How do I implement this Custom Equality?

Question

I have a Symbol entity that when compared to another has this behavior:

If FileName & FileDate are Equal, return True
If FileDate is different, then Compare the CRC32 of each and return that Value

I'm wondering how to implement this Equality especially the GetHashCode() in this case.

Is your question how to implement a GetHashCode() in particular, or which fields to include? The main thing with hash codes is that its okay for different things to have same hash code (though not optimal) but its very bad for equivalent things to have different hash codes. So in the minimum you want to pick the least common denominator of equivalence fields for a hash code. In your case, it looks like FileName. (I'm assuming on #2 when you say "If FileDate is different" you infer that the FileName is still the same, right? — James Michael Hare
– James Michael Hare, Commented Jul 15, 2011 at 14:41
What have you written so far in your override of Equals()? — Joel B Fant
– Joel B Fant, Commented Jul 15, 2011 at 14:47
Ok I was concerned about the GetHashCode(). This clears it up. — Sunit
– Sunit, Commented Jul 15, 2011 at 14:58

James Michael Hare · Accepted Answer · 2011-07-15 14:53:39Z

I'd say (based on my understanding of your example), something like this. You could include a more complex hash code that parallels your FileDate vs CRC32, but really since the common glue is always the FileName you could just use that as your surrogate hash code.

Remember, Equal() objects should never have different hash codes, but !Equal() objects may have the same one (it's just a potential collision).

Also you'll want to be careful about the fields that are part of the hash code being mutable, otherwise the hash code of the object can "change" which could be very bad in a Dictionary...

    public sealed class Symbol : IEquatable<Symbol>
    {
        public string FileName { get; set; }
        public DateTime FileDate { get; set; }
        public long CRC32 { get; set; }

        public bool Equals(Symbol other)
        {
            if (other == null)
            {
                return false;
            }

            return FileName == other.FileName &&
                   (FileDate == other.FileDate || CRC32 == other.CRC32);
        }

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

        public override int GetHashCode()
        {
            // since FileName must be equal (others may or may not)
            // can use its hash code as your surrogate hash code.
            return FileName.GetHashCode();
        }
    }

Security Hound · Accepted Answer · 2011-07-15 14:52:07Z

0

     public override bool Equals(object obj)     
    {         
    var file = obj as Symbol;
    if ( file.FileName == FileName && file.FileDate == FileDate )
        return true
    else    
return Boolean Value of [Compare CRC Here];      
      }

Here is how you can calculate the CRC on a file.

http://damieng.com/blog/2006/08/08/calculating_crc32_in_c_and_net

This is basically what James Michael Hare suggested I was just slower.

answered Jul 15, 2011 at 14:52

Security Hound

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