Linq的Lambda声明

Question

Is it possible to do something like the following in Linq?

List<Group> groupsCollection = new List<Group>();
groups.Select( g => {
    String id = g.Attributes["id"].Value;
    String title = g.Attributes["title"].Value;
    groupsCollection.Add( new Group(id, title) );
} );

This is just an example. I know a Foreach loop would be sufficient but I was querying whether it would be possible or not.

I tried it, and it's saying:

Cannot convert lambda expression to type System.Collections.IEnumerable<CsQuery.IDomObject> because it's not a delegate type

Edit: Group is my custom class. Groups are a collection of CsQuery Dom Objects. You can think of them as a collection of html anchor elements. They are IEnumerable.

Answer 1

I think you're looking for this:

groupsCollection = groups.Select(g =>
    new Group(g.Attributes["id"].Value,
              g.Attributes["title"].Value)).ToList();

Explanation:

Select() projects each element of a sequence into a new form. (From MSDN )

It's basically a transformation function that takes one IEnumerable and transforms it, in whatever way you like, to another IEnumerable, which seems to be exactly what you want here.

Answer 2

Select() takes a lambda expression that returns a value ; it shouldn't have any sife effects.

You want

var groupsCollection = groups.Select(g => {
    return ...;
}).ToList();

Answer 3

You can use the constructor to initialize it with the query:

var query = groups.Select( g => {
    String id = g.Attributes["id"].Value;
    String title = g.Attributes["title"].Value;
    return new Group(id, title);
});
List<Group> groupsCollection = new List<Group>(query);

Do not modify colections in a linq-query, instead select data that can be used to modify collections.

Answer 4

As you said, you know using ForEach would be relevant. However you are just curious if you can do it another way. As others pointed out, you can use Select but the code in the {...} should return some value. However, you can do it this way which is better if you want to stop or break the loop depending on some condition using TakeWhile :

groups.TakeWhile( g => {
       String id = g.Attributes["id"].Value;
       String title = g.Attributes["title"].Value;
       groupsCollection.Add( new Group(id, title) );
       return true;//If you want to break the loop, just return false accordingly.
    });