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Linq: find similar objects from two different lists

I've got two separate lists of custom objects. In these two separate lists, there may be some objects that are identical between the two lists, with the exception of one field ("id"). I'd like to know a smart way to query these two lists to find this overlap. I've attached some code to help clarify. Any suggestions would be appreciated.
namespace ConsoleApplication1
{
class userObj
{
public int id;
public DateTime BirthDate;
public string FirstName;
public string LastName;
}
class Program
{
static void Main(string[] args)
{
List<userObj> list1 = new List<userObj>();
list1.Add(new userObj()
{
BirthDate=DateTime.Parse("1/1/2000"),
FirstName="John",
LastName="Smith",
id=0
});
list1.Add(new userObj()
{
BirthDate = DateTime.Parse("2/2/2000"),
FirstName = "Jane",
LastName = "Doe",
id = 1
});
list1.Add(new userObj()
{
BirthDate = DateTime.Parse("3/3/2000"),
FirstName = "Sam",
LastName = "Smith",
id = 2
});
List<userObj> list2 = new List<userObj>();
list2.Add(new userObj()
{
BirthDate = DateTime.Parse("1/1/2000"),
FirstName = "John",
LastName = "Smith",
id = 3
});
list2.Add(new userObj()
{
BirthDate = DateTime.Parse("2/2/2000"),
FirstName = "Jane",
LastName = "Doe",
id = 4
});
List<int> similarObjectsFromTwoLists = null;
//Would like this equal to the overlap. It could be the IDs on either side that have a "buddy" on the other side: (3,4) or (0,1) in the above case.
}
}
}

I don't know why you want a List<int>, i assume this is what you want:
var intersectingUser = from l1 in list1
join l2 in list2
on new { l1.FirstName, l1.LastName, l1.BirthDate }
equals new { l2.FirstName, l2.LastName, l2.BirthDate }
select new { ID1 = l1.id, ID2 = l2.id };
foreach (var bothIDs in intersectingUser)
{
Console.WriteLine("ID in List1: {0} ID in List2: {1}",
bothIDs.ID1, bothIDs.ID2);
}
Output:
ID in List1: 0 ID in List2: 3
ID in List1: 1 ID in List2: 4

You can implement your own IEqualityComparer<T> for your userObj class and use that to run a comparison between the two lists. This will be the most performant approach.
public class NameAndBirthdayComparer : IEqualityComparer<userObj>
{
public bool Equals(userObj x, userObj y)
{
return x.FirstName == y.FirstName && x.LastName == y.LastName && x.BirthDate == y.BirthDate;
}
public int GetHashCode(userObj obj)
{
unchecked
{
var hash = (int)2166136261;
hash = hash * 16777619 ^ obj.FirstName.GetHashCode();
hash = hash * 16777619 ^ obj.LastName.GetHashCode();
hash = hash * 16777619 ^ obj.BirthDate.GetHashCode();
return hash;
}
}
}
You can use this comparer like this:
list1.Intersect(list2, new NameAndBirthdayComparer()).Select(obj => obj.id).ToList();

You could simply join the lists on those 3 properties:
var result = from l1 in list1
join l2 in list2
on new {l1.BirthDate, l1.FirstName, l1.LastName}
equals new {l2.BirthDate, l2.FirstName, l2.LastName}
select new
{
fname = l1.FirstName,
name = l1.LastName,
bday = l1.BirthDate
};
Instead of doing a simple join on just one property (column), two anonymous objects are created new { prop1, prop2, ..., propN}, on which the join is executed.
In your case we are taking all properties, except the Id, which you want to be ignored and voila:
Output:
And Tim beat me to it by a minute

var similarObjectsFromTwoLists = list1.Where(x =>
list2.Exists(y => y.BirthDate == x.BirthDate && y.FirstName == x.FirstName && y.LastName == x.LastName)
).ToList();
This is shorter, but for large list is more efficient "Intersect" or "Join":
var similarObjectsFromTwoLists =
list1.Join(list2, x => x.GetHashCode(), y => y.GetHashCode(), (x, y) => x).ToList();
(suposing GetHashCode() is defined for userObj)

var query = list1.Join (list2,
obj => new {FirstName=obj.FirstName,LastName=obj.LastName, BirthDate=obj.BirthDate},
innObj => new {FirstName=innObj.FirstName, LastName=innObj.LastName, BirthDate=innObj.BirthDate},
(obj, userObj) => (new {List1Id = obj.id, List2Id = userObj.id}));
foreach (var item in query)
{
Console.WriteLine(item.List1Id + " " + item.List2Id);
}

How to intersect list in c#?

