Close Connection / MySqlDataReader doesn't work

Question

I have a class and my MysqlConnection is in there:

public class DB
{
    private static MySqlConnection _Connection;
    public static MySqlConnection Connection
    {
        get
        {
            if(_Connection == null)
            {
                string cs = string.Format("SERVER={0}; DATABASE={1}; UID={2}; PWD={3};", SERVER_ADRESS, DATABASE, UID, PWD);
                _Connection = new MySqlConnection(cs);
            }

            if(_Connection.State == System.Data.ConnectionState.Closed)
                try
                {
                    MessageBox.Show("MySQL Connection ist geschlossen. Öffne Sie");
                    _Connection.Open();
                }
                catch(MySqlException ex)
                {
                    switch (ex.Number)
                    {
                        case 0:
                            MessageBox.Show("Verbindung zum Server konnte nicht hergestellt werden.");
                            break;
                        case 1045:
                            MessageBox.Show("Ungültiger Benutzername/Passwort.");
                            break;
                        default:
                            MessageBox.Show(ex.Message);
                            break;
                    }
                }
            return _Connection;
        }
    }
}

So i can use this connection in all the other classes with DB.Connection .

But now I get "DataReader is already open". But all my DataReader's are in usings.

We start at my login page:

using (loginreader = cmd.ExecuteReader())
            {
                if (loginreader.Read())
                {
                    DB.Connection.Close();
                    return true;
                }
                else
                {
                    DB.Connection.Close();
                    return false;
                }
               loginreader.Close();
            } 

I guess this doesn't work. But the first Error Message after log in i get on another class on line 83:

DataTable schema = null;

            using (var schemaCommand = new MySqlCommand("SELECT * FROM " + firmCustomerTablename, connection))
            {
                using (var reader = schemaCommand.ExecuteReader(CommandBehavior.SchemaOnly))
                {
                    schema = reader.GetSchemaTable();
                }
            }

which is in a using too. So I don't understand why I get this Error. I guess closing the connections / the DataReaders dont work.

Before this change, i had a connection for every site. But my program had no good performance. So I decided to make 1 connection which is always Open and just call querys to this open connection. And now I get DataReader Errors.

Can someone explain me, why using is not closing the DataReader? And line 83 isn't a DataReader it's a var so i don't know why I get this Error at this line.

Answer 1

It sounds like your issues are regarding connection state management? I may not be fully understanding what you're asking but by design using statements within the context of connections will close the connection. They are syntactic sugar for try {} catch {} finally . Far too often I see examples of Connection objects, Command Objects, etc. not utilizing IDisposable and not being properly disposed/closed.

In this code, I don't see a connection being opened again for the command to execute.

 DataTable schema = null;

        using (var schemaCommand = new MySqlCommand("SELECT * FROM " + firmCustomerTablename, connection))
        {
            using (var reader = schemaCommand.ExecuteReader(CommandBehavior.SchemaOnly))
            {
                schema = reader.GetSchemaTable();
            }
        }

Here is a basic idea:

        using (var conn = new SqlConnection(connectionString: ""))
        {
            conn.Open();

            using (var cmd = new SqlCommand(cmdText: "cmdText", connection: conn))
            {
                using (var reader = cmd.ExecuteReader())
                {
                    while (reader.Read())
                    {
                        //
                    }
                }
            }

        }

Documentation: MSDN SqlConnection Class