Cascading combo box

I am trying to bind 2 combo box, being the value of the second dependent from de first.

All the solutions that i saw to this problem were by the use of Sql.Data (Datatable), which i cant use, due to the architecture of the application.

I can do it 2 ways, by c# or through ASP Object Data Source. I’ve trying this approach with any sucess. It is possible to do this?

if (!Page.IsPostBack)
{
 recursohumano rh = new RecursoHumano();
 rdpUnidade.DataValueField= "ID"
 rdpUnidade.DataTextField= "NomeUnidade"
 rdpUnidade.DataSource= new BLLUnidade().GetAll();
 rdpUnidade.DataBind();

 rdpInvestigador.DataValueField= "ID"
 rdpInvestigador.DataTextField= "Nome"
 rdpInvestigador.DataSource= new BLLRecursoHumano().GetAll();
 rdpInvestigador.DataBind();
 rdpInvestigador.Items.Insert(0, new RadComboBoxItem("", ""));

 //rdp investigador should depend on rdpUnidade

 private void rdpUnidade_SelectedIndexChanged(object sender, EventArgs e)
 {
 recursohumano rh = new RecursoHumano();

 var InvUnidade = from recursohumano in rh.recursohumano where recursohumano.id == Convert.ToInt32(rdpUnidade.SelectedValue) select recursohumano;
 rdpInvestigador.DataValueField= "ID";
 rdpInvestigador.DataTextField= "Nome";
 rdpInvestigador.DataSource = new BLLRecursoHumano().GetAll()
 }

Answer

The original approach was right, but i was missing a query to List the wanted values:

var res = RecHumano.GetAll().Where(x => x.IDUnidade == id).ToList()