How does Distinct() work?

Question

Lets say i have this:

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

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

Now, when i do this:

list = list.Distinct().Tolist();

It obviously returns 4 elements. I would like a method, that compares all the data i have in class, and returns unique elements, something that checks every property of the class. Do i need to write my own comparer, or is there something that is built-in that works this way?

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