Put the below code in to separate class.

public DataTable Select() {
            SqlConnection conn = new SqlConnection(myconnstrng);
            DataTable dt = new DataTable();
            try {
                string sql = "SELECT * FROM tablename";
                SqlCommand cmd = new SqlCommand(sql, conn);
                SqlDataAdapter adapter = new SqlDataAdapter(cmd);
                conn.Open();
                adapter.Fill(dt);
            }
            catch (Exception ex){
            }
            finally{
                conn.Close();
            }
            return dt;
        }

Below code for form code section
private void frmDeparmentChoose_Load(object sender, EventArgs e) {
            DataTable dt = c.Select();
            comboBoxDepartmentChoose.DisplayMember = "department_name";
            comboBoxDepartmentChoose.DataSource = dt;
        }

IDE Used To Test This Code : Visual Studio.

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