How do i add items to a generic collection?

Question

Given:

public static void DoStuff<T>(ICollection<T> source)
{
    Customer c = new Customer();
    ....
    source.Add(c);
}

except c is not of type <T>.

So how do i add an item to a generic collection?

i tried having:

public static void DoStuff(ICollection<Human> source)
{
    Customer c = new Customer();
    ....
    source.Add(c);
}

but i don't use it because nobody can call DoStuff:

ICollection<Customer> c;
DoStuff(c); <---error

because something about covariance, and .NET doesn't realize that Customer descends from Human:

class Customer : Human {}

Are you actually doing anything with elements contained in `source`, or do you just want the collection populated? Would it be possible to just return a newly created collection? — Clockwork-Muse, Jan 10 '12 at 19:55
i'm not populating a new list, i'm modifying (adding, modifying, deleting) items in the passed list. — Ian Boyd, Jan 10 '12 at 20:36

James Michael Hare · Accepted Answer · 2012-01-10T20:02:07.410

Just so you know why you get that error, an ICollection<Customer> can not be passed to an ICollection<Human> because they are not the same thing. Think of it this way, if you had an ICollection<Human> you could Add(new Deadbeat()) if Deadbeat derived from Human.

The other answers on ways to avoid your issue with a generic solves your problem (so they should get the answer credit):

public static void DoStuff<T>(ICollection<T> source) where T : new()
{
    T c = new T();
    ...
    source.Add(c);
}

but I just wanted to throw this answer out to explain why you get that error. Think of it, if you could pass a collection of Customer as a collection of Human, it would let you add ANY kind of human, and this would violate the original collection.

Thus, even though Customer extends Human, this does not mean that ICollection<Customer> extends ICollection<Human> and ICollection<T> is not covariant/contravariant because it uses T for both in and out operations.

score 7 · Answer 2 · answered Jan 10 '12 at 19:53

7

should be just:

T t = new T();
....
source.Add(t);

Also you will have to add the new constraint to guarantee that T has a public parameterless constructor:

public static void DoStuff<T>(ICollection<T> source) where T: new()
{
   //...
}

answered Jan 10 '12 at 19:53

BrokenGlass

158,293
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Would there be any recommendation on how to accomplish what i want? i don't think i can set the properties of my **T** customer with that syntax. – Ian Boyd Jan 10 '12 at 20:39
@IanBoyd: Do you want to specify all Customer properties? Or just Human ones? – James Michael Hare Jan 11 '12 at 14:51

score 1 · Answer 3 · answered Jan 10 '12 at 19:55

1

You probably want something like this:

public static void DoStuff<T>(ICollection<T> source)
    where T : new()
{
    T c = new T();
    source.Add(c);
}

answered Jan 10 '12 at 19:55

That Chuck Guy

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4
12

How do i add items to a generic collection?

3 Answers3