Get values of many dropdownlists

Hi 
I would like to know how i can get the values of many dropdownlists and then executed them to bind the datalist depend on the values, what i am looking from c# code to check each dropdownlist if it has value then get the value of it, if it does not have value check others dropdownlist values and show the products depending on the dropdownlist values that users choose it 

<asp:DropDownList ID="DropDownList1" runat="server"> <asp:ListItem Selected="True">Gear:</asp:ListItem> <asp:ListItem Value="Auto">Auto</asp:ListItem> <asp:ListItem Value="Manual">Manual</asp:ListItem> </asp:DropDownList> <asp:DropDownList ID="DropDownList2" runat="server"> ...