Code:
void AutoCompleteText() {
            string myconnstrng = ConfigurationManager.ConnectionStrings["connstrng"].ConnectionString;
            SqlConnection conn = new SqlConnection(myconnstrng);
            string sql = "SELECT product_code FROM ProductMaster";
                SqlCommand cmd = new SqlCommand(sql, conn);
                conn.Open();
                SqlDataReader reader = cmd.ExecuteReader();
                AutoCompleteStringCollection Collection = new AutoCompleteStringCollection();
                while (reader.Read()) {
                    Collection.Add(reader["product_code"].ToString());
                }
                txtInvoiceProductCode.AutoCompleteMode=AutoCompleteMode.SuggestAppend;
                txtInvoiceProductCode.AutoCompleteSource = AutoCompleteSource.CustomSource;
                txtInvoiceProductCode.AutoCompleteCustomSource = Collection;
                conn.Close();
        }

After that call AutoCompleteText() function within Load function in a form.

IDE Used To Test This Code : Microsoft Visual Studio.

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