Question

I have a list that i want to group based on properties on an object. But it seems when i output the grouped list it groups on every property in the object, even if i dont assign them any values. Since the default values should be the same for the unassigned values they should be grouped together, right?

I know i can solve this by using anonymous types ( new { x.SentDate.Year } ) and stuff, but the idea is to pass a GroupBy function as a parameter in PopulateMessages method for the sake of being more dynamic.

public void PopulateMessages()
    {
        List<Message> Messages = new DataRepository().GetMessages();

        Func<Message, GroupProperties> func = x => new GroupProperties
            {
                Year = x.SentDate.Year
            };

        IEnumerable<IGrouping<GroupProperties, Message>> GroupedMessages = Messages.OrderByDescending( x => x.SentDate )
            .GroupBy( func )
            .ToList();
    } 

    public class GroupProperties
    {
        public int Year { get; set; }
        public int Month { get; set; }
        public int Day { get; set; }
    }

Answer 1

I’d be surprised if you found your current code was grouping at all. Your current grouping key type is a class which doesn’t override Equals or GetHashCode, which means you’ll basically get reference equality. Every instance of GroupProperties is non-equal to every other instance, and as you create a new instance for each Message, there’ll be no grouping.

You need to override those methods appropriately (ideally implement IEquatable<GroupProperties>) and then grouping will work. You shouldn’t need to care about which properties have been set, so long as your key extraction function always sets a consistent set of them.