GetHashCode on null fields?

Question

How do I deal with null fields in GetHashCode function?

Module Module1
  Sub Main()
    Dim c As New Contact
         


        

Answer 1

As Jeff Yates suggested, the override in the answer would give the same hash for (name = null, address = "foo") as (name = "foo", address = null). These need to be different. As suggested in link, something similar to the following would be better.

public override int GetHashCode()
{
    unchecked // Overflow is fine, just wrap
    {
        int hash = 17;
        hash = hash * 23 + (Name == null ? 0 : Name.GetHashCode());
        hash = hash * 23 + (Address == null ? 0 : Address.GetHashCode());
    }
    return hash;
}

What is the best algorithm for an overridden System.Object.GetHashCode?

Answer 2

Typically, you check for null and use 0 for that "part" of the hash code if the field is null:

return (Name == null ? 0 : Name.GetHashCode()) ^ 
  (Address == null ? 0 : Address.GetHashCode());

(pardon the C#-ism, not sure of the null check equivalent in VB)