double hashtable or double hashing hashtable in vb.net

Question

im trying to creat a double hashing hashtable in vb.net and i am getting a few error that i don't know how to solve. hopefuly you guys can help me out. the problems im having have are anywhere i have dbnull.value or mod= i get errors in the editor and i dont' know how to fix them. put this code in vb to see what i mean. here is the code:

Public Class DoubleHashing

Class DataItem
    Private data As Integer

    Public Sub New(ByVal i As Integer)
        data = i
    End Sub
    Public Function getKey() As Integer
        Return data
    End Function

End Class

Private hashArray() As DataItem
Private arraySize As Integer
Private bufItem As DataItem

Public Sub New(ByVal size As Integer)

    arraySize = size
    hashArray(arraySize) = New DataItem(arraySize)
    Dim bufItem(-1) As DataItem
End Sub

Public Function hashFunc1(ByVal key As Integer) As Integer
    Return key Mod arraySize
End Function

Public Function hashFunc2(ByVal key As Integer) As Integer
    Return 6 - key Mod 6
End Function


Public Sub insert(ByVal key As Integer, ByVal item As DataItem)
    Dim hashVal As Integer = hashFunc1(key) ' hash the key
    Dim stepSize As Integer = hashFunc2(key) ' get step size

    ' until empty cell or -1
    While hashArray(hashVal) <> DBNull.Value & hashArray(hashVal).getKey() <> -1
        hashVal += stepSize ' add the step
        hashVal mod= arraySize ' for wraparound
    End While
    hashArray(hashVal) = item ' insert item

End Sub

Public Function delete(ByVal key As Integer) As DataItem
    Dim hashVal As Integer = hashFunc1(key)
    Dim stepSize As Integer = hashFunc2(key) ' get step size

    While hashArray(hashVal) <> DBNull.Value
        If hashArray(hashVal).getKey() = key Then
            Dim temp As DataItem = hashArray(hashVal) ' save item
            hashArray(hashVal) = bufItem '  delete item
            Return temp '  return item
        End If

        hashVal += stepSize ' add the step
        hashVal mod= arraySize '  for wraparound
    End While

    Return DBNull.Value
End Function

Public Function find(ByVal key As Integer) As DataItem
    Dim hashVal As Integer = hashFunc1(key)
    Dim stepSize As Integer = hashFunc2(key)

    While hashArray(hashVal) <> DBNull.Value
        If hashArray(hashVal).getKey() = key Then
            Return hashArray(hashVal)
        End If

        hashVal += stepSize
        hashVal mod= arraySize
    End While

    Return DBNull.Value
End Function

End Class

Answer 1

Normally the second hash function must never return 0.

I don't think DBNull.Value inherits from DataItem. Also, your array is initialized to an array of some size containing references to Nothing.