Casting List<T> - covariance/contravariance problem

问题

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.)

回答1:

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>();

回答2:

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;
    }
 } 

回答3:

(.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