To interface or not to interface

[Kagamin]

Steven Schveighoffer Wrote:

> On Tue, 25 May 2010 02:26:02 -0400, Kagamin <spam at here.lot> wrote:
> 
> > Recently I've hit a problem with collections in C#. Given classes
> > class A {}
> > class B {}
> > And two collections Collection<A> and Collection<B> it's possible to  
> > concat them into an array A[]. The efficient way to do it is to use  
> > CopyTo(T[],int) method, but it accepts only array of exact collection  
> > item's type, so I had to change the Collection<B> type to Collection<A>.
> 
> Did you mean
> 
> class B : A {} ?

Ah, yes.

> According to this page, it says that an exception could be thrown if "Type  
> T cannot be cast automatically to the type of the destination array."   
> This implies that a destination array type to which T could automatically  
> be cast should work.

ICollection<A> acoll;
ICollection<B> bcoll;
A[] cat;
ICollection<A>.CopyTo(A[],int)
ICollection<B>.CopyTo(B[],int) - note the signature, the destination array can't be an array of supertype. There's no chance to throw an exception because the code doesn't pass type check at compile time.