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The classic function in C# to create a hash value from an array looks like this or similar:
public int GetHashCode(T[] array)
            if (array == null)
                return 0;
            int hash = 17;
            foreach (T item in array)
                hash = hash * 31 + item.GetHashCode;
            return hash;
But since unchecked is not available in VB.Net I will quickly get an exception when it overflows.
I thought I could solve it using bitshifting, but in VB its only arithmetic and not circular so every bit that is shifted out is dropped.

And while I can't believe this has never been solved in VB my google foo is failing me.

What is the standard solution for getting a hash value for an array in VB?
Updated 19-May-14 4:49am

Couldn't you just declare hash as Int64 and clear the upper part when needed?
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Jörgen Andersson 19-May-14 12:27pm    
That would mean that the first items influence on the hash would be shifted out after just seven more items added.
This is the reason I wanted to use a circular bitshift, but that is also not available in VB.
CPallini 19-May-14 12:33pm    
However that's exactly what happens in the C# code, right?
Jörgen Andersson 19-May-14 12:37pm    
No, that's why you want the 'Unchecked' operator, it overflows without an exception.
CPallini 19-May-14 15:43pm    
Well, 'overflows without an exceptions' doesn't prevent it to discard the bits.
Jörgen Andersson 19-May-14 16:33pm    
Dammit, you're right.
That means I've tried to fix something that wasn't as broken as I thought. Either that, or the implementation is faulty.
For future reference, this is how my solution looks like:
Public Overrides Function GetHashCode() As Integer
     Dim hash As Int64 = 19
     For Each Item As T In Me
         hash = hash * 31 + Item.GetHashCode
         hash = ((hash << 32) >> 32) + hash >> 32
     hash = ((hash << 32) >> 32)
     Return CInt(hash)
 End Function
There is some room for improving performance, the overflow only needs to be handled every 6 iterations for example
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