Casting ICollection of a concrete type to an ICollection of the concrete type interface

Question

What is the recommended way to cast an ICollection<Bar> to ICollection<IBar> where Bar implements IBar?

Is it as simple as

collection = new List<Bar>();
ICollection<IBar> = collection as ICollection<IBar>?

or is there a better way of doing it?

It's not quite an exact duplicate, but read https://stackoverflow.com/questions/13107071 to see why your solution won't work. (It has syntax errors already, but even if it didn't, it would give `null` as a result of the `as` operator.) — Jon Skeet, Jan 18 '16 at 15:04
You cannot cast to `ICollection`, but you can cast to `IEnumerable`. — Henrik, Jan 18 '16 at 15:24
@HenkHolterman yes, thank you. I see now that none of the other answers mention this possibility. — Henrik, Jan 19 '16 at 10:53

Tim Schmelter · Accepted Answer · 2016-01-19T08:22:16.170

3

You have to cast every item in the list and create a new one, for example with Cast:

ICollection<IBar> ibarColl = collection.Cast<IBar>().ToList();

In .NET 4, using the covariance of IEnumerable<T>:

ICollection<IBar> ibarColl = collection.ToList<IBar>();

ICollection<IBar> ibarColl = collection.ConvertAll(b => (IBar)b);

The latter might be a little bit more efficient since it knows the size beforehand.

edited Jan 19 '16 at 08:22

answered Jan 18 '16 at 15:03

Tim Schmelter

That would create a copy of the original list. Is there a way to reuse the same `ICollection` instance to add items and manipulate the collection? – Shimmy Weitzhandler Dec 15 '18 at 15:58
@Shimmy only if it's an instance of IList like an array or list. Then you can use a for loop and replace them or use Array.ConvertAll or list.ConvertAll as shown in the answer – Tim Schmelter Dec 15 '18 at 16:25

score 2 · Answer 2 · answered Jan 19 '16 at 10:50

You cannot cast to ICollection<IBar>, but you can cast to IEnumerable<IBar>.

So if you don't intend to add something to the list, you can do this:

IEnumerable<IBar> enumeration = (IEnumerable<IBar>)collection;

The solutions from the other answers don't actually cast but create a new list that will not reflect subsequent changes to the original list.

score 0 · Answer 3 · answered Jan 18 '16 at 15:10

0

Just convert all entries with

ICollection<IBar> ibars = collection.ConvertAll(bar => (IBar)bar);

I think this variant is also readable. Perhaps there are ways to cast it with more performance...

answered Jan 18 '16 at 15:10

Matthias Burger

3 Answers3