is-operator on generic list

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?

Checking to see whether a list of VW objects is a list of IVehicle objects seems silly, since VW inherits from IVehicle amd you are therefore writing if(true). Besides, since VW inherits from IVehicle and myCars is a List<VW>, foreach (IVehicle v in myCars) will simply work. — Steven
– Steven, Commented Feb 19, 2012 at 20:10
@Steven: a list of VW is emphatically not a list of IVehicle, because you can add a Ford to a list of IVehicle, but not to a list of VW. — phoog
– phoog, Commented Feb 20, 2012 at 0:49

Marc Gravell · Accepted Answer · 2012-02-19 20:05:02Z

11

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.

answered Feb 19, 2012 at 20:05

Marc Gravell

1.1m273 gold badges2.6k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Steven Over a year ago

And if(myCars is IEnumerable<IVehicle>) will work as well in .NET 4.0.

Marc Gravell Over a year ago

Just for completeness, I should note that arrays can be covariant (for reference-types), but in a can-error-at-runtime way.

Marc Gravell Over a year ago

@Steven indeed; I just hate doing the test twice ;p

Pavel Krymets · Accepted Answer · 2012-02-19 20:06:34Z

2

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

answered Feb 19, 2012 at 20:06

Pavel Krymets

6,3041 gold badge25 silver badges37 bronze badges

Comments

phoog · Accepted Answer · 2012-02-20 00:51:37Z

0

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>) //...

edited Feb 20, 2012 at 0:51

answered Feb 20, 2012 at 0:46

phoog

43.2k6 gold badges84 silver badges125 bronze badges

Collectives™ on Stack Overflow

is-operator on generic list

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related