August 21, 2008 04:17pm pdt

Member Login

Hall of Fame

Yearly

1TheLearn... 114,130
2Dhaest 100,545
3jaime_oliv... 99,658
4apeter 97,595
5jpaulino 69,979
6angelIII 52,750
7emoreau 52,618
8jcoehoorn 46,325
9Fernando... 45,654
10naspinski 32,000
11elimesika 31,125
12graye 29,830
13GreenGho... 28,795
14PockyMas... 28,698
15TimCottee 28,359
16mlmcc 27,730
17gregoryyo... 23,000
18p_davis 22,240
19dweppen... 22,050
20Idle_Mind 22,015
21kaylanreilor 21,150
22PaulHews 21,040
23ZachSmith 21,010
24VBRocks 20,360
25chapmandew 20,175

	[x] Attachment Details

Overloading the == operator and the GetHashCode() method...

Zones: .Net Editors & IDEs, Microsoft Visual C#.Net, .NET Framework 2.0

Tags: int, method, unique

Hi,

I have a class called foo (below) that overloads the == operator. But I'm getting a warning on compile:

'foo' defines operator == or operator != but does not override Object.GetHashCode()

This makes sense since I could have two totally seperate instances in memory, but if they have the same Id, they would return true for ==, but would generate two different hash codes since the base implementation is based on their location in memory. The question is how can I reliably convert my unique Guid Id field into a reliable int hash code to use in my overriden GetHashCode() method?

         
             public class foo {
  public Guid Id;
  public static bool operator ==(foo f1, foo f2) {
    return f1.Id == f2.Id;
  }
}

Start your free trial to view this solution

[x]

The Solution Rating System

With so many solutions, how can you tell which solutions are most likely to help you and which ones are not? To provide you with a tool to use, we rate our solutions based on various elements that most accurately determine if a solution is a quality solution. To explain what factors affect the solution rating, here are the elements we take into consideration when formulating our solution rating.

The Grade of the Solution
The Zone Rank of the Expert Providing the Solution
The Number of Author and Expert Comments
The Number of Experts Contributing
The Feedback of the Community

Your Input Matters
Because of the way the system is set up, the most important variable in this equation is you. As a member of Experts Exchange, you are able to cast your vote on the quality of the solutions in regard to how complete, accurate, helpful and easy to understand each solution is. When you provide your feedback, each rating is adjusted accordingly. So, if you see a solution that has a poor rating that you think is a good solution, let us know by rating it. As you do, the rating will be adjusted and will become more accurate for other members of our site.

If you have any suggestions that you would like to make for our rating system, please ask a question in the Suggestions Zone of Community Support.

Thank you!

6.8

Question Stats

Zone: Programming

Question Asked By: kalliopi

Solution Provided By: GreenGhost

Participating Experts: 2

Solution Grade: A

Translate:

Loading Advertisement...

Open Discussion

Comment by GreenGhost

> I suppose that if you implement the value part of the hashcode as a bucket...

The implementations in .NET, like HashTable and Dictionary<> already do that.

This can be demonstrated by creating a dictionary that uses the same hash code for all key values. The dictionary still works, but all values are stored in the same bucket, so it performs as bad as a plain list:

public class Key {
      private int _code;
      public Key(int code) { _code = code; }
      public class EqualityComparer : IEqualityComparer<Key> {
            public bool Equals(Key x, Key y) { return x._code == y._code; }
            public int GetHashCode(Key obj) { return 1; }
      }
}

Dictionary<Key, string> list = new Dictionary<Key, string>(new Key.EqualityComparer());

list.Add(new Key(1), "Hello world");
list.Add(new Key(2), "Test");
list.Add(new Key(42), "Answer");

Console.WriteLine(list[new Key(1)]);
Console.WriteLine(list[new Key(2)]);
Console.WriteLine(list[new Key(42)]);

20080723-EE-VQP-34 / EE_QW_EXPERT_20070906