Question

Basically, GethashCode is different even though they contain the SAME values for the properties… so why is the default to return diff hashcodes?

public class User
{
    public Int32 Id { get; set; }
    public String Username { get; set; }
}

User a = new User();
a.Id = 1;
a.Username = "Hello";

User b = new User();
b.Id = 1;
b.Username = "Hello";

Console.WriteLine("Hash A: {0} | Hash B: {1}", a.GetHashCode(), b.GetHashCode());
//Hash A: 37121646 | Hash B: 45592480 <-- these values change each time I rerun the app?

Is there a more proper way to make it so I don’t break how Object.Equals works for my objects, but am still able to have my own equality checking based on the parameter values?

The reason I ask is because I have a service: SynchronizeUsers() which downloads an array of users. Instead of clearing out my cache of users, I’d rather just update the ones that need to be updated, remove the ones that the synch says to, and add the new ones. But, I can’t just do Object.Equals() on these objects.

Answer 1

Have you tried implementing your own IEqualityComparer? You can pass this to an .Equals() overload to define your own custom equality logic, as in

User A = User B even if they are distinct instances, if properties x, y, z are the same.

See this:
MSDN

Edit: I should have written you can instantiate your EqualityComparer and pass two instances to its Equals() method and get a bool. Basic console app… will show true, false, false. Thing is trivial, has the two properties shown.

var comparer = new ThingEqualityComparer();

Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, new Thing() { Id = 1, Name = "1" }));
Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, new Thing() { Id = 2, Name = "2" }));
Console.WriteLine(comparer.Equals(new Thing() { Id = 1, Name = "1" }, null));


class ThingEqualityComparer : IEqualityComparer<Thing>
{
    public bool Equals(Thing x, Thing y)
    {
        if (x == null || y == null)
            return false;

        return (x.Id == y.Id && x.Name == y.Name);
    }

    public int GetHashCode(Thing obj)
    {
        return obj.GetHashCode();
    }
}