`GetHashCode`

Hashing is often done in VB.NET programs—we often use strings as keys to a Dictionary. Other types (any class) can also be hashed with GetHashCode.

With the Overrides keyword, we can implement our own GetHashCode on a class. This can be useful if the class has a unique field we want to base the hash on.

Example

Consider this example VB.NET program—it builds up a string by appending the value "x." This creates a new string each time.

And The result of GetHashCode is different each time—this means the integer returned is unique among these strings.

Module Module1
    Sub Main()
        Dim value As String = ""
        ' Create some strings and print their hash codes.
        For i = 0 To 10
            value += "x"
            ' Print value of GetHashCode.
            Console.WriteLine("GETHASHCODE: {0}", value.GetHashCode())
        Next
    End Sub
End ModuleGETHASHCODE: 588487099
GETHASHCODE: 2037659917
GETHASHCODE: -2047743330
GETHASHCODE: 1240005642
GETHASHCODE: -1113563208
GETHASHCODE: 494194757
GETHASHCODE: -763422986
GETHASHCODE: -1835225738
GETHASHCODE: -876106756
GETHASHCODE: -1704566245
GETHASHCODE: 992105103

Overrides

How can we specify our own GetHashCode function? This is usually not needed, and is often not a good idea, but it can be done.

Part 1 We create a BlogEntry class, and print its custom GetHashCode result, which is 100.

Part 2 We create a BlogEntryDefault class, which has no custom GetHashCode, so the default hash code algorithm for objects is used.

Class BlogEntry
    Public Overrides Function GetHashCode() As Integer
        Return 100
    End Function
End Class

Class BlogEntryDefault
End Class

Module Module1
    Sub Main()
        ' Part 1: use class with overrides GetHashCode function.
        Dim blog = New BlogEntry()
        Console.WriteLine(blog.GetHashCode())

        ' Part 2: use class without a custom GetHashCode.
        Dim blogDefault = New BlogEntryDefault()
        Console.WriteLine(blogDefault.GetHashCode())
    End Sub
End Module100
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It is possible to add a custom GetHashCode Function on VB.NET classes with the Overrides keyword. This can help in certain cases when objects are used as keys to Dictionaries.