c#4.0: int a real subtype of object? covariance, ienumerable and value types

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