Question

I wonder why IEnumerable<int> can’t be assigned to a IEnumerable<object>. After all IEnumerable is one of the few interfaces that supports covariance…

• The subtype relation and covariance stuff works with reference types
• int seems to be a proper subtype of object

The combination of both features doesn’t work however…

class A
{
}

class B : A
{
}

class Program
{
    static void Main(string[] args)
    {
        bool b;
        b = typeof(IEnumerable<A>).IsAssignableFrom(typeof(List<B>));
        Console.WriteLine("ienumerable of ref types is covariant: " + b); //true

        b = typeof(IEnumerable<object>).IsAssignableFrom(typeof(List<int>));
        Console.WriteLine("ienumerable of value tpyes is covariant: " + b); //false

        b = typeof(object).IsAssignableFrom(typeof(int));
        Console.WriteLine("int is a subtype of object: " + b); //true
    }
}

thanks for your help!
sebastian

Answer 1

Value types aren’t LSP-subtypes of object until they’re boxed.

Variance doesn’t work with value types. At all.

---

Demonstration that int is not a proper subtype (subtype in the LSP sense) of object:

Works:

object x = new object();
lock (x) { ... }

Does not work (substitutability violated):

int y = new int();
lock (y) { ... }

Returns true:

object x = new object();
object a = x;
object b = x;
return ReferenceEquals(a, b);

Returns false (substitutability violated):

int y = new int();
object a = y;
object b = y;
return ReferenceEquals(a, b);

Of course, the topic of the question (interface variance) is a third demonstration.