C# linq groupby returns incorrect groups

Question

I am trying to understand how linq works. i wrote a test app and it is not working the way i expect it to. from the following code, i am expecting to see items "test1" and "test4" grouped together, but i am not getting that. instead I am getting back 4 separate groups. meaning one of the items are getting grouped together. can someone explain what i am doing wrong? Thanks.

public class linqtest
{   public int x1;
    public int x2;
    public string x3;

    public linqtest(int a, int b, string c)
    {
        x1 = a;
        x2 = b;
        x3 = c;

    }

    public bool Equals(linqtest other)
    {

        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;

        return x1 == other.x1 &&
                x2 == other.x2;

    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != typeof(linqtest)) return false;
        return Equals((linqtest)obj);
    }
}
linqtest tc14 = new linqtest(1, 4, "test1");
inqtest tc15 = new linqtest(3, 5, "test2");
linqtest tc16 = new linqtest(3, 6, "test3");
linqtest tc16a = new linqtest(1, 4, "test4");

List<linqtest> tclistitems = new List<linqtest>();
tclistitems.Add(tc14);
tclistitems.Add(tc15);
tclistitems.Add(tc16);
tclistitems.Add(tc16a);

IEnumerable<IGrouping<linqtest, linqtest>> tcgroup = tclistitems.GroupBy(c => c);

why does tcgroup contain 4 groups? i was expecting 3 groups.

Answer 1

The error happens because you override Equals without overriding GetHashCode. These two must be overriden together, otherwise GroupBy will not work.

Add this code to your class to fix the issue:

public override int GetHashCode()
{
    // You are ignoring x3 for equality, so hash code must ignore it too
    return 31*x1+x2;
}