SqlDataSource parameter from codebehind

i am using sql datasource and a checkboxlist to filter a datalistview. i want to make a parameter for sqldatasource from code behind and use SQL IN to filter datalistview rows. when i check 1 checkbox in checkboxlist everything work great... but when i check more than 1 checkbox datalist view takes no data. this is my onselecting code for sql datasource:

protected void DS_subgroupID_changed_Selecting(object sender,SqlDataSourceSelectingEventArgs e)
{
    List<String> selected = new List<String>();
    foreach (ListItem item in CheckBoxList_subgroup.Items)
        if (item.Selected)
        {
            selected.Add(item.Value);
        }

    String commaselect = String.Join(",", selected.ToArray());
    e.Command.Parameters["@product_subgroup"].Value = commaselect;


}

and here is my sqldatasource control:

<asp:SqlDataSource ID="DS_subgroupID_changed" runat="server"
   ConnectionString="<%$ ConnectionStrings:nimrokhDBConnectionString %>" 
   SelectCommand="SELECT * FROM [tbl_product] WHERE (([product_group] = @product_group) AND ([product_subgroup] IN (@product_subgroup)))"
   OnSelecting="DS_subgroupID_changed_Selecting">
        <SelectParameters>
            <asp:ControlParameter ControlID="radio_list_group" Name="product_group" PropertyName="SelectedValue" Type="String" />
            <asp:Parameter Name="product_subgroup" />
        </SelectParameters>
        </asp:SqlDataSource>

i have 1 table subgroup that contains subgroupID as int and subgroupname, and i have a table product that contan subgroup to store subgroup ID but this is in nvarchar. i wonder if this cause the problem. because my checkboxlist value is set to subgroupID form subgroup table.

i appretiate any help. thank you guys.