Suppose you bound your combobox to a List<Person>
List<Person> pp = new List<Person>();
pp.Add(new Person() {id = 1, name="Steve"});
pp.Add(new Person() {id = 2, name="Mark"});
pp.Add(new Person() {id = 3, name="Charles"});
cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;
At this point you cannot set the Text property as you like, but instead you need to add an item to your list before setting the datasource
pp.Insert(0, new Person() {id=-1, name="--SELECT--"});
cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;
cbo1.SelectedIndex = 0;
Of course this means that you need to add a checking code when you try to use the info from the combobox
if(cbo1.SelectedValue != null && Convert.ToInt32(cbo1.SelectedValue) == -1)
MessageBox.Show("Please select a person name");
else
......
The code is the same if you use a DataTable instead of a list. You need to add a fake row at the first position of the Rows collection of the datatable and set the initial index of the combobox to make things clear. The only thing you need to look at are the name of the datatable columns and which columns should contain a non null value before adding the row to the collection
In a table with three columns like ID, FirstName, LastName with ID,FirstName and LastName required you need to
DataRow row = datatable.NewRow();
row["ID"] = -1;
row["FirstName"] = "--Select--";
row["LastName"] = "FakeAddress";
dataTable.Rows.InsertAt(row, 0);