Question

interface IVehicle 
{
    void DoSth();
}

class VW : IVehicle
{
    public virtual void DoSth() { ... }
}

class Golf : VW { }

class Lupo : VW
{
    public override void DoSth()
    {
        base.DoSth();
        ...
    }  
}

in my code i have:

List<VW> myCars = new List<VW>();
myCars.Add(new Golf());
myCars.Add(new Lupo());

now i want to evaluate if i have a list of vehicles. something like:

if(myCars is List<IVehicle>)
{
    foreach(IVehicle v in myCars)
        v.DoSth();
}

how can i do this? the is-operator on the generic list does not work. is there another way?

Answer 1

Even with 4.0 variance rules, a list-of-VW is not ever a list-of-IVehicle, even if a VW is an IVehicle. That isn’t how variance works.

However, in 4.0, you could use:

var vehicles = myCars as IEnumerable<IVehicle>;
if(vehicles != null) {
     foreach(var vehicle in vehicles) {...}
}

Since IEnumerable<out T> exhibits covariance.