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Select all from one List, replace values that exist on another List

Few days ago I asked same question with SQL, but now it arises in C# code
Lets say we have this kind of class for holding different id/text pairs:
public class Text {
public int id { get; set; }
public string text { get; set; }
...
}
Now lets populate some data,
ListA gets a lot of data:
List<Text> ListA = new List<Text>{
new () {id = 1, text = "aaa1"},
new () {id = 2, text = "aaa2"},
new () {id = 3, text = "aaa3"},
new () {id = 4, text = "aaa4"},
new () {id = 5, text = "aaa5"},
new () {id = 6, text = "aaa6"},
};
And ListB gets just a little bit of data:
List<Text> ListB = new List<Text>{
new () {id = 4, text = "bbb4"},
new () {id = 5, text = "bbb5"},
};
And now what we are looking:
var result = ... // Some Linq or Lambda magic goes here
// and if we do:
foreach(var item in result){
Console.WriteLine(item.Id + " " + item.Text);
}
// Result will be:
1 : aaa1
2 : aaa2
3 : aaa3
4 : bbb4
5 : bbb5
6 : aaa6

You can try looking for id within ListB:
var result = ListA
.Select(a => ListB.FirstOrDefault(b => b.id == a.id) ?? a);
Here for each a within ListA we try to find corresponding by id (b.id == a.id) item within ListB. If no such item is found we just return ListA item: ?? item
In case of .Net 6 you can use overloaded .FirstOrDefault version (we can pass a as a default value):
var result = ListA
.Select(a => ListB.FirstOrDefault(b => a.id == b.id, a));

It might be more efficient to convert ListB to a Dictionary first:
var dictB = ListB.ToDictionary(x=> x.id)
Then you can write
var result = ListA.Select(x => dictB.TryGetValue(x.id, out var b) ? b : x)
UPD Rewrote taking comment suggestions into account

One option is to do an Union operation, by specifying an EqualityComparer. If the order is important, you can do an OrderBy operation at the end.
class TextIdComparer : EqualityComparer<Text> {
public override bool Equals(Text x, Text y) => x.id == y.id;
}
var result = ListB.Union(ListA, new TextIdComparer()).OrderBy(x => x.id)

Updating property values in one list with a property value average of matching items in another list

I have two Lists and need to update a property value of all the items in the 1st list with a property value average of all the matching items in another list.
class transaction
{
public string orderId;
public string parentOrderId;
public int quantity;
public decimal marketPrice;
public decimal fillPrice;
}
List<transaction> makerTransactions = new List<transaction>()
{
new transaction(){
orderId = "1",
parentOrderId = "1",
quantity = 100,
marketPrice = 75.87M,
fillPrice = 75.87M
}
};
List<transaction> takerTransactions = new List<transaction>()
{
new transaction(){
orderId = "2",
parentOrderId = "1",
quantity = 50,
marketPrice = 75.97M,
fillPrice = 75.97M
},
new transaction(){
orderId = "3",
parentOrderId = "1",
quantity = 50,
marketPrice = 75.85M,
fillPrice = 75.85M
}
};
Trying to make this work with LINQ extension methods but cant figure out the correct way.
makerTransactions.All(mt => mt.fillPrice = takerTransactions
.Where(tt => tt.parentOrderId == mt.orderId)
.Average(ta => ta.fillPrice));

try this:
makerTransactions.ForEach(mt => mt.fillPrice = takerTransactions
.Where(tt => tt.parentOrderId == mt.orderId)
.Average(ta => ta.fillPrice));
All is an extension method. It tells you if all the elements in a collection match a certain condition and, apparently, it's not what you need.
To make it more efficient, first create a dictionary and use that to take the averages from:
var priceDictionary = takerTransactions
.GroupBy(tt => tt.parentOrderId)
.ToDictionary(grp => gr.Key, grp => grp.Average(ta => ta.fillPrice));
makerTransactions.ForEach(mt => mt.fillPrice = priceDictionary[mt.orderId]);

Select Last non null-able item per product?

Let's say I have,
class Product
{
public int Id {get; set;}
public string Name {get; set;}
public int Order {get; set;}
}
and my data have,
products[0] = new Product { Id = 1, Name = "P1", Order = 1 };
products[1] = new Product { Id = 1, Name = "P2", Order = 2 };
products[2] = new Product { Id = 1, Name = null, Order = 3 };
products[3] = new Product { Id = 2, Name = "P3", Order = 4 };
products[4] = new Product { Id = 2, Name = null, Order = 5 };
products[5] = new Product { Id = 2, Name = null, Order = 6 };
What I need is the last(order by Order desc) non-nullable value of Name per Product.Id. So my final output will look like,
items[0] = new { Id = 1, Name = "P2"};
items[1] = new { Id = 2, Name = "P3"};
If Id=1, I have 3 Names (P1, P2, null) and non-nullable Names (P1, P2) but last one is P3.