I have following list of Item objects in c#:
public class Item
{
public int Id { get; set; }
public List<string> Orders { get; set; }
}
List<Item> item = new List<Item>() {
new Item() { Id = 1, Code = 23, Orders = new List<string>() { "A", "B" }},
new Item() { Id = 2, Code = 24, Orders = new List<string>() { "C", "D" }},
new Item() { Id = 1, Code = 23, Orders = new List<string>() { "E", "F" }},
new Item() { Id = 3, Code = 25, Orders = new List<string>() { "G", "H" }}
};
I want to concat the Orders whose Id is same, so the output of above list should be:
{
new Item() { Id = 1, Code = 23, Orders = new List<string>() { 'A', 'B', 'E', 'F' },
new Item() { Id = 2, Code = 24, Orders = new List<string>() { 'C', 'D' },
new Item() { Id = 3, Code = 25, Orders = new List<string>() { 'G', 'H' }
};
How can i do this efficiently in c# ( using linq if possible ) ?

You want to group the items based on their ID, and then create a new sequences based on all of the Orders for that group.
var query = items.GroupBy(item => item.Id)
.Select(group => new Item
{
Id = group.Key,
Orders = group.SelectMany(item => item.Orders).ToList()
});
Note that this is not the intersection of any data. You're getting the union of all data within each group.

It appears what you want is something like this:
var output = items.GroupBy(i => i.Id)
.Select(g => new Item()
{
Id = g.Key
Orders = g.SelectMany(i => i.Orders)
.ToList()
});
Or in query syntax:
var output =
from i in items
group i by i.Id into g
select new Item()
{
Id = g.Key
Orders = g.SelectMany(i => i.Orders).ToList()
};

You can group your items by their id, then create new item for each id concatenate the orders:
item.GroupBy(x => x.Id)
.Select(x => new Item
{
Id = x.Key,
Orders = x.SelectMany(a => a.Orders).ToList()
}).ToList();

Descending sorting of a list of object based on a count from a different class

I'm stuck on this problem where I need to do descending sort based on other list. l_lstNames need to update by age descending.
public class Test
{
public String Name;
public Int32 Age;
}
List<String> l_lstNames = new List<String> { "A1", "A3", "A2", "A4", "A0" };
List<Test> l_lstStudents = new List<Test>
{
new Test { Age = 33, Name = "A0" },
new Test { Age = 10, Name = "A1" },
new Test { Age = 50, Name = "A2" },
new Test { Age = 8, Name = "A3" },
new Test { Age = 25, Name = "A4" },
};
// Output
List<String> l_lstNames = new List<String> { "A2", "A0", "A4", "A1", "A3" };
Found few sames samples but not matching what I'm looking for. Thank you for help.

Create Dictionary<string, int> with Name to Age mapping and use it within order method:
var dict = students.ToDictionary(x => x.Name, x => x.Age);
var ordered = source.OrderByDescending(x => dict[x.Name]).ToList();
or you can just order students collection and then select Name only:
var ordered = students.OrderByDescending(x => x.Age)
.Select(x => x.Name)
.ToList();

If you just want the names in order descending:
var sorted = l_lstStudents // From the list of students
.OrderByDescending(l => l.Age) // with the oldest student first
.Select(s => s.Name) // give me just the names
.ToList(); // in a list!

