How does Distinct() work?

Question

Lets say i have this:

class Foo
{
    public Guid id;
    public string description;
}

var list = new List();
list.Add(new Foo() { id = Guid.Empt         


        

Answer 1

You have to override Foo.Equals (and subsequently Foo.GetHashCode) to explicitly compare each field. Otherwise it will use the default implementation, Object.Equals (ReferenceEquals).

Or, you can explicitly pass an IEqualityComparer to the Distinct() method.

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Note though that using anonymous classes does return 3 elements. Depending on where you want to use Foo and how much compile-time type safety you need, you could do:

var list = new List<dynamic>();
list.Add(new { id = Guid.Empty, description = "empty" });
list.Add(new { id = Guid.Empty, description = "empty" });
list.Add(new { id = Guid.NewGuid(), description = "notempty" });
list.Add(new { id = Guid.NewGuid(), description = "notempty2" });

list = list.Distinct().ToList(); //3 elements selected

Answer 2

It compares each two items using EqualityComparer.Default until specified another implementation of IEqualityComparer