Linq - Remove one anonymous list from another

Question

I've seen many answers to this - but can't seem to get them to work:

 var numinqtoday = bc.SSRecs.Where(x => x.DateTime == id).Select(n => new
       {
           refer = n.@ref
       }).GroupBy(x => x.refer).ToList();

 var numinqprev = bc.SSRecs.Where(x => x.DateTime < id).Select(n => new
       {
           refer = n.@ref
       }).GroupBy(x => x.refer).ToList();


 var filtered = numinqtoday.Except(numinqprev).ToList();

In my code above:

 numinqtoday.Count() = 184
 numinqprev.Count() = 155

 filtered.Count() = 184

I have checked and there are definitely occurrences within numinqprev within numinqtoday - so it appears the .Except isn't removing them before putting in filtered.

Could anyone advise please where I'm going wrong?

Thank you,

Mark

Answer 1

You've got a list of groups there - and I suspect you'll find that one group is never equal to another. I suspect you want to perform the Except operation before the grouping. For example:

var numinqtoday = bc.SSRecs.Where(x => x.DateTime == id).Select(n => new
   {
       refer = n.@ref
   }).ToList();
var numinqprev = bc.SSRecs.Where(x => x.DateTime < id).Select(n => new
   {
       refer = n.@ref
   }).ToList();
var filtered = numinqtoday.Except(numinqprev).GroupBy(x => x.refer).ToList();

Another option would be to create a set of keys, and use Where :

// Code as per your original, including the GroupBy calls. Change the string
// part as per the type of refer, which we don't know...
var keys = new HashSet<string>(numinqprev.Select(g => g.Key));
var filtered = numinqtoday.Where(g => !keys.Contains(g.Key)).ToList();

Also note that if you've only got one property, there's not much point in using an anonymous type.