Dropdown List appears empty when using #parameter - asp.net

Question

I am trying to make a dropdownlist work for me. A user can be in many 'Field1's, so I wanted the dropdown to show those Field1s.
Currently what happens is I just get a blank list. Here is a sample of the code:
<asp:SqlDataSource ID="SqlDataSource2" runat="server"
ConnectionString="<%$ ConnectionStrings:MyConnectionString %>"
SelectCommand="SELECT DISTINCT Field1 FROM table
WHERE (Field3= #Field3)">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList1" Name="Field3" Type="String" />
</SelectParameters>
</asp:SqlDataSource>
If I remove the select parameters and the WHERE clause the list of all DHBs appears - it just seems to have issues with #Field3.
Edit: I have tested the query in SQL Server and it works for me (by replacing #Field3 with a known value)
Any suggestions? I am very new to asp.net, and have next to nothing in the .cs file (besides some authorisation and such), so if people could point me in the right direction that would be fantastic.
Edit 2: It would probably help if I gave you the whole template:
<asp:TemplateField HeaderText="Field1" SortExpression="Field2">
<EditItemTemplate>
<asp:Label ID="Label2" runat="server" Text='<%# Eval("Field2") %>'></asp:Label>
</EditItemTemplate>
<ItemTemplate>
<asp:DropDownList ID="DropDownList1" runat="server"
DataSourceID="SqlDataSource2" DataTextField="Field1" DataValueField="Field1">
</asp:DropDownList>
<asp:SqlDataSource ID="SqlDataSource2" runat="server"
ConnectionString="<%$ ConnectionStrings:MyConnectionString %>"
SelectCommand="Field1 FROM table
WHERE (Field3 = #Field3)">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList1" Name="Field3" Type="String" />
</SelectParameters>
</asp:SqlDataSource>
</ItemTemplate>
</asp:TemplateField>

Answer 1

You're populating another control using Dropdownlist selectedValue as a parameter, that is dictated by your code. In this case
You're missing property in the controlParameter
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList1" Property="SelectedValue" Name="Field3" Type="String" />
</SelectParameters>
If you're trying to populate DropDownList1 you're completely derailed. In this case you need to show up more markup code and details.
Update: You cannot take the parameters value from the DropDownList control which you're populating. Either remove the parameter or use another control to provide the value