Equivalent of control parameters in oracle when feeding from a sqldatasource

[darkgen]

So I've been mucking about with MS sql server and building up a simple web application with a number of queries, however I have now been asked to generate some reports from a oracle dB and I've run in to a dead end when trying to use control parameters.

As an example within asp;

<asp:SqlDataSource ID="SqlDataSource3" runat="server" 
        ConnectionString="<%$ ConnectionStrings:foo %>" 
        SelectCommand="SELECT a , b , c , d FROM footable WHERE (d = @controld OR @controld = '-1' OR @controld IS NULL)">
    <SelectParameters>
        <asp:ControlParameter ControlID="DropDownList2" Name="controld" 
                PropertyName="SelectedValue" Type="String" />
        </SelectParameters>
</asp:SqlDataSource>

So the above query returns rows where d equals controld in a drop down list. The above approach works fine when running it against MS sql server but oracle fails with "illegal variable name/number". Is there an equivalent means/syntax to accomplish the above when interacting with oracle?

Cheers, Paul.