I think this is what you are looking for
List<String> l_lstNames1 = (from student in l_lstStudents
where l_lstNames.Any(a => student.Name == a)
orderby student.Age descending
select student.Name ).ToList();
OR
List<String> l_lstNames2 = l_lstStudents.OrderByDescending(a => a.Age)
.Where(a => l_lstNames.Any(b => b == a.Name))
.Select(a => a.Name).ToList();

Remove duplicate items and calculate average values using LINQ

For example I have a list of objects (properties: Name and value)
item1 20;
item2 30;
item1 50;
I want the result:
item1 35 (20+50)/2
item2 30
How can I do this?
Sorry guys, duplicate is based on item.Name.

var results =
from kvp in source
group kvp by kvp.Key.ToUpper() into g
select new
{
Key= g.Key,
Value= g.Average(kvp => kvp.Value)
}
or
var results = source.GroupBy(c=>c.Name)
.Select(c => new (c.Key, c.Average(d=>d.Value)));

You could do it using average and group by:
public class myObject
{
public string Name {get;set;}
public double Value {get;set;}
}
var testData = new List<myObject>() {
new myObject() { Name = "item1", Value = 20 },
new myObject() { Name = "item2", Value = 30 },
new myObject() { Name = "item1", Value = 50 }
};
var result = from x in testData
group x by x.Name into grp
select new myObject() {
Name=grp.Key,
Value= grp.Average(obj => obj.Value)
};

Select top N records after filtering in each group

I am an old bee in .NET but very new to Linq! After some basic reading I have decided to check my skill and I failed completely! I don't know where I am making mistake.
I want to select highest 2 order for each person for while Amount % 100 == 0.
Here is my code.
var crecords = new[] {
new {
Name = "XYZ",
Orders = new[]
{
new { OrderId = 1, Amount = 340 },
new { OrderId = 2, Amount = 100 },
new { OrderId = 3, Amount = 200 }
}
},
new {
Name = "ABC",
Orders = new[]
{
new { OrderId = 11, Amount = 900 },
new { OrderId = 12, Amount = 800 },
new { OrderId = 13, Amount = 700 }
}
}
};
var result = crecords
.OrderBy(record => record.Name)
.ForEach
(
person => person.Orders
.Where(order => order.Amount % 100 == 0)
.OrderByDescending(t => t.Amount)
.Take(2)
);
foreach (var record in result)
{
Console.WriteLine(record.Name);
foreach (var order in record.Orders)
{
Console.WriteLine("-->" + order.Amount.ToString());
}
}
Can anyone focus and tell me what would be correct query?
Thanks in advance

Try this query:
var result = crecords.Select(person =>
new
{
Name = person.Name,
Orders = person.Orders.Where(order => order.Amount%100 == 0)
.OrderByDescending(x => x.Amount)
.Take(2)
});
Using your foreach loop to print the resulting IEnumerable, the output of it is:
XYZ
-->200
-->100
ABC
-->900
-->800

This has already been answered but if you didn't want to create new objects and simply modify your existing crecords, the code would look like this alternatively. But you wouldn't be able to use anonymous structures like shown in your example. Meaning you would have to create People and Order classes
private class People
{
public string Name;
public IEnumerable<Order> Orders;
}
private class Order
{
public int OrderId;
public int Amount;
}
public void PrintPeople()
{
IEnumerable<People> crecords = new[] {
new People{
Name = "XYZ",
Orders = new Order[]
{
new Order{ OrderId = 1, Amount = 340 },
new Order{ OrderId = 2, Amount = 100 },
new Order{ OrderId = 3, Amount = 200 }
}
},
new People{
Name = "ABC",
Orders = new Order[]
{
new Order{ OrderId = 11, Amount = 900 },
new Order{ OrderId = 12, Amount = 800 },
new Order{ OrderId = 13, Amount = 700 }
}
}
};
crecords = crecords.OrderBy(record => record.Name);
crecords.ToList().ForEach(
person =>
{
person.Orders = person.Orders
.Where(order => order.Amount%100 == 0)
.OrderByDescending(t => t.Amount)
.Take(2);
}
);
foreach (People record in crecords)
{
Console.WriteLine(record.Name);
foreach (var order in record.Orders)
{
Console.WriteLine("-->" + order.Amount.ToString());
}
}
}