GroupBy on complex object (e.g. List<T>)

Question

Using GroupBy() and Count() > 1 I'm trying to find duplicate instances of my class in a list.

The class looks like this:

public class SampleObject
{
    public string Id;
    public IEnumerable<string> Events;
}

And this is how I instantiate and group the list:

public class Program
{
    private static void Main(string[] args)
    {
        var items = new List<SampleObject>()
        {
            new SampleObject() { Id = "Id", Events = new List<string>() { "ExampleEvent" } },
            new SampleObject() { Id = "Id", Events = new List<string>() { "ExampleEvent" } }
        };

        var duplicates = items.GroupBy(x => new { Token = x.Id, x.Events })
                         .Where(g => g.Count() > 1)
                         .Select(g => g.Key)
                         .ToList();
    }
}

The duplicates contains no items. How can I make the grouping work?

Answer 1

To get objects to work with many of LINQ's operators, such as GroupBy or Distinct, you must either implement GetHashCode & Equals, or you must provide a custom comparer.

In your case, with a property as a list you probably need a comparer, unless you made the list read only.

Try this comparer:

public class SampleObjectComparer : IEqualityComparer<SampleObject>
{
    public bool Equals(SampleObject x, SampleObject y)
    {
        return x.Id == y.Id && x.Events.SequenceEqual(y.Events);
    }

    public int GetHashCode(SampleObject x)
    {
        return x.Id.GetHashCode() ^ x.Events.Aggregate(0, (a, y) => a ^ y.GetHashCode());
    }
}

Now this code works:

    var items = new List<SampleObject>()
    {
        new SampleObject() { Id = "Id", Events = new List<string>() { "ExampleEvent"} },
        new SampleObject() { Id = "Id", Events = new List<string>() { "ExampleEvent" } }
    };

    var comparer = new SampleObjectComparer();

    var duplicates = items.GroupBy(x => x, comparer)
                     .Where(g => g.Count() > 1)
                     .Select(g => g.Key)
                     .ToList();