DropdownList selected item not maintained after postback using C# in ASP.Net

Dear Programmer,

I have an issue related to my Select field, When i select my Dropdown text field that comes on Pageload is auto redirect to Select option pLease help me out.

code.aspx.cs 

            //Get Department Country
            {
                //connectionstring
                cnn.Open();
                SqlCommand cmd = new SqlCommand("Select * from Department", cnn);
                SqlDataAdapter sda = new SqlDataAdapter(cmd);
                DataSet ds = new DataSet();
                sda.Fill(ds);
                cnn.Close();
                ddl_ctry.DataSource = ds.Tables[0].DefaultView;
                ddl_ctry.DataTextField = "Name";
                ddl_ctry.DataValueField = "Value";
                ddl_ctry.DataBind();
            }
            //Default Field
            ddl_ctry.Items.Insert(0, new ListItem("--Select--", "0"));