Given the following types:
public interface IMyClass { }
public class MyClass : IMyClass { }
I wonder how can I convert a List<MyClass> to a List<IMyClass>? I am not completely clear on the covariance/contravariance topics, but I understand that I cannot just plainly cast the List because of that.
I could come up with this trivial solution only; lacking any elegance, wasting resources:
...
public List<IMyClass> ConvertItems(List<MyClass> input)
{
var result = new List<IMyClass>(input.Count);
foreach (var item in input)
{
result.Add(item);
}
return result;
}
....
How can you solve it in a more elegant/performant way?
(Please, mind that I need .NET 2.0 solution, but for completeness, I would be happy to see the more elegant solutions using newer framework versions, too.)
The simplest way is probably to use ConvertAll:
List<IMyClass> converted = original.ConvertAll<IMyClass>(x => x);
Even if you're using .NET 2, you can use lambda syntax if you're using VS2008 or higher. Otherwise, there's always anonymous methods:
List<IMyClass> converted = original.ConvertAll<IMyClass>(
delegate (MyClass x) { return x; });
In .NET 3.5 you could use LINQ with Cast, OfType or even just Select:
var converted = original.Cast<IMyClass>().ToList();
var converted = original.OfType<IMyClass>().ToList();
var converted = original.Select(x => (IMyClass) x).ToList();
In .NET 4.0 you can use ToList directly without an intermediate cast, due to the covariance of IEnumerable<T>:
var converted = original.ToList<IMyClass>();
Does it need to be a list? An IEnumerable solution could be more efficient:
public IEnumerable<IMyClass> ConvertItems(List<MyClass> input)
{
foreach (var item in input)
{
yield return (IMyClass)item;
}
}
(.NET 3.5 solution)
List<MyClass> list = new List<MyClass> { ... };
List<IMyClass> converted = list.Cast<IMyClass>().ToList();
来源：https://stackoverflow.com/questions/4931789/casting-listt-covariance-contravariance-problem