LINQ Enumerable中的Enumerable-为什么？

Question

I have a Ticket class that contains a List of User objects:

class Ticket {
    public List<User> Users { get; set; }
}

I want to get all Users that are related to tickets. I do this:

var u = from t in db.Tickets
        where t.Users.Count() > 0
        select t.Users

That works but returns a Enumerable with 1 element. And that element is an Enumerable with the Users.

Why is it nested?

Answer 1

What you need to do is

  var u = (from t in db.Tickets from user in t.Users select user).ToList();

Then you will have a flattened List of Users. There's no need to check for where t.Users.Count() > 0 and if you did need to you should be using where t.Users.Any() for efficiency.

Answer 2

Because you select a IEnumerable<Users> of List<User> ...

The second Enumerable is your List<User>

You can flatten that with the following query.

db.Tickets.Where(t => t.Users.Any()).SelectMany(t => t.Users)

Any() is much faster than Count() > 0 because it will stop couting at the first occurence, Count iterate on the full list of users you have

Answer 3

Because it doesn't merge in one list all t.Users (t0.Users, t1.Users...). You need to combine them.

Answer 4

您可以将其合并为IEnumerable，如下所示：db.Tickets.Where（t => t.Users.Count（）> 0）.SelectMany（t => t.Users）

Answer 5

I think you are looking for

var u = (from t in db.Tickets
         from u in t.Users
         select u).Distinct();

Answer 6

From the query i supose you want to have all the user in the tickets.

You can solve this like this:

List<User> users = new List<User>();

foreach(List<user> u in  db.Tickets.where(ticket => ticktet.Users.Count() > 0).select(ticket => ticket.Users)){
users.AddAll(u);
}            

Answer 7

The SelectMany method is what you want. From the MSDN documentation:

"Projects each element of a sequence to an IEnumerable(Of T) and flattens the resulting sequences into one sequence."

So you take all the tickets and get all the users for each ticket so the resulting list will be a list of users. You also might want to use the Distinct method as well to remove duplicate users. I'm assuming that a user might be associated with more than one ticket.

Answer 8

First: A LINQ-Query will always return an enumerable.

Second: Your t.Users is a List, which implements IEnumerable aswell.

So the outer enumerable is the linq resultset and the inner one the actual first result that can be cast back to a list of users like

var u = from t in db.Tickets
       where t.Users.Count() > 0
       select (t.Users as List<User)