c# - 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 tried this

 List<Project> model = notre_admin.Get_List_Project_By_Expert(u.Id_user);
 if (model != null) model = model.Distinct().ToList();

The list model still contains 4 identical objects Project.

What is the reason of this? How can i fix it?

Answer 1

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;
    }
}