Rewritting C# GetHashCode in Visual Basic with a few Questions

Question

User686339638 posted

Hello,

I found the following code online, but i don't know the Visual Basic equivalent.  I found some pieces but i'm not sure i understand it.

// Note: Not quite FNV!
public override int GetHashCode()
{
    unchecked // Overflow is fine, just wrap
    {
        int hash = (int) 2166136261;
        // Suitable nullity checks etc, of course :)
        hash = hash * 16777619 ^ field1.GetHashCode();
        hash = hash * 16777619 ^ field2.GetHashCode();
        hash = hash * 16777619 ^ field3.GetHashCode();
        return hash;
    }
}

Found here: http://stackoverflow.com/questions/263400/what-is-the-best-algorithm-for-an-overridden-system-object-gethashcode

I reformatted this code in VB below to correct for the lack of a unchecked method 

Public Overrides Function GetHashCode() As Integer
 
   Dim hash As Long = 17
   hash = hash * 16777619 ^ If(IsNothing(field1), 1, field1.GetHashCode())
   hash = hash * 16777619 ^ If(IsNothing(field2), 1, field2.GetHashCode())
   hash = hash * 16777619 ^ If(IsNothing(field3), 1, field3.GetHashCode())
 
   Return CInt(hash And &H7FFFFFFFL)
End Function

I got part of this from: http://stackoverflow.com/questions/4654227/overriding-gethashcode-in-vb-without-checked-unchecked-keyword-support

Few Questions:

1) Is this written correctly?

2) What does And &H7FFFFFFFL do? I don't understand this.

3) Why choose 2166136261 over 17 for the initial Long value? It seems that the longer number will reach Arithmetic Overflow a lot quicker than using smaller numbers such as 17. Is there an advantage?

Thanks very much! 

Answer 1

User397347636 posted

The '^' symbol in C# is the exclusive or operator - the equivalent in VB is 'XOr'.

"And &H7FFFFFFL" is a bitwise 'And' operation.

Answer 2

User614698185 posted

Hi JacobPressures,

hash = hash * 16777619 ^ field1.GetHashCode();

As David Anton said, The '^' symbol in C# is the exclusive or operator - the equivalent in VB is 'XOr':

hash = hash * 16777619 Xor field1.GetHashCode()

2) What does And &H7FFFFFFFL do? I don't understand this.

If you calculate hash as Long, but then return it as Integer with Return CInt(hash And &H7FFFFFFFL). This way could avoid the overflow.

Best Regards,

Candice Zhou

Answer 3

User686339638 posted

Thanks very much guys. One last question:

3) Why choose 2166136261 over 17 for the initial Long value? It seems that the longer number will reach Arithmetic Overflow a lot quicker than using smaller numbers such as 17. Is there an advantage?

Candice, you said in answer to #2: "If you calculate hash as Long, but then return it as Integer with Return CInt(hash And &H7FFFFFFFL). This way could avoid the overflow."

So does that mean that the size of the integers being multiplied doesn't matter? Should i still avoid the overflow perhaps because of degradation of precision or quality?

Thanks a lot guys!

Answer 4

User686339638 posted

This did not work. It resulted in an overflow.  I'm not sure that code is working right.

Answer 5

User614698185 posted

Hi JacobPressures,

If you want to use GetHashCode method, please refer to the below link:

https://documentation.devexpress.com/#CodeRush/CustomDocument17203

Best Regards,

Candice Zhou

Answer 6

User686339638 posted

Thanks for the link Candice!  Ok the following 2 code samples were overflowing on the Long int.

Public Overrides Function GetHashCode() As Integer
            
            Dim hash As Long = 2166136261 


            hash = hash * 16777619 Xor If(IsNothing(PayerID), 1, PayerID.GetHashCode())
            hash = hash * 16777619 Xor If(IsNothing(InsuredPersonID), 1, InsuredPersonID.GetHashCode())
            hash = hash * 16777619 Xor If(IsNothing(CaseID), 1, CaseID.GetHashCode())
            hash = hash * 16777619 Xor If(IsNothing(PersonPayerID), 1, PersonPayerID.GetHashCode())
            hash = hash * 16777619 Xor If(IsNothing(AppointmentID), 1, AppointmentID.GetHashCode())
            hash = hash * 16777619 Xor If(IsNothing(EncounterID), 1, EncounterID.GetHashCode())

            Return CInt(hash And &H7FFFFFFFL)  
End Function

Public Overrides Function GetHashCode() As Integer

            Dim hash As Long = 2166136261 

            hash = hash * 16777619 + If(IsNothing(PayerID), 1, PayerID.GetHashCode())
            hash = hash * 16777619 + If(IsNothing(InsuredPersonID), 1, InsuredPersonID.GetHashCode())
            hash = hash * 16777619 + If(IsNothing(CaseID), 1, CaseID.GetHashCode())
            hash = hash * 16777619 + If(IsNothing(PersonPayerID), 1, PersonPayerID.GetHashCode())
            hash = hash * 16777619 + If(IsNothing(AppointmentID), 1, AppointmentID.GetHashCode())
            hash = hash * 16777619 + If(IsNothing(EncounterID), 1, EncounterID.GetHashCode())

            Return CInt(hash And &H7FFFFFFFL)  
End Function

I purposely used these numbers to test if they overflowed.

While i did not understand everything discussed in that link, I was able to get this much out, which does not overflow:

Public Overrides Function GetHashCode() As Integer

            Dim hash As Long = 2166136261 

            hash = CInt((hash * 16777619 Xor If(IsNothing(PayerID), 1, PayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 Xor If(IsNothing(InsuredPersonID), 1, InsuredPersonID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 Xor If(IsNothing(CaseID), 1, CaseID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 Xor If(IsNothing(PersonPayerID), 1, PersonPayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 Xor If(IsNothing(AppointmentID), 1, AppointmentID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 Xor If(IsNothing(EncounterID), 1, EncounterID.GetHashCode())) And &H7FFFFFFFL)

            Return CInt(hash And &H7FFFFFFFL)  
End Function

Public Overrides Function GetHashCode() As Integer
            
            Dim hash As Long = 2166136261 

            hash = CInt((hash * 16777619 + If(IsNothing(PayerID), 1, PayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 + If(IsNothing(InsuredPersonID), 1, InsuredPersonID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 + If(IsNothing(CaseID), 1, CaseID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 + If(IsNothing(PersonPayerID), 1, PersonPayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 + If(IsNothing(AppointmentID), 1, AppointmentID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 16777619 + If(IsNothing(EncounterID), 1, EncounterID.GetHashCode())) And &H7FFFFFFFL)

            Return CInt(hash And &H7FFFFFFFL)  
End Function

Since the two above did not overflow, I assume the smaller numbers should certainly  be safe.

So here is the code I ended up using. 

Public Overrides Function GetHashCode() As Integer
            
            Dim hash As Long = 17

            hash = CInt((hash * 23 + If(IsNothing(PayerID), 1, PayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 23 + If(IsNothing(InsuredPersonID), 1, InsuredPersonID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 23 + If(IsNothing(CaseID), 1, CaseID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 23 + If(IsNothing(PersonPayerID), 1, PersonPayerID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 23 + If(IsNothing(AppointmentID), 1, AppointmentID.GetHashCode())) And &H7FFFFFFFL)
            hash = CInt((hash * 23 + If(IsNothing(EncounterID), 1, EncounterID.GetHashCode())) And &H7FFFFFFFL)

            Return CInt(hash And &H7FFFFFFFL)
End Function