Linq - Using GroupBy with my own type vs anonymous type

Question

I have a list of objects I want to group with Linq. The objects type is GroupRating. I want to group them by their "Params" property.

public class GroupRating
{
    public long Id { get; set; }
    public Parameters Params { get; set; }
}

public class Parameters
{
    public int CarrierId { get; set; }
    public int CustomerId { get; set; }
}

The thing is that this works: (ie. I get only one group with all the ids)

        var myList = new List<GroupRating>();
        ... blahblah code...

        var groupedList = myList.GroupBy(i => new {
             CarrierId = i.Params.CarrierId,
             CustomerId = i.Params.CustomerId
         }, i => i.Id).ToArray();

But this doesn't work: (ie. I get as many groups as there is Ids)

        var myList = new List<GroupRating>();
        ... blahblah code...

        var groupedList = myList.GroupBy(i => new Params {
             CarrierId = i.Params.CarrierId,
             CustomerId = i.Params.CustomerId
         }, i => i.Id).ToArray();

Any idea why?

Thanks

Answer 1

Your class must override Equals(object) and GetHashCode() correctly. Otherwise two new Params { ... } will not be "equal" even if they look the same.

Anonymous types override these two methods automatically.

You could also use a struct instead of a class because a struct uses the overrides of Equals(object) and GetHashCode() that exist in System.ValueType . If you choose a struct , consider making the type immutable , ie make the properties read-only (or with private set; ).

Answer 2

You must use class or anonymous type.Params is not class.You can use it.

Try this:

  var groupedList = myList.GroupBy(i => new Parameters {
                 CarrierId = i.CarrierId,
                 CustomerId = i.CustomerId
             }, i => i.Id).ToArray();