Equals, GetHashCode, EqualityComparers and fuzzy equality

Question

For an object with properties A, B, C, D, StartDate and EndDate if I wanted to implement something where any two objects are equal if they have identical A, B and C and over

Answer 1

If you are using the items in a hashtable then their GetHashCode method will be used initially to test for equality and if their hashes are found to be equal a call to their Equals method will be made to establish whether they are equal.

From the documentation:

"But is that enough for the Hashtable to determine they are identical objects? Unfortunately, no. If the Hashtable finds two objects with the same hash, it calls their Equals method to see whether the two objects are in fact equal. Again, the default implementation of Object.Equals will return false if the two objects are two different instances of the same class. So we need to also add an override of the Equals method to our class.

Answer 2

It is possible. The only rule for GetHashCode is that A.GetHashCode() must equal B.GetHashCode() if A == B. The opposite, if A == B A.GetHashCode() == B.GetHashCode() does not have to be true.

So you can simply make GetHashCode like so

public override int GetHashCode()
{
    return A.GetHashCode() ^ B.GetHashCode() ^ C.GetHashCode();
}

GetHashCode is not for identity!! It is used for grouping 'similar' objects.

Proof:

string a = "a";
string b = "EUKCnPMLpp";
Console.WriteLine("a = '{0}', b = '{1}', Same = {2}", a, b, a == b);
Console.WriteLine("a.GetHashCode() = {0}, b.GetHashCode() = {1}, Same = {2}", a.GetHashCode(), b.GetHashCode(), a.GetHashCode() == b.GetHashCode());

Answer 3

This is not possible.

Equality in .Net must be transitive, meaning that if a == b and b == c, then a == c.