Remove duplicates from list of object

Question

I have MyObject with field: id, a, b, c, e, f and I have List with 500 000 items, now how can I remove all duplicate items with of the same value of the parameter a, c, f? <

Answer 1

Well you can always use LINQ Distinct() like this :

var matches = list.Distinct(new Comparer()).ToList();

But for Ditsinct() to work you need to impletemnt Comparer for your Class:

class Comparer : IEqualityComparer<MyObject>
{
    public bool Equals(MyObject x, MyObject y)
    {
        return x.a == y.a && x.c == y.c && x.f == y.f;
    }

    public int GetHashCode(MyObject obj)
    {
        return (obj.a + obj.c + obj.f).GetHashCode();
    }
}

Answer 2

If what you are looking for is speed, and don't mind using up some memory then I would recommend that you use a HashSet, if you are interested in doing some custom comparison, then you can make an IEqualityComparer<T>, something like this:

var original = new ArrayList(); // whatever your original collection is 
var unique = new HasSet<YourClass>(new MyCustomEqualityComparer());

foreach(var item in original)
{
    if(!unique.Contains(item))
        unique.Add(item);
}

return unique;

the issue here is that you may end up gobbling up twice the original memory.

Update:

I made some extra research and I think you can achieve just what you want by simply doing:

var original // your original data
var unique = new HashSet<YourClass>(origin, new CustomEqualityComparer());

that should take care of removing duplicated data as no duplication is allowed in a HashSet. I'd recommend that you also take a look at this question about GetHasCode implementation guidelines.

If you want to know some more about the HashSet class follow these links:

About HashSet
About IEqualityComparer constructor
IEqualityComparer documentation

hope this helps

Answer 3

One efficient method would be first to to a quicksort (or similar n Log n sort), based on a hash of (a, c, f) and then you can iterate through the resultant list, picking one every time the value of (a, c, f) changes.

That would give a n log n speed solution, which is probably the best you can do.

Answer 4

Drakko! You can use the Distinct() method to get only the values that has different values for the properties you specify.
You could do something like this:

List<MyObj> list = new List<MyObj>();

//Run the code that is going to populate your list.
var result = list.Select(myObj => new { myObj.a, myObj.c, myObj.f})
                 .Distinct().ToList();

//Result contains the data based on the difference.

Answer 5

Code from this link worked great for me. https://nishantrana.me/2014/08/14/remove-duplicate-objects-in-list-in-c/

public class MyClass
{
public string ID { get; set; }
public string Value { get; set; }

}

List<MyClass> myList = new List<MyClass>();
var xrmOptionSet = new MyClass();
xrmOptionSet.ID = "1";
xrmOptionSet.Value = "100";
var xrmOptionSet1 = new MyClass();
xrmOptionSet1.ID = "2";
xrmOptionSet1.Value = "200";
var xrmOptionSet2 = new MyClass();
xrmOptionSet2.ID = "1";
xrmOptionSet2.Value = "100";
myList.Add(xrmOptionSet);
myList.Add(xrmOptionSet1);
myList.Add(xrmOptionSet2);

// here we are first grouping the result by label and then picking the first item from each group
var myDistinctList = myList.GroupBy(i => i.ID)
.Select(g => g.First()).ToList();