How to implement GetHashCode() in a C# struct

Question

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();

`System.Int32.GetHashCode()` performs a simple `return this` (at least in mscorlib v4), so your two code snippets would be equivalent anyway. — Frédéric Hamidi, Sep 10 '15 at 12:43
This question is not specific to `struct`. The core of the problem is combining multiple hash codes into one, which is explained in the top answer to the "What is the best algorithm for an overridden System.Object.GetHashCode?" question. — Sergey Kalinichenko, Sep 10 '15 at 12:49

score 3 · Accepted Answer · edited Sep 10 '15 at 12:55

3

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.

edited Sep 10 '15 at 12:55

stakx - no longer contributing

83,039
20
168
268

answered Sep 10 '15 at 12:45

Blindy

65,249
10
91
131

4

Don't forget to wrap the whole expression in an `unchecked(…)` block, otherwise you might get an exception due to possible arithmetic overflows. – stakx - no longer contributing Sep 10 '15 at 12:46
1

See also [this answer by Jon Skeet](http://stackoverflow.com/a/263416/240733) to the question, ["What is the best algorithm for an overridden System.Object.GetHashCode?"](http://stackoverflow.com/q/263400/240733). – stakx - no longer contributing Sep 10 '15 at 12:47
Didn't know he commented on the subject, yeah `unchecked` makes sense. – Blindy Sep 10 '15 at 12:50
2

I hope you don't mind the code edit. You don't need a `unchecked { … }` block. You can use the variant for just one in-line expression (`unchecked (…)`, i.e. with parentheses). – stakx - no longer contributing Sep 10 '15 at 12:55

How to implement GetHashCode() in a C# struct

1 Answers1

Linked