This example demonstrates how to bind programatically RadComboBox in load on demand scenario.
        Also when user types it in the input field a callback is made to the server in order to request all customer names that match the typed text. 
    
    
protected void RadComboBox2_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e)
		{
            using (NorthwindReadOnlyDataContext northwindEntities = new NorthwindReadOnlyDataContext())
            {
               
                var allCustomers = northwindEntities.Customers.
                    OrderBy(i => i.ContactName);
                if (!String.IsNullOrEmpty(e.Text))
                {
                    allCustomers = northwindEntities.Customers.Where(i =>
                      i.ContactName.Contains(e.Text.Trim()))
                      .OrderBy(i => i.ContactName);
                }
				var customers = allCustomers.Skip(e.NumberOfItems).Take(10);
				RadComboBox2.DataSource = customers;
				RadComboBox2.DataBind();
				int endOffset = e.NumberOfItems + customers.Count();
				int totalCount = allCustomers.Count();
				if (endOffset == totalCount)
					e.EndOfItems = true;
				e.Message = String.Format("Items 1-{0} out of {1}",
					endOffset, totalCount);
			}
		}