How to group by custom types with linq

Question

I have this class

public class Item
{
       public Coordinate coordinate { get; set; }
        ...
        ...
}

With Coordinate being define like this:

public class Coordinate
{
        public Coordinate(float latitude, float longitude)
        {
            Latitude = latitude;
            Longitude = longitude;
        }

        public float Latitude { get; private set; }
        public float Longitude { get; private set; }
}

And I want to have a linq query like that :

var grouped = from it in items
              group it by it.Coordinate into grp
              select grp;

As mentioned here by MSDN I thought this was possible if I would override Equals on my Coordinate class :

Use a named type if you must pass the query variable to another method. Create a special class using auto-implemented properties for the keys, and then override the Equals and GetHashCode methods. You can also use a struct, in which case you do not strictly have to override those methods. For more information see How to: Implement an Immutable Class That Has Auto-Implemented Properties

Equals implementation for Coordinate class :

public override bool Equals(object obj)
{
       var coord = obj as Coordinate;
       if(coord == null) return false;
       return (Latitude == coord.Latitude && Longitude == coord.Longitude);
}

still I cant get my linq query to group by similar coordinates, as my failing test shows :

[TestMethod]
public void GroupBy_3ItemsWith2DifferentCoordinates_Returns2Groups()
{
    var items = new List<Item>
        {
            new Item {Coordinate = new Coordinate(10, 10)},
            new Item {Coordinate = new Coordinate(10, 10)},
            new Item {Coordinate = new Coordinate(12, 10)},
        };
    var grouped = from it in items
                  group it by it.Coordinate into g
                  select g;
    Assert.AreEqual(2, grouped.Count());
}

There is an overload to the GrouBy method that takes an IEqualityComparer as a parameter, but is there the equivalent using the group clause? Am I doing something wrong?? Any thoughts?

Answer 1

You've shown the Equals implementation, but not GetHashCode. You need to override both (and in a consistent way) for grouping to work.

Sample GetHashCode implementation:

public override int GetHashCode()
{
    int hash = 23;
    hash = hash * 31 + Latitude.GetHashCode();
    hash = hash * 31 + Longitude.GetHashCode();
    return hash;
}

Note that comparing float values for exact equality is always somewhat risky - but I'd at least expect your unit tests to pass, given that they're not performing any calculations.