The most common way to connect to the database is through string connection, as follows:

public static string connstr = "server = localhost; uid = root; pwd = 123456; database = db_sms;";

 MySqlConnection conn = new MySqlConnection(connstr);
 MySqlCommand comm = new MySqlCommand(sqlstr, conn);

However, this method is not conducive to the later maintenance after the application is released.

All of them connect through another way -- by reading the configuration file App.config

(1) First write App.config:

<configuration>
  <connectionStrings>
    <add name ="connstr" connectionString ="server = localhost; uid = root; pwd = 123456; database = db_sms;"/>
  </connectionStrings>
</configuration>

Be sure to edit in the configuration node and write the string connecting to the database.

(2) Add reference System.Configuration

(3) Use the ConfigurationManger class:

 public static string connstr = ConfigurationManager.ConnectionStrings["connstr"].ToString();

Next is the normal operation (add, delete, modify and query)

matters needing attention:

(1) The following code must be written strictly without any errors. Even if one character is wrong (one more character, one less character, case), the configuration file reading will fail.

<configuration>
  <connectionStrings>
    <add name ="" connectionString =""/>
  </connectionStrings>
</configuration>

(2) The name of the database connection string (in this case, "connstr") must be consistent in App.config and in the code.

Attach the complete code of database operation class:

1. using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Text;
   using System.Threading.Tasks;
   using System.Data;
   using MySql.Data;
   using MySql.Data.MySqlClient;
   using System.Configuration;
   
   namespace DAL
   {
       public class SQLHelper
       {
           //public static string connstr = "server = localhost; uid = root; pwd = 123456; database = db_sms;";
   
           /*
            1.Add in the App.config configuration file under SMS:
            <connectionStrings>
               <add name ="connstr" connectionString ="server = localhost; uid = root; pwd = 123456; database = db_sms;"/>
            </connectionStrings>
   
            2.Add reference under DAL: System.Configuration
   
            3.Add namespace, use ConnectionStrings under ConfigurationManager class
            */
   
           // Database connection by reading configuration file
           public static string connstr = ConfigurationManager.ConnectionStrings["connstr"].ToString();
   
           /// <summary>
           ///Execute addition, deletion and modification
           /// </summary>
           /// <param name="sqlstr"></param>
           /// <returns></returns>
           private static int UpDate(string sqlstr)
           {
               MySqlConnection conn = new MySqlConnection(connstr);
               MySqlCommand comm = new MySqlCommand(sqlstr, conn);
               try
               {
                   conn.Open();
                   int result = comm.ExecuteNonQuery();
                   return result;
               }
               catch (System.Exception ex)
               {
                   // Write to system log
                   throw ex;
               }
               finally
               {
                   conn.Close();
               }
           }
   
           /// <summary>
           ///Queries: getting a single result
           /// </summary>
           /// <param name="sql"></param>
           /// <returns></returns>
           public static object GetSingleResult(string sqlstr)
           {
               MySqlConnection conn = new MySqlConnection(connstr);
               MySqlCommand comm = new MySqlCommand(sqlstr, conn);
               try
               {
                   conn.Open();
                   object result = comm.ExecuteScalar();
                   return result;
               }
               catch (System.Exception ex)
               {
                   // Write to system log
                   throw ex;
               }
               conn.Close();
           }
   
           /// <summary>
           ///Queries: getting result sets
           /// </summary>
           /// <param name="sqlstr"></param>
           /// <returns></returns>
           public static MySqlDataReader GetDataReader(string sqlstr)
           {
               MySqlConnection conn = new MySqlConnection(connstr);
               MySqlCommand comm = new MySqlCommand(sqlstr, conn);
               try
               {
                   conn.Open();
                   return comm.ExecuteReader(CommandBehavior.CloseConnection);
               }
               catch (System.Exception ex)
               {
                   // Write to system log
                   conn.Clone();
                   throw ex;
               }
               finally
               {
                   conn.Close();
               }
           }
       }
   }