Custom Class as key for Dictionary in C# not working

Question

I recently started studying C# but am stuck on this particular case. Yes I have gone over the previous questions but was not able to get what the issue in my code is. Custom Class used as key in Dictionary but key not found This is the closest one I could find, but the answer given here does not seem to work for me.

I have a Dictionary registry<Customers, string>. Customers is a class that has 2 strings: s1 and s2. A customer can match either s1 or s2 (both are not required).

Class Customers:

public class Customers
    {
        public string _s1;
        public string _s2;

        public Customers(string s1, string s2)
        {
            _s1 = s1;
            _s2 = s2;
        }

        public class EqualityComparer : IEqualityComparer<Customers>
        {
            public bool Equals(Customers x, Customers y)
            {
                Console.WriteLine("Inside Equals");
                return ((x._s1.Equals(y._s1)) || (x._s2.Equals(y._s2)));
            }
            public int GetHashCode(Customers x)
            {
                return x._s1.GetHashCode() ^ x._s2.GetHashCode();
            }
        }
    }

Program.cs:

Dictionary<Customers, string> registry = new Dictionary<Customers, string>(new Customers.EqualityComparer());

Customers key = new Customers("1", "2");
Customers key2 = new Customers("12", "21");
registry.Add(key, "12");
registry.Add(key2, "22");
Customers lookUp1 = new Customers("1", "2");
Customers lookUp2 = new Customers("1", "32");
if (registry.ContainsKey(lookUp2))
{
    Console.WriteLine("Found");
}
else
{
    Console.WriteLine("Not Found");
}

The issue is that when both s1 and s2 are matching, then I get "Found", but when only one of them does I get "Not found" despite having changed Equals() accordingly and giving the reference of the new EqualityComparer in the Constructor as well.

Also, For lookup1 I do get "Inside Equals" but not for lookup2 and I'm not sure why.

Dmitry Bychenko · Answer 1 · 2022-06-20T17:29:36.007

You have a problem with equality design. Each equality correspondence must follow these rules:

A.Equals(A) == true for all A.
if A.Equals(B) == true then B.Equals(A) == true for all A, B.
if A.Equals(B) && B.Equals(C) == true then A.Equals(C) == true for all A, B, C.

As we can see, the rule #3 (transitive rule) is broken:

   Customer A = new Customer("x", "y");
   Customer B = new Customer("x", "z");
   Customer C = new Customer("p", "z");

Since equality is implemented incorrectly, Dictionary<K, V> doesn't work properly.

Guru Stron · Accepted Answer · 2022-06-20T16:59:14.410

2

This happens because Dictionary is basically a hash table:

Retrieving a value by using its key is very fast, close to O(1), because the Dictionary<TKey,TValue> class is implemented as a hash table.

and first rule for matching is that hashcodes should be equal and "1".GetHashCode() ^ "2".GetHashCode() (for key) will definitely differ from "1".GetHashCode() ^ "32".GetHashCode()(for lookUp2).

edited Jun 20 '22 at 16:59

answered Jun 20 '22 at 16:54

Guru Stron

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Thank you! that does seem to be the issue. I changed the return x._s1.GetHashCode() ^ x._s2.GetHashCode(); to return 0 and it seems to work. But I read that this could result in a not so efficient code? – Sanju Jun 20 '22 at 16:55
@Sanju I can't think of implementation for `GetHashcode` supporting "or" logic from top of my head. I would suggest just using simple `Dictionary` with multiple keys referring to the same value. – Guru Stron Jun 20 '22 at 16:57
1

The problem is *deeper*: even when we put `public int GetHashCode(Customers x) => 0;` - awful but *correct* implementation the dictionary will not work properly. The basic equality design is incorrect. – Dmitry Bychenko Jun 20 '22 at 17:01
@DmitryBychenko yes, there are multiple problems with the code/approach. – Guru Stron Jun 20 '22 at 17:02

Custom Class as key for Dictionary in C# not working

2 Answers2