linq，计算计数

Question

I'm trying to figure out if I have a list of objects that have their own list of objects, how to get the total count. For example:

public class FileWrapper {
   List<File> Files
}

public class File {
   ...
}

So with a List wrappers, how do I get the total count of files.

wrappers.ForEach(f => f.Files.Count).Count()

Answer 1

要Sum每个列表的数量，不Count的每个表的计数：

var sum = wrappers.Sum(file => file.Files.Count);

Answer 2

wrappers.Select(f => f.Files.Count).Sum()

Answer 3

You also can use SelectMany .

var sum = wrappers.SelectMany(wrapper => wrapper.Files).Count();

I do not have access to VS. So it is eyes-compiled.

Like @Servy says, it is a bad idea :

This forces every single inner list to be iterated to determine its count, when they're capable of just returning the already-known Count.