Default value for combobox in C#.net

public void FillCmbo(String Sql, ComboBox cmb, String name, String id)
       {
           try
           {
               if (Con.State == ConnectionState.Open)
                   Con.Close();
               Con.Open();

               OleDbDataAdapter Ada = new OleDbDataAdapter(Sql, Con);
               DataTable Dte = new DataTable();
               Ada.Fill(Dte);
               DataRow Drw;
               Drw = Dte.NewRow();
               Drw.ItemArray = new object[] { 0, "<----Select---->" };
               Dte.Rows.InsertAt(Drw, 0);
               cmb.DisplayMember =name;
               cmb.ValueMember = id;
               cmb.DataSource = Dte;
           }
           catch (System.Exception ex)
           {

               MessageBox.Show(ex.Message, "Check", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }
       }

/................you can also use
ComboBox.selectIndex=0;

Please refer these links:
1) http://social.msdn.microsoft.com/forums/en-US/winforms/thread/1bbfc2b0-f099-4572-be70-e70a27ec2eed/[^]
2) http://stackoverflow.com/questions/7309164/setting-combobox-default-value-using-selectedindex[^]

In the Designer (definition for InitializeComponent() in your form), just put

this.comboBox1.SelectedIndex = 0; // or whatever default value

before

this.comboBox1.SelectedIndexChanged += new System.EventHandler(this.comboBox1_SelectedIndexChanged);

just to ensure you don't accidentally trigger the index changed event.

If we are binding the database from table to combobox means it can't set a default value.But its not the right thing to do because it won't be secure.Writing Query is the best option for retriving datas from table.The below code is to get the datas from table to combobox,
OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=d:\\xxx.acdb;Persist Security Info=False;");
con.Open();
OleDbCommand cmd = new OleDbCommand("SELECT Distinct aaa FROM bbb", con);
dr = cmd.ExecuteReader();
comboBox4.Items.Clear();
while (dr.Read())
{
comboBox4.Items.Add(dr[0].ToString());
}
dr.Close();
con.Close()