LINQ: Distinct values

Question

I have the following item set from an XML:

id           category

5            1
5            3
5            4
5            3
5            3

<

Answer 1

Are you trying to be distinct by more than one field? If so, just use an anonymous type and the Distinct operator and it should be okay:

var query = doc.Elements("whatever")
               .Select(element => new {
                             id = (int) element.Attribute("id"),
                             category = (int) element.Attribute("cat") })
               .Distinct();

If you're trying to get a distinct set of values of a "larger" type, but only looking at some subset of properties for the distinctness aspect, you probably want DistinctBy as implemented in MoreLINQ in DistinctBy.cs:

 public static IEnumerable DistinctBy(
     this IEnumerable source,
     Func keySelector,
     IEqualityComparer comparer)
 {
     HashSet knownKeys = new HashSet(comparer);
     foreach (TSource element in source)
     {
         if (knownKeys.Add(keySelector(element)))
         {
             yield return element;
         }
     }
 }

(If you pass in null as the comparer, it will use the default comparer for the key type.)