Typecasting lists

[Thomas Lorenz]

Hi,

I want to pass a List<string> to a method which requires a List<object>
parameter. I wonder if there is a comfortable way for typecasting the
stringlist without having to copy each item.

 

[Barry Kelly]

I want to pass a List<string> to a method which requires a List<object>
parameter. I wonder if there is a comfortable way for typecasting the
stringlist without having to copy each item.

You need to use List<T>.ConvertAll<TOutput>() or some other copying /
conversion - there's no way to typecast, because covariance isn't
supported by C#'s generics.

-- Barry

 

[Marc Gravell]

Do you control the method in question? If so, you could make it:

void SomeMethod<T>(List<T> list) {}

This would allow you to pass *any* list to it in a type-safe manner.
For instance, inside SomeMethod you will only be able to treat items as
"T" instances, so you won't be able to add random items (but you can if
you can guarantee the item is a T, e.g. by adding the "where T : new()"
clause and creating the new item yourself). Also, unless you need the
full List<T> functionality, it may be adviseable to make your parameter
IList<T> or even IEnumerable<T>.

Note that the T does not have to be specified in usage; the compiler
will infer it:

List<string> strings = new List<string>();
strings.Add("abc");
strings.Add("def");
SomeMethod(strings);

Marc