You need to add a Row to your datatable like this
try
{
DataTable dt = fillcombo();
dt.Rows.Add(new object[] {"All"});
comboBox1.DataSource = dt.DefaultView;
comboBox1.DataSource = dt;
comboBox1.ValueMember = "Work_Location";
}
catch (Exception ee)
{
MessageBox.Show(ee.Message);
}
By the way you should also Close your Database connection in your FillCombo method, as you Open it but it never gets closed. Your best bet would be to use a Using statement like this
using (OleDbConnection con = new OleDbConnection("yourConnectionString"))
{
string sqlstr = "SELECT Work_Location FROM Employees";
con.Open();
OleDbDataAdapter da = new OleDbDataAdapter();
da.SelectCommand = new OleDbCommand(sqlstr, con);
da.SelectCommand.ExecuteNonQuery();
DataTable ds = new DataTable();
da.Fill(ds);
return ds;
}
Hope this helps