Nicholas Paldino [.NET/C# MVP]]

Ashish,

You would have to declare the GCollection<Address> as
GCollection<IBaseInterface>.

The reason you have to do this is say you could perform the cast. Then,
when you try to add another class which implements IBaseInterface, but is
not Address, you would get an exception because of the invalid cast.

Hope this helps.

 

[Jon Skeet [C# MVP]]

so i declare a collection of Address type

GCollection<Address> mcol = new GCollection<Address>();

then i try to type cast it to

GCollection<IBaseInterface> mysecondcol = (GCollection<IBaseInterface>)
mcol; // this throws type cast exception !

can someone explain why this is not allowed and what can be a way around ?

Does it even compile? I'd be surprised.

Generics aren't covariant in that way - and for good reason. Imagine
you wrote the code above, and then:

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

That will compile, but at runtime it would either have to throw an
exception, or end up with mcol not *really* being a collection of
Address.

 

[Ashish]

hi Nicholas,
thanks for the reply,

this compiles properly but throws error at runtime, i can understand if
i try to add object of some other class and it throws error, but
IBaseEntity is a braoder type of Address so i should be able to hold a
reference ....

TIA

 

[Ashish]

it compiles properly, and its okay if this throws runtime error on

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

but i should be able to hold a reference since Address is a narrower
type of IBaseEntity...

so i declare a collection of Address type

GCollection<Address> mcol = new GCollection<Address>();

then i try to type cast it to

GCollection<IBaseInterface> mysecondcol = (GCollection<IBaseInterface>)
mcol; // this throws type cast exception !

can someone explain why this is not allowed and what can be a way around ?

Does it even compile? I'd be surprised.

Generics aren't covariant in that way - and for good reason. Imagine
you wrote the code above, and then:

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

That will compile, but at runtime it would either have to throw an
exception, or end up with mcol not *really* being a collection of
Address.

 

[Nicholas Paldino [.NET/C# MVP]]

Ashish,

No, you shouldn't. There is no relation between
GCollection<IBaseInterface> and GCollection<Address>. The relation of the
type parameters does not ensure that you can cast between the two.


--
- Nicholas Paldino [.NET/C# MVP]
- (e-mail address removed)

it compiles properly, and its okay if this throws runtime error on

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

but i should be able to hold a reference since Address is a narrower type
of IBaseEntity...

so i declare a collection of Address type

GCollection<Address> mcol = new GCollection<Address>();

then i try to type cast it to

GCollection<IBaseInterface> mysecondcol = (GCollection<IBaseInterface>)
mcol; // this throws type cast exception !

can someone explain why this is not allowed and what can be a way around
?

Does it even compile? I'd be surprised.

Generics aren't covariant in that way - and for good reason. Imagine you
wrote the code above, and then:

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

That will compile, but at runtime it would either have to throw an
exception, or end up with mcol not *really* being a collection of
Address.

 

[Bruce Wood]

it's okay if this throws runtime error on

mysecondcol[0] = new SomeOtherImplementationOfIBaseInterface();

but i should be able to hold a reference since Address is a narrower
type of IBaseEntity...

No, you shouldn't, because the whole point of generics is to catch
these things at compile time and therefore to avoid having to generate
code to trap invalid casts at runtime (which costs). If you were
allowed to do what you're proposing, then everywhere you used a generic
structure the compiler would also have to write in run-time type
checking, which would slow things down considerably.

You're looking at the from the point of view of a reader: "If I'm going
to read this collection, I don't care whether it's declared to be a
collection of type A, or a collection of some base type of A." All well
and good, but for the purposes of writing to the collection, it does
matter.

Don't forget: there's already a way to have a collection that you can
regulate using run-time type checking of its contents: use an
ArrayList. Remember that in the 1.1 world there were collections in
which you could place any type of object and you took care of type
checking yourself.

In the 2.0 world, generics added the capability to have collections
that were locked down to a particular type (or any type derived from
that particular type). There is no capability to "broaden the scope" of
what's allowed in the collection for certain purposes. A collection is
of a particular type and that's all there is to it. Any relaxation of
that rule comes with (undesirable) run-time costs.

The only reasonable exception I can see would be to allow you to cast a
*read-only* collection of class A to a *read-only* collection of any
ancestor of class A, but that's probably too specific to justify the
work it would take to shoehorn it into the language.

 

[Ashish]

I see your point, but in my logic i dont know what type the collection
would be, apart from that it would be a type implementing IBaseEntity,

Is there a work around i could implement so somehow type cast into a
more generic collection ?

thanks again for your help

regards

 

[Jon Skeet [C# MVP]]

it compiles properly

Please provide the code then. Here's some code which *doesn't* compile,
which is very similar to your code:

using System;
using System.IO;
using System.Collections.Generic;

class Test
{
static void Main (string[] args)
{
List<MemoryStream> m = new List<MemoryStream>();
List<IDisposable> i = (List<IDisposable>)m;
}
}

That's about as close to your code as I can get without seeing the
definition of GCollection etc.