Question

I have the following code:

var foo = (from data in pivotedData.AsEnumerable()
                   select new
                   {
                     Group = data.Field<string>("Group_Number"),
                     Study = data.Field<string>("Study_Name")
                   }).Distinct();

As expected this returns distinct values. However, what I want is to return a strongly-typed collection as opposed to an anonymous type, so when I do:

var foo = (from data in pivotedData.AsEnumerable()
                   select new BarObject
                   {
                     Group = data.Field<string>("Group_Number"),
                     Study = data.Field<string>("Study_Name")
                   }).Distinct();

This does not return the distinct values, it returns them all. Is there a way to do this with actual objects?

Answer 1

For Distinct() (and many other LINQ features) to work, the class being compared (BarObject in your example) must implement implement Equals() and GetHashCode(), or alternatively provide a separate IEqualityComparer<T> as an argument to Distinct().

Many LINQ methods take advantage of GetHashCode() for performance because internally they will use things like a Set<T> to hold the unique items, which uses hashing for O(1) lookups. Also, GetHashCode() can quickly tell you if two objects may be equivalent and which ones are definitely not – as long as GetHashCode() is properly implemented of course.

So you should make all your classes you intend to compare in LINQ implement Equals() and GetHashCode() for completeness, or create a separate IEqualityComparer<T> implementation.