How to implement GetHashCode() in a C# struct [dup

Question

This question already has an answer here:

• What is the best algorithm for an overridden System.Object.GetHashCode? 19 answers

I have a struct that overrides the Equals() method and the compiler complains about GetHashCode() not being overridden.

My struct:

  private struct Key
  {
    ...

    public override int GetHashCode()
    {
      return ?;
    }

    public int FolderID;
    public MyEnum SubItemKind;
    public int SubItemID;
  }

What is the right way to implement the GetHashCode() method?

a)

    return FolderID ^ SubItemKind.GetHashCode() ^ SubItemID;

or b)

    return FolderID.GetHashCode() ^ SubItemKind.GetHashCode() ^ SubItemID.GetHashCode();

Answer 1

Always the latter. The former isn't sufficient because most bits are 0 (your numbers are most likely small), and those zeroes are in the most significant bits. You'd be wasting a lot of the hash code, thus getting a lot more collisions.

Another common way of doing it is to multiply each item by a prime number and relying on overflows:

return unchecked(FolderID.GetHashCode() * 23 * 23 
                 + SubItemKind.GetHashCode() * 23 
                 + SubItemID.GetHashCode());

Edit: Updated to use unchecked for explicit overflow support as per stakx's comment.