Question

I got tree objects.

A object EFacebook, very simple, with a property called id.

class EFacebook
{
   public string id { get; set; }
}

A EUser, more complex. The EUser gets a list of EProvider inside, and each EProvider gets a type. Like:

class EUser
{
    public string name { get; set; }
    public List<EProvider> EProviders { get; set; }
}

class EProvider
{
   public enum EnumType
   {
      facebook = 1,
      twitter = 2
   }
   public string id { get; set; }
   public EnumType type { get; set; }
}

And two lists: (the list of providers, inside users, is optional, nullable):

// list of facebooks
List<EFacebook> facebooks = new List<EFacebook>();

facebooks.Add(new EFacebook { id = "1" });
facebooks.Add(new EFacebook { id = "2" });
facebooks.Add(new EFacebook { id = "3" });

// list of users
List<EUser> users = new List<EUser>();
List<EProvider> ps1 = new List<EProvider>();

ps1.Add(new EProvider { id = "1", type = EProvider.EnumType.facebook });
ps1.Add(new EProvider { id = "2", type = EProvider.EnumType.twitter });

List<EProvider> ps2 = new List<EProvider>();
ps2.Add(new EProvider { id = "3", type = EProvider.EnumType.facebook });
ps2.Add(new EProvider { id = "4", type = EProvider.EnumType.twitter });

EUser u1 = new EUser { name = "somea", EProviders = ps1 };
EUser u2 = new EUser { name = "someb", EProviders = ps2 };
EUser u3 = new EUser { name = "somec" };

users.Add(u1);
users.Add(u2);
users.Add(u3);

Now, I need to obtain two children lists of facebooks based on the relation between facebook.id and user.eprovider(of enum facebook).id. It means, a list of EFacebook containing only the object facebook with id 2, because I don’t have a user with a provider of type facebook and id 2, and a list of EFacebook containing the objects facebook with id 1 and 3, because I do have users with a provider of type facebook and id 1 and id 3.

I tried a lot, but It’s not working:

/*var xaa = users.Select(z => z.EProviders.Where(x => x.ProviderType == 
    EProvider.EnumProviderType.Facebook).Select(x => x.Ip).Zip(z);*/

/*var a = users.Select(x => x.EProviders.Where(y => y.ProviderType == 
    EProvider.EnumProviderType.Facebook));*/


/*
var outra = facebooks.Where(x=>                 
var a = from i in users
    where i.EProviders != null && i.EProviders.Any(j => j.ProviderType == 
    EProvider.EnumProviderType.Facebook)
from j in i.EProviders
join k in facebooks on j.Ip equals k.id
select k;

var b = facebooks.Except(a);
*/

Any help will be appreciated.

Answer 1

How about:

// Create an enumerable of all IDs of Facebook providers from the users list
var facebookIds = users
    // Exclude all users with a null EProviders list
    .Where(u => u.EProviders != null)
    // For each user, select all EProviders with type == facebook
    // and use SelectMany to flatten them into a single enumerable
    .SelectMany(u => u.EProviders.Where(p => p.type == EProvider.EnumType.facebook));

// Use Join to find all facebooks whose IDs also exist in the facebookIds set constructed above
var facebooksWithUsers = facebooks.Join(facebookIds, f => f.id, p => p.id, (f, p) => f);

// Use Except to find the opposite subset
var facebooksWithoutUsers = facebooks.Except(facebooksWithUsers);

// Write the contents of the two sets to the console
Console.WriteLine("facebooksWithUsers:");
foreach (var fb in facebooksWithUsers)
{
    Console.WriteLine(fb.id);
}

Console.WriteLine();
Console.WriteLine("facebooksWithoutUsers:");
foreach (var fb in facebooksWithoutUsers)
{
    Console.WriteLine(fb.id);
}

This outputs:

facebooksWithUsers:
1
3

facebooksWithoutUsers:
2