通用列表上的is-operator

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.

Answer 2

In .net 4 it is posible using generic parameter variance. Read more about it here

Answer 3

You could do this:

if (typeof(IVehicle).IsAssignableFrom(myCars.GetType().GetGenericArguments[0]))
    foreach (IVehicle v in myCars)
        //...

This assumes that you know myCars is a generic type. If you don't know that for sure, you would need to do an additional check or two first.

However, since you aren't using any member of list other than GetEnumerator, you can do this:

if (myCars is IEnumerable<IVehicle>) //...