Question

Suppose I have this LINQ query (on .NET 4.0) :

IEnumerable<Service> listService = (from MyObject myObj in new MyObjects()
                                        select myObj.Services);

As you can see, listService is a collection of “collection” (Services is a collection of “Service”, which contains a Title, an ID (what I need), and some other fields.).

What I’d like to do in LINQ is to have, with that query, an IEnumerable<int>, with the Distinct list of ID for each Service for each Services.

Is there a way to do this on LINQ or I need to cycle with some foreach and manage with another arrays?

Example :

my first myObj (so, MyObjects[0]) have got a collection, called Service, which contain single Service. Every Service have got a single id, respectively : "1", "2", "4"

my second myObj (so, MyObjects[1]) have got a collection, called Service, which contain single Service. Every Service have got a single id, respectively : "4", "5", "1", "2"}

What I need is a “single” collection which contains the list of ID from each Service collection from each myObj. This list must be with Distinct value.

Answer 1

Since you haven’t shown the content of your classes, I implemented what I could guess from your description. Does this match your objects?

class Service
{
    public int ID { get; set; }
    public string Name { get; set; }
}

class MyObject
{
    public IEnumerable<Service> Services { get; set; }
}

class MyObjects : List<MyObject>
{

}

If so, then if I stick the following in the constructor for MyObjects:

class MyObjects : List<MyObject>
{
    public MyObjects()
    {
        Add(new MyObject 
            { 
                Services = new List<Service>()
                    {
                        new Service { ID = 1, Name = "foo" },
                        new Service { ID = 2, Name = "bar" },
                    }
            });
        Add(new MyObject
        {
            Services = new List<Service>()
                    {
                        new Service { ID = 3, Name = "baz" },
                        new Service { ID = 4, Name = "foo1" },
                        new Service { ID = 1, Name = "dup 1"}
                    }
        });
    }
}

I can get the distinct IDs like so:

var distinctIDs = (from myObj in new MyObjects()
               from service in myObj.Services
               select service.ID).Distinct();

The following test program:

static void Main(string[] args)
{
    var objects = new MyObjects();
    var distinctIDs = (from myObj in new MyObjects()
                       from service in myObj.Services
                       select service.ID).Distinct();
    var notDistinctIDs = from myObj in new MyObjects()
                         from service in myObj.Services
                         select service.ID;
    foreach (var id in distinctIDs)
    {
        Console.WriteLine("Distinct ID: {0}", id);
    }
    Console.WriteLine("---");
    foreach (var id in notDistinctIDs)
    {
        Console.WriteLine("Not Distinct ID: {0}", id);
    }
}

prints:

Distinct ID: 1
Distinct ID: 2
Distinct ID: 3
Distinct ID: 4
---
Not Distinct ID: 1
Not Distinct ID: 2
Not Distinct ID: 3
Not Distinct ID: 4
Not Distinct ID: 1