public static void FillService<T>(T control, IEnumerable dataSource, string dataTextField,
string dataValueField) where T : ListControl

{
FillService<T>(control, dataSource, dataTextField, dataValueField, String.Empty);
}

public static void FillService<T>(T control, IEnumerable dataSource, string dataTextField,
string dataValueField,string defaultText) where T : ListControl
{
control.DataSource = dataSource;
control.DataTextField = dataTextField;
control.DataValueField = dataValueField;

if (!String.IsNullOrEmpty(defaultText))
{
control.Items.Insert(0, new ListItem(defaultText, "0"));
}

control.DataBind();
}

And here is the usage:

IVRRepository repository = new VRRepository();
SiteHelper.FillService<DropDownList>(ddlCourses, repository.GetCoursesByDepartmentId(
Int32.Parse(ddlDepartments.SelectedValue)), "Name", "CourseID","Select a course");

IVRRepository repository = new VRRepository();
SiteHelper.FillService<ListBox>(lbSections, repository.GetSectionsByCourseId(
ddlCourses.SelectedValue.ToInt()), "SectionNo", "SectionID","Select a section");