Say I've got this interface:
public interface IFoo
{
string Text {get;}
}
And this class that implements it:
public class Bar : IFoo
{
public string Text
{
get {return "Hello World!";}
}
}
I've got a function that takes as an argument a List<IFoo>
. When I try to pass it a List<Bar>
, I get this compiler error:
Argument 1: cannot convert from System.Collections.Generic.List<Bar>
to System.Collections.Generic.List<IFoo>
Why is this? It seems like that should work for the same reason as if I passed a list of a class that inherits from Bar
directly.
Why is this? It seems like that should work for the same reason as if I passed a list of a class that inherits from Bar directly.
Because this disaster can happen:
class EvilBar : IFoo { }
List<Bar> list = new List<Bar>();
List<IFoo> foos = list; // suppose it were allowed
foos.Add(new EvilBar());
And now you've add an EvilBar
to a list than can only accept Bar
. Oops!
Basically, you've discovered that T -> List<T>
is not covariant in T
.
Because generic contravariance was not supported in.Net 2.0. You can do it with arrays though.
IFoo[] foos = new Bar[5];
Another option is to do
List<IFoo> myList = myBarList.ConvertAll(b => (IFoo)b);
Do not pass List<Bar>
. Use cast and pass this:
yourObj.YourMethod(yourListOfBar.Cast<IBar>().ToList());
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