Question

I have 2 lists that I need to consolidate. List 1 has only the dates, and List 2 may have the time element as well:

var List1 = new[] { 
  new ListType{ val = new DateTime(2012, 1, 1)}, 
  new ListType{ val = new DateTime(2012, 1, 2)} 
};

List2 = new[] { new ListType{ val = new DateTime(2012, 1, 1, 5, 0, 0)} };

FinalList = new[] { 
  new ListType{ val = new DateTime(2012, 1, 1, 5, 0, 0)}, 
  new ListType{ val = new DateTime(2012, 1, 2)} 
};

The way I’m going about this is:

foreach (var l in List1) {
  var match = List2.FirstOrDefault(q => q.val.Date == l.val);
  if (match == null) continue;
  l.val = match.val;
}

Is there a better way than iterating through List1, using FirstOrDefault and then reassigning the val? It works, so this is just more a curiosity if Linq has a more elegant way (i.e. I am missing something obvious).

Thanks

Answer 1

You can use Enumerable.Union with a custom IEqualityComparer<ListType>:

class ListType
{
    public DateTime val { get; set; }

    public class DateComparer : IEqualityComparer<ListType>
    {
        public bool Equals(ListType x, ListType y)
        {
            if (ReferenceEquals(x, y))
                return true;
            else if (x == null || y == null)
                return false;
            return x.val.Date == y.val.Date;
        }

        public int GetHashCode(ListType obj)
        {
            return obj.val.Date.GetHashCode();
        }
    }
}

and then …

var finalList = List2.Union(List1, new ListType.DateComparer());