Generating Hash Code for Object

Question

I have a custom object (DataPointCollection) with two Integer properties and a Guid property. I want that object to generate a HashCode so that no two objects with the same values in those properties are added to a HashSet. I know I need to override the GetHashCode() method, but how do I do generate a hash code to accomplish this?

Here's how I want to use it.

Dim dataPointCollections As New HashSet(Of DataPointCollection)() For Each row As DataRow In myDataSet.Tables(0).Rows Dim dataPointCollection As New DataPointCollection() dataPointCollection.ProjectID = row("ProjectID") 'Integer' dataPointCollection.RoleID = row("RoleID") 'Integer' dataPointCollection.ResourceGUID = row("ResourceGUID") 'Guid' If Not dataPointCollections.Contains(dataPointCollection) Then dataPointCollections.Add(dataPointCollection) End If Next

I'm open to other ideas, but I thought this might be faster than doing a LINQ query on a collection of objects (there could be a very large number of these objects).

Jon Skeet · Accepted Answer · 2010-11-16 15:39:19Z

You need to override both GetHashCode and Equals - it's the combination of the two which will be used by the HashSet.

Your equality check should:

Check that the other object is of the same type as this object (it's simpler if DataPointCollection is a sealed type; equality and inheritance are messy when combined)
Compare the three fields for equality

Your hash code check needs to combine the three fields; the result doesn't have to be unique; for performance it's better if two unequal objects have a different hash code, but that's not required. The absolute requirement is that two equal objects do have the same hash code. I'd do something like this (C#, but easy to convert to VB):

public override int GetHashCode() { int hash = 17; hash = hash * 31 + projectId; hash = hash * 31 + roleId; hash = hash * 31 + resourceGuid.GetHashCode(); return hash; }

Community · Accepted Answer · 2017-05-23 12:29:03Z

If I understand you correctly, you just need to override the GetHashCode and Equals methods in your DataPointCollection class, and use the method in this question to generate the hash code.

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