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Hashtable Code Examples

Listing 4 is some code for filling a Hashtable with one order and its order lines. In this case, the method receives a DataReader and uses that for filling the Hashtable.

Listing 4: Code for Filling a Hashtable

    Dim aDataReader As SqlDataReader = _

    Dim anOrder As New Hashtable()
    anOrder.Add(OrderColumns.Id, _
    anOrder.Add(OrderColumns.CustomerId, _
    anOrder.Add(OrderColumns.OrderDate, _

    Dim i As Integer
    While aDataReader.Read
      anOrder.Add(OrderLineColumns.ProductId * _
 Hack.Factor + i, aDataReader.GetInt32 _
      anOrder.Add(OrderLineColumns.PriceForEach * _
 Hack.Factor + i, aDataReader.GetSqlMoney _
      anOrder.Add(OrderLineColumns.NoOfItems * _
 Hack.Factor + i, aDataReader.GetInt32 _
      anOrder.Add(OrderLineColumns.Comment * _
 Hack.Factor + i, aDataReader.GetString _
      i += 1
    End While

    Return anOrder

As I said earlier in this article, I need a little hack for storing both the main information about an order and the order lines in the same Hashtable. To solve that, I added a factor (Hack.Factor) to the keys for the order lines. Of course, that is a problematic choice of what that value should be. In the code in Listing 4, I chose 1, 000,000. (You can't see that in the code; that value is in a public enumeration.) If I chose to store several orders in the same Hashtable, that would mean that I couldn't have more than 1,000,000 orders. You probably don't want to have that many orders anyway in memory, but again, this is a hack.


My friend Joe Cleland asked why I didn't use a separate Hashtable for each order's details, added to the first Hashtable. That's a good question and one that made me blush. I guess I could have said that I wanted to have just one Hashtable instead of two for each order, but I can't lie to my readers, can I?

Joe's suggestion is, of course, a much better one than my fast hack. I will add it to my tests for Part 5 in this series.

Finally, Listing 5 gives the code for when the client is inspecting the order information and navigating through all the order lines.

Listing 5: Code for Browsing the Hashtable

    Dim anOrder As Hashtable = _

    _id = DirectCast _
  (anOrder.Item(OrderColumns.Id), Integer)
    _customerId = DirectCast _
    (anOrder.Item(OrderColumns.CustomerId), Integer)
    _orderDate = DirectCast _
  (anOrder.Item(OrderColumns.OrderDate), Date)

    Dim i As Integer
    While anOrder.ContainsKey _
    (OrderLineColumns.ProductId * Hack.Factor + i)
      _productId = DirectCast(anOrder.Item _
 (OrderLineColumns.ProductId * _
 Hack.Factor + i), Integer)
      _priceForEach = DirectCast(anOrder.Item _
 (OrderLineColumns.PriceForEach * _
 Hack.Factor + i), Decimal)
      _noOfItems = DirectCast(anOrder.Item _
 (OrderLineColumns.NoOfItems * _
 Hack.Factor + i), Integer)
      _comment = DirectCast(anOrder.Item _
 (OrderLineColumns.Comment * _
 Hack.Factor + i), String)
      i += 1
    End While

The code in Listing 5 is much messier than the "same" code for the wrapped DataSet. This is the price you pay when you choose to go with something generic instead of something specific. The way I see it, in the generic case, you pay with more client-side code (and also with quite a lot of server-side code). In my opinion, having much client-side code is a very high price to pay. On the other hand, you will soon see that the Hashtable has good performance. Trade-offs, trade-offs....

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