如何隐式转换列表 <Foo> 列出 <Bar> 鉴于定义了强制转换运算符？

Question

I have a type Bar that defines an implicit cast operator:

public static implicit operator Bar(Foo foo)
{
    return new Bar(foo.property);
}

Converting single objects works great, but now I have a List<Foo> and I need a List<Bar> .

I found this workaround:

List<Bar> barList = fooList.ConvertAll<Bar>(item => item);

which seems to do the trick, but it's somewhat defeating the point of an implicit cast operator if I have to apply it in this somewhat explicit sense. Is there any way to make the implicit conversion work for lists?

Answer 1

Unfortunately, you cannot make this cast implicit, although the reason is somewhat unexpected: when you explicitly provide a type argument to a generic method, C# requires you to provide all remaining arguments as well. It is this all-or-nothing approach that lets you write this

List<Bar> barList = fooList.Select(item => (Bar)item).ToList();

or this

List<Bar> barList = fooList.Select<Foo,Bar>(item => item);

but it does not let you write this:

List<Bar> barList = fooList.Select<...,Bar>(item => item);
//                                 ^^^
//                                  |
// Figure this out from the context |

Theoretically, C# could capture Foo from the type of item , but it does not have a feature to do it.

LINQ's Cast<T> is not going to work either, because it treats items being cast as plain object s, thus ignoring any conversion operators that may be defined on them.

Since you are required to specify both the "from" and the "to" types, a solution based on generics is not going to be any better than your solution based on ConvertAll .