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

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());