I am working on a c# application which connects to MySql server. There are about 20 functions which will connect to database. This application will be deployed in 200 over machines. I am using the below code to connect to my database which is identical for all the functions. The problem is, i can some connections were not closed and still alive when deployed in 200 over machines.

Connection String :

<add key="Con_Admin" value="server=test-dbserver; database=test_admindb; uid=admin; password=1Password; Use Procedure Bodies=false;" />

Declaration of the connection string Globally in application [Global.cs] :

public static MySqlConnection myConn_Instructor = new MySqlConnection(ConfigurationSettings.AppSettings["Con_Admin"]);

Function to query database :

  public static DataSet CheckLogin_Instructor(string UserName, string Password)
        {
            DataSet dsValue = new DataSet();
            //MySqlConnection myConn = new MySqlConnection(ConfigurationSettings.AppSettings["Con_Admin"]);
            try
            {
                string Query = "SELECT accounts.str_nric AS Nric, accounts.str_password AS `Password`," +
                        " FROM accounts " +
                        " WHERE accounts.str_nric = '" + UserName + "' AND accounts.str_password = '" + Password + "\'";

                MySqlCommand cmd = new MySqlCommand(Query, Global.myConn_Instructor);
                MySqlDataAdapter da = new MySqlDataAdapter();
                if (Global.myConn_Instructor.State == ConnectionState.Closed)
                {
                    Global.myConn_Instructor.Open();
                }

                cmd.ExecuteScalar();
                da.SelectCommand = cmd;
                da.Fill(dsValue);
                Global.myConn_Instructor.Close();
            }
            catch (Exception ex)
            {
                Global.myConn_Instructor.Close();
                ExceptionHandler.writeToLogFile(System.Environment.NewLine + "Target  :  " + ex.TargetSite.ToString() + System.Environment.NewLine + "Message :  " + ex.Message.ToString() + System.Environment.NewLine + "Stack   :  " + ex.StackTrace.ToString());
            }

            return dsValue;
        }