This should get the last products in order.
var lastOrders = products
.Where(x => x.Name != null) // Remove inapplicable data
.OrderBy(x => x.Order) // Order by the Order
.GroupBy(x => x.Id) // Group the sorted Products
.Select(x => x.Last()); // Get the last products in the groups

var result = products
.GroupBy(p => p.Id)
.Select(g => g.OrderBy(x => x.Order).Last(x => x.Name != null));

this will give you your desired output:
products.GroupBy(p => p.Id)
.Select(g => g.OrderByDescending(gg => gg.Name)
.Where(gg => gg.Name != null)
.Select(gg => new { gg.Id, gg.Name })
.First());

The task can be solved using the following Linq statement.
var Result = products.OrderBy().Where( null != iProduct.Name ).First();
This requires products to contain at least one item where Name is null, otherwise an Exception will be thrown. Alternatively,
var Result = products.OrderBy().Where( null != iProduct.Name ).FirstOrDefault();
will return null if products contains no such item.

Try with :
var expectedProduct =products.Where(p => p.Id != null).OrderByDescending(p => p.Order).GroupBy(p => p.Id).Last()

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);
}

Assign values from one list to another using LINQ

Hello I have a little problem with assigning property values from one lists items to anothers. I know i could solve it "the old way" by iterating through both lists etc. but I am looking for more elegant solution using LINQ.
Let's start with the code ...
class SourceType
{
public int Id;
public string Name;
// other properties
}
class DestinationType
{
public int Id;
public string Name;
// other properties
}
List<SourceType> sourceList = new List<SourceType>();
sourceList.Add(new SourceType { Id = 1, Name = "1111" });
sourceList.Add(new SourceType { Id = 2, Name = "2222" });
sourceList.Add(new SourceType { Id = 3, Name = "3333" });
sourceList.Add(new SourceType { Id = 5, Name = "5555" });
List<DestinationType> destinationList = new List<DestinationType>();
destinationList.Add(new DestinationType { Id = 1, Name = null });
destinationList.Add(new DestinationType { Id = 2, Name = null });
destinationList.Add(new DestinationType { Id = 3, Name = null });
destinationList.Add(new DestinationType { Id = 4, Name = null });
I would like to achieve the following:
destinationList should be filled with Names of corresponding entries (by Id) in sourceList
destinationList should not contain entries that are not present in both lists at once (eg. Id: 4,5 should be eliminated) - something like inner join
I would like to avoid creating new destinationList with updated entries because both lists already exist and are very large,
so no "convert" or "select new".
In the end destinationList should contain:
1 "1111"
2 "2222"
3 "3333"
Is there some kind of elegant (one line Lambda? ;) solution to this using LINQ ?
Any help will be greatly appreciated! Thanks!

I would just build up a dictionary and use that:
Dictionary<int, string> map = sourceList.ToDictionary(x => x.Id, x => x.Name);
foreach (var item in destinationList)
if (map.ContainsKey(item.Id))
item.Name = map[item.Id];
destinationList.RemoveAll(x=> x.Name == null);

Hope this will your desired result. First join two list based on key(Id) and then set property value from sourceList.
var result = destinationList.Join(sourceList, d => d.Id, s => s.Id, (d, s) =>
{
d.Name = s.Name;
return d;
}).ToList();

Barring the last requirement of "avoid creating new destinationList" this should work
var newList = destinationList.Join(sourceList, d => d.Id, s => s.Id, (d, s) => s);
To take care of "avoid creating new destinationList", below can be used, which is not any different than looping thru whole list, except that it probably is less verbose.
destinationList.ForEach(d => {
var si = sourceList
.Where(s => s.Id == d.Id)
.FirstOrDefault();
d.Name = si != null ? si.Name : "";
});
destinationList.RemoveAll(d => string.IsNullOrEmpty(d.Name));

Frankly, this is the simplest:
var dictionary = sourceList.ToDictionary(x => x.Id, x => x.Name);
foreach(var item in desitnationList) {
if(dictionary.ContainsKey(item.Id)) {
item.Name = dictionary[item.Id];
}
}
destinationList = destinationList.Where(x => x.Name != null).ToList();
You could do something ugly with Join but I wouldn't bother.

I hope this will be useful for you. At the end, destinationList has the correct data, without creating any new list of any kind.
destinationList.ForEach(x =>
{
SourceType newSource = sourceList.Find(s=>s.Id == x.Id);
if (newSource == null)
{
destinationList.Remove(destinationList.Find(d => d.Id == x.Id));
}
else
{
x.Name = newSource.Name;
}
});