Compare 2 lists using linq - linq

Question

I'm a linq noob.... can someone please some me how to achieve this using linq... I'm trying to compare 2 lists in both directions...
internal void UpdateUserTeams(int iUserID)
{
UserTeamCollection CurrentTeams = GetUserTeams(iUserID);
UserTeamCollection UpdatedTeams = this;
foreach (UserTeam ut in CurrentTeams)
{
if(!UpdatedTeams.ContainsTeam(ut.ID))
{
RemoveTeamFromDB();
}
}
foreach (UserTeam ut in UpdatedTeams)
{
if (!CurrentTeams.ContainsTeam(ut.ID))
{
AddTeamToDB();
}
}
}
public bool ContainsTeam(int iTeamID)
{
return this.Any(t => t.ID == iTeamID);
}

Answer 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Linqage
{
class Program
{
static void Main(string[] args)
{
UserTeamCollection currentTeams = new UserTeamCollection()
{
new UserTeam(1),
new UserTeam(2),
new UserTeam(3),
new UserTeam(4),
new UserTeam(5)
};
UserTeamCollection updatedTeams = new UserTeamCollection()
{
new UserTeam(2),
new UserTeam(4),
new UserTeam(6),
new UserTeam(8)
};
currentTeams.Except(updatedTeams).All(u =>
{
//Console.WriteLine("Item ID: {0}",u.ID);
//RemoveFromDB()
return true;
});
updatedTeams.Except(currentTeams).All(u =>
{
//Console.WriteLine("Item ID: {0}", u.ID);
//AddToDB()
return true;
});
}
}
public class UserTeamCollection
: List<UserTeam>
{
}
//Either overwrite the GetHashCode and Equals method OR create a IComparer
public class UserTeam
{
public int ID { get; set; }
public UserTeam(int id)
{
ID = id;
}
public override bool Equals(object obj)
{
UserTeam iOther = obj as UserTeam;
if (iOther != null)
{
return this.ID == iOther.ID;
}
return false;
}
public override int GetHashCode()
{
return ID.GetHashCode();
}
}
}

Answer 2

So converting your initial question to an english requirement:
foreach (UserTeam ut in CurrentTeams) // for each current team
{
if(!UpdatedTeams.ContainsTeam(ut.ID)) // that is not in the updated teams list
{
RemoveTeamFromDB(); // remove it from the database
}
}
foreach (UserTeam ut in UpdatedTeams) //for each of the teams in the updated teams list
{
if (!CurrentTeams.ContainsTeam(ut.ID)) //if the current team does not contain the updated team
{
AddTeamToDB(); //add the team to the database
}
}
Therefore, you want to do:
//select all current teams that are not in updated teams list
CurrentTeam.Except(UpdatedTeams).All(team => { RemoveTeamFromDB(team); return true; });
//select all updated teams that are not in the current teams list
UpdatedTeam.Except(CurrentTeams).All(team => { AddTeamToDB(team); return true; });
Make sure your UserTeam object has proper overrides for the Equals and GetHashCode methods, so that comparison between two UserTeams is accurate :)

Answer 3

You would normally use Enumerable.Except both ways to get the differences. Then add and remove as needed.

Answer 4

var addedTeams = UpdatedTeams.Except(CurrentTeams);
var removedTeams = CurrentTeams.Except(UpdatedTeams);

Answer 5

You're trying to get the outer parts from a full outer join. Here's a rough way to achieve that.
ILookup<int, UserTeam> currentLookup = CurrentTeams
.ToLookup(ut => ut.ID);
ILookup<int, UserTeam> updatedLookup = UpdatedTeams
.ToLookup(ut => ut.ID);
List<int> teamIds = CurrentTeams.Select(ut => ut.ID)
.Concat(UpdatedTeams.Select(ut => ut.ID))
.Distinct()
.ToList();
ILookup<string, UserTeam> results =
(
from id in teamIds
let inCurrent = currentLookup[id].Any()
let inUpdated = updatedLookup[id].Any()
let key = inCurrent && inUpdated ? "No Change" :
inCurrent ? "Remove" :
inUpdated ? "Add" :
"Inconceivable"
let teams = key == "Remove" ? currentLookup[id] :
updatedLookup[id]
from team in teams
select new {Key = key, Team = team)
).ToLookup(x => x.Key, x => x.Team);
foreach(UserTeam ut in results["Remove"])
{
RemoveTeamFromDB();
}
foreach(UserTeam ut in results["Add"])
{
AddTeamToDB();
}