c# - linq & distinct, implementing the equals & gethashcode

Question

so I'm trying to make this work and I can't seem to know why it doesn't work

demo code;

namespace ConsoleApplication1
{
class Program
{
    static void Main(string[] args)
    {
        var myVar = new List<parent >();
        myVar.Add(new parent() { id = "id1", blah1 = "blah1", c1 = new child() { blah2 = "blah2", blah3 = "blah3" } });
        myVar.Add(new parent() { id = "id1", blah1 = "blah1", c1 = new child() { blah2 = "blah2", blah3 = "blah3" } });

        var test = myVar.Distinct();

        Console.ReadKey();

    }
}


public class parent : IEquatable<parent>
{
    public String id { get;set;}
    public String blah1 { get; set; }
    public child c1 { get; set; }

    public override int GetHashCode()
    {
        unchecked // Overflow is fine, just wrap
        {
            int hash = 17;
            // Suitable nullity checks etc, of course :)
            hash = hash * 23 + id.GetHashCode();
            hash = hash * 23 + blah1.GetHashCode();
            hash = hash * 23 + (c1 == null ? 0 : c1.GetHashCode());
            return hash;
        }
    }

    public bool Equals(parent other)
    {
        return object.Equals(id, other.id) &&
            object.Equals(blah1, other.blah1) &&
            object.Equals(c1, other.c1);
    }

}

public class child : IEquatable<child>
{
    public String blah2 { get; set; }
    public String blah3 { get; set; }

    public override int GetHashCode()
    {
        unchecked // Overflow is fine, just wrap
        {
            int hash = 17;
            // Suitable nullity checks etc, of course :)
            hash = hash * 23 + blah2.GetHashCode();
            hash = hash * 23 + blah3.GetHashCode();
            return hash;
        }
    }

    public bool Equals(child other)
    {
        return object.Equals(blah2, other.blah2) &&
            object.Equals(blah3, other.blah3);
    }

}
}

anyone could spot my error(s) ?

Answer 1

You need to override the Equals(object) method:

public override bool Equals(object obj) {
    return Equals(obj as parent);
}

The object.Equals method (unlike EqualityComparer<T>.Default) does not use the IEquatable interface. Therefore, when you write object.Equals(c1, other.c1), it doesn't call your Child.Equals(Child) method.

You don't absolutely need to do that for parent as well, but you really should.