Remove duplicates using except alternative

Question

I try to obtain a except-wise list using Linq, that is, I want to subtract list of elements from another list if exists. But the following doesn't remove duplicates.

var simplifiedList = request.CountryExceptionRecords
                .Where(e => !request.CopyInitialCountryExceptionRecords
                        .Any(c =>
                                    c.PropString1 == e.PropString1
                                    && c.PropString2.Trim()  == e.PropString2.Trim()
                                    && c.PropString3 == e.PropString3));

Edit, adding .Distinct() to the final(after Where) works like a charm. But I don't understand why it works because the classes used by the Records(Lists) implement neither GetHash nor Equal method.

@KingOfArrows yes, I get whole list (CountryExceptionRecords) including the duplicate one which must be removed. — Hax, Apr 13 '20 at 06:49
You say it doesn't remove duplicates. Does that mean that the code above is doing what you want (subtracting one list of elements from another based on those 3 properties) but it isn't deduplicating the resultant list? — ProgrammingLlama, Apr 13 '20 at 06:49
Do you actually have any duplicates that match the Any LINQ in list 1 compared to list 2? Could we see the contents of the two lists being compared please. — KingOfArrows, Apr 13 '20 at 07:00
@Hax, could you add some sample data to make question more understandable? — Selim Yildiz, Apr 13 '20 at 07:09
@LongChalk how does it work? Come on! Even it doesn't have equals and hashcode methods. Please, could you explain? It will be nice if you can write it as the answer to be accepted. — Hax, Apr 13 '20 at 07:44

Selim Yildiz · Accepted Answer · 2020-04-13T08:52:24.500

Adding .Distinct() to the final(after Where) works like a charm. But I don't understand why it works because the classes used by the Records(Lists) implement neither GetHash nor Equal method.

It's because from Distinct not working with LINQ to Objects

The Distinct() method checks reference equality for reference types. This means it is looking for literally the same object duplicated, not different objects which contain the same values.

So let me show you in example:

foo testA = new foo() { PropString1 = "A", PropString2 = "A", PropString3 = "A" };
foo testB = new foo() { PropString1 = "B", PropString2 = "B", PropString3 = "B" };
foo testC = new foo() { PropString1 = "C", PropString2 = "C", PropString3 = "C" };

List<foo> CountryExceptionRecords = new List<foo>();
CountryExceptionRecords.Add(testA);
CountryExceptionRecords.Add(testB);
CountryExceptionRecords.Add(testC);
CountryExceptionRecords.Add(testC); //adding duplicate item

List<foo> CopyInitialCountryExceptionRecords = new List<foo>();
CopyInitialCountryExceptionRecords.Add(testA); //same object adding second list

var simplifiedList = CountryExceptionRecords
                        .Where(e => !CopyInitialCountryExceptionRecords
                                        .Any(c =>
                                            c.PropString1 == e.PropString1
                                            && c.PropString2.Trim() == e.PropString2.Trim()
                                            && c.PropString3 == e.PropString3)).Distinct().ToList();

In this case the simplifiedList count would be 2 since lists have the objects with same reference.

But on the other hand following example does not remove duplicates it's because they are not same object reference wheares the object has same values. In this case you need to use IEqualityComparer

List<foo> CountryExceptionRecords = new List<foo>();
CountryExceptionRecords.Add(new foo() { PropString1 = "A", PropString2 = "A", PropString3 = "A" });
CountryExceptionRecords.Add(new foo() { PropString1 = "B", PropString2 = "B", PropString3 = "B" });
CountryExceptionRecords.Add(new foo() { PropString1 = "C", PropString2 = "C", PropString3 = "C" });
CountryExceptionRecords.Add(new foo() { PropString1 = "C", PropString2 = "C", PropString3 = "C" });

List<foo> CopyInitialCountryExceptionRecords = new List<foo>();
//This is another object wheares the values are same
CopyInitialCountryExceptionRecords.Add(new foo() { PropString1 = "A", PropString2 = "A", PropString3 = "A" });

var simplifiedList = CountryExceptionRecords
                        .Where(e => !CopyInitialCountryExceptionRecords
                                        .Any(c =>
                                            c.PropString1 == e.PropString1
                                            && c.PropString2.Trim() == e.PropString2.Trim()
                                            && c.PropString3 == e.PropString3)).Distinct().ToList();

@SelimYıldız You should go on to say provided that the type being used doesn't implement `IEquatable.Equals` method, it uses `Object.Equals` thereby checking the reference equality. Sadece bit tavsiye. — Soner from The Ottoman Empire, Apr 14 '20 at 21:51

Remove duplicates using except alternative

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