Get distinct list values

Question

i have a C# application in which i\'d like to get from a List of Project objects , another List which contains distinct objects.

i trie

Answer 1

Check this example: you need to use either Comparator or override Equals()

class Program
{
    static void Main( string[] args )
    {
        List<Item> items = new List<Item>();
        items.Add( new Item( "A" ) );
        items.Add( new Item( "A" ) );
        items.Add( new Item( "B" ) );
        items.Add( new Item( "C" ) );

        items = items.Distinct().ToList();
    }
}

public class Item
{
    string Name { get; set; }
    public Item( string name )
    {
        Name = name;
    }

    public override bool Equals( object obj )
    {
        return Name.Equals((obj as Item).Name);
    }
    public override int GetHashCode()
    {
        return Name.GetHashCode();
    }
}

Answer 2

You need to define "identical" here. I'm guessing you mean "have the same contents", but that is not the default definition for classes: the default definition is "are the same instance".

If you want "identical" to mean "have the same contents", you have two options:

• write a custom comparer (IEqualityComparer<Project>) and supply that as a parameter to Distinct
• override Equals and GetHashCode on Project

There are also custom methods like DistinctBy that are available lots of places, which is useful if identity can be determined by a single property (Id, typically) - not in the BCL, though. But for example:

if (model != null) model = model.DistinctBy(x => x.Id).ToList();

With, for example:

public static IEnumerable<TItem>
    DistinctBy<TItem, TValue>(this IEnumerable<TItem> items,
    Func<TItem, TValue> selector)
{
    var uniques = new HashSet<TValue>();
    foreach(var item in items)
    {
        if(uniques.Add(selector(item))) yield return item;
    }
}

Answer 3

Here's an answer from basically the same question that will help.

Explanation:

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.

Credits to @Rex M.

Answer 4

The object's reference aren't equal. If you want to be able to do that on the entire object itself and not just a property, you have to implement the IEqualityComparer or IEquatable<T>.

Answer 5

var newList = 
(
from x in model
select new {Id_user= x.Id_user}
).Distinct();

or you can write like this

var list1 = model.DistinctBy(x=> x.Id_user);

Answer 6

How do you define identical? You should override Equals in Project with this definition (if you override Equals also override GetHashCode). For example:

public class Project
{
    public int ProjectID { get; set; }

    public override bool Equals(object obj)
    {
        var p2 = obj as Project;
        if (p2 == null) return false;
        return this.ProjectID == m2.ProjectID;
    }

    public override int GetHashCode()
    {
        return ProjectID;
    }
}

Otherwise you are just checking reference equality.