Why doesn't .Net 4 allow me to pass in an array of an interface?

Question

I have an array of Circle objects (where Circle implements IShape interface and i have a function that has a parameter of List<IShape> . . why can't i pass in my array of Circles into this ?

Visual studio gives me a build error saying can't convert List<Circle> to List<IShape>

You mean you have a `List` and a function `Foo(List c)` and `Circle : IShape`? Consider if `Foo` had the line: `c.Add(new Square())`. — Travis Gockel, Mar 22 '13 at 02:44
This is an often asked question. Search for questions involving covariance on enumerable types. — Anthony Pegram, Mar 22 '13 at 02:44
http://stackoverflow.com/questions/2033912/c-sharp-variance-problem-assigning-listderived-as-listbase — Anthony Pegram, Mar 22 '13 at 02:46
Because you cant convert directly convert `List` to `List`, But you should be able to pass it in as `myCircles.OfType().ToList()` — sa_ddam213, Mar 22 '13 at 02:49

Travis Gockel · Accepted Answer · 2013-03-22T03:31:19.750

5

The short answer is because a function Foo can be implemented like this:

void Foo(IList<IShape> c)
{
    c.Add(new Square());
}

If you passed a List<Circle> to Foo, the provided type would not be capable of storing the Square, even though the type signatures claim it is okay. IList<T> is not covariant: the general IList<Circle> cannot be an IList<IShape> because it cannot support the addition of arbitrary shapes.

The fix is to use IEnumerable<IShape> to accept arguments in Foo, but that won't work in all cases. IEnumerable<T> is covariant: the specialized IEnumerable<Circle> fits the contract of the general IEnumerable<IShape>.

This behavior is also a Good Thing. The classic example of something that is covariant when it should not be is an array. The following code will compile, but will fail at runtime:

void Bar()
{
    // legal in C#:
    object[] o = new string[10];
    // fails with ArrayTypeMismatchException: can't store Int in a String[]
    o[0] = 10;
}

edited Mar 22 '13 at 03:31

answered Mar 22 '13 at 02:52

Travis Gockel

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Actually, `IEnumerable` will not be accepted by a method with the parameter `IEnumerable` you have to cast the collection you pass to the function to an instance of `IEnumerable` if the method signature is `Foo(IEnumerable shapes)`. – Francis R. Griffiths-Keam Mar 22 '13 at 03:53
2

@FrancisR.Griffiths-Keam, you are incorrect. As of .NET 4, IEnumerable is covariant on T, which means IEnumerable can be substituted as long as X derives from (or implements) T. – Anthony Pegram Mar 22 '13 at 04:07
@AnthonyPegram Ahh I missed that memo, thanks for putting the record straight. – Francis R. Griffiths-Keam Mar 22 '13 at 04:10
1

It is probably also important to note that `IList` does not have this same property (and *can't*). – Travis Gockel Mar 22 '13 at 07:21

Why doesn't .Net 4 allow me to pass in an array of an interface?

1 Answers1