Overriding GetHashCode in VB without checked/unchecked keyword support?

Question

So I\'m trying to figure out how to correctly override GetHashCode() in VB for a large number of custom objects. A bit of searching leads me to this wonderful

Answer 1

After researching that VB had not given us anything like unchecked and raging for a bit (c# dev now doing vb), I implemented a solution close to the one Hans Passant posted. I failed at it. Terrible performance. This was certainly due to my implementation and not the solution Hans posted. I could have gone back and more closely copied his solution.

However, I solved the problem with a different solution. A post complaining about lack of unchecked on the VB language feature requests page gave me the idea to use a hash algorithm already in the framework. In my problem, I had a String and Guid that I wanted to use for a dictionary key. I decided a Tupple(Of Guid, String) would be a fine internal data store.

Original Bad Version

Public Structure HypnoKey
  Public Sub New(name As String, areaId As Guid)
    _resourceKey = New Tuple(Of Guid, String)(resourceAreaId, key)
  End Sub

  Private ReadOnly _name As String
  Private ReadOnly _areaId As Guid

  Public ReadOnly Property Name As String
    Get
      Return _name 
    End Get
  End Property

  Public ReadOnly Property AreaId As Guid
    Get
      Return _areaId 
    End Get
  End Property

  Public Overrides Function GetHashCode() As Integer
    'OMFG SO BAD
    'TODO Fail less hard
  End Function

End Structure

Much Improved Version

Public Structure HypnoKey
  Public Sub New(name As String, areaId As Guid)
    _innerKey = New Tuple(Of Guid, String)(areaId , key)
  End Sub

  Private ReadOnly _innerKey As Tuple(Of Guid, String)

  Public ReadOnly Property Name As String
    Get
      Return _innerKey.Item2
    End Get
  End Property

  Public ReadOnly Property AreaId As Guid
    Get
      Return _innerKey.Item1
    End Get
  End Property

  Public Overrides Function GetHashCode() As Integer
    Return _innerKey.GetHashCode() 'wow! such fast (enuf)
  End Function

End Structure

So, while I expect there are far better solutions than this, I am pretty happy. My performance is good. Also, the nasty utility code is gone. Hopefully this is useful to some other poor dev forced to write VB who comes across this post.

Cheers

Answer 2

Improved answer Overriding GetHashCode in VB without checked/unchecked keyword support?

Public Overrides Function GetHashCode() as Integer
  Dim hashCode as Long = 0
  If myReplacePattern IsNot Nothing Then _
    hashCode = ((hashCode*397) Xor myField.GetHashCode()) And &HffffffffL
  If myPattern IsNot Nothing Then _
    hashCode = ((hashCode*397) Xor myOtherField.GetHashCode()) And &HffffffffL
  Return CInt(hashCode)
End Function

There is a trimming after each multiplication. And literal is defined explicitly as Long because the And operator with an Integer argument does not zeroize the upper bytes.

Answer 3

I've also found that RemoveIntegerChecks MsBuild property affects /removeintchecks VB compiler property that prevents compiler from emitting runtime checks:

  <PropertyGroup>
    <RemoveIntegerChecks>true</RemoveIntegerChecks>   
  </PropertyGroup>

Answer 4

Use Long to avoid the overflow:

Dim hash As Long = 17
'' etc..
Return CInt(hash And &H7fffffffL)

The And operator ensures no overflow exception is thrown. This however does lose one bit of "precision" in the computed hash code, the result is always positive. VB.NET has no built-in function to avoid it, but you can use a trick:

Imports System.Runtime.InteropServices

Module NoOverflows
    Public Function LongToInteger(ByVal value As Long) As Integer
        Dim cast As Caster
        cast.LongValue = value
        Return cast.IntValue
    End Function

    <StructLayout(LayoutKind.Explicit)> _
    Private Structure Caster
        <FieldOffset(0)> Public LongValue As Long
        <FieldOffset(0)> Public IntValue As Integer
    End Structure
End Module

Now you can write:

Dim hash As Long = 17
'' etc..
Return NoOverflows.LongToInteger(hash)

Answer 5

I had the same problem implementing Mr. Skeet's solution in vb.net. I ended up using the Mod operator to get there. Each Mod by Integer.MaxValue should return just the least significant component up to that point and will always be within Integer.MaxValue and Integer.MinValue -- which should have the same effect as unchecked. You probably don't have to mod as often as I do (it's only when there's a chance of getting bigger than a long (which would mean combining a LOT of hash codes) and then once at the end) but a variant of this works for me (and lets you play with using much bigger primes like some of the other hash functions without worrying).

Public Overrides Function GetHashCode() As Int32
    Dim hash as Int64 = 17
    hash = (hash * 23 + _Name.GetHashCode()) Mod Integer.MaxValue
    hash = (hash * 23 + _Value) Mod Integer.MaxValue
    hash = (hash * 23 + _Type.GetHashCode()) Mod Integer.MaxValue
    Return Convert.ToInt32(hash)
End Function

Answer 6

Here is an implementation combining Hans Passant's answer and Jon Skeet's answer.

It works even for millions of properties (i.e. no integer overflow exceptions) and is very fast (less than 20 ms to generate hash code for a class with 1,000,000 fields and barely measurable for a class with only 100 fields).

Here is the structure to handle the overflows:

<StructLayout(LayoutKind.Explicit)>
Private Structure HashCodeNoOverflow
    <FieldOffset(0)> Public Int64 As Int64
    <FieldOffset(0)> Public Int32 As Int32
End Structure

And a simple GetHashCode function:

Public Overrides Function GetHashCode() As Integer

    Dim hashCode As HashCodeNoOverflow

    hashCode.Int64 = 17

    hashCode.Int64 = CLng(hashCode.Int32) * 23 + Field1.GetHashCode
    hashCode.Int64 = CLng(hashCode.Int32) * 23 + Field2.GetHashCode
    hashCode.Int64 = CLng(hashCode.Int32) * 23 + Field3.GetHashCode

    Return hashCode.Int32

End Function

Or if your prefer:

Public Overrides Function GetHashCode() As Integer

    Dim hashCode = New HashCodeNoOverflow With {.Int32 = 17}

    For Each field In Fields
        hashCode.Int64 = CLng(hashCode.Int32) * 23 + field.GetHashCode
    Next

    Return hashCode.Int32

End Function