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Invalid Attempt To Call Read when reader is closed .NET

I currently have a DB library used for database access and I use it across several projects. I currently use the following code to get a recordset.
METHODS
public static IDataReader GetRs(string sql)
{
using (var con = NewSqlConnection())
{
con.Open();
return GetRs(sql, con);
}
}
public static IDataReader GetRs(string sql, SqlConnection dbconn)
{
using (var cmd = new SqlCommand(sql, dbconn))
{
int tries = 1;
while (tries <= 3)
{
try
{
if (dbconn.State == ConnectionState.Closed)
{
dbconn.Open();
}
DataTable myTable = new DataTable();
var reader = cmd.ExecuteReader();
myTable.Load(reader);
return myTable.CreateDataReader();
//return cmd.ExecuteReader();
}
catch (SqlException ex)
{
if (ex.Message.Contains("Timeout expired") || ex.Number == 1205) // Deadlock
{
Thread.Sleep(1000);
if (tries == 3)
{
throw ex;
}
tries += 1;
cmd.CommandTimeout *= 10;
}
else
{
throw ex;
}
}
}
}
throw new Exception("Could not get RecordSet");
}
USAGE
public static void Test()
{
using(var reader = GetRs("SELECT Col FROM TABLE"))
{
while(reader.Read())
{
// do stuff with data here e.g. var value = reader[0];
}
}
}
While this method works, as you can see it loads the entire dataset into memory thus causing issues with scaling.
I tried replacing the following code in the GetRs(string sql, SqlConnection con) method
DataTable myTable = new DataTable();
var reader = cmd.ExecuteReader();
myTable.Load(reader);
return myTable.CreateDataReader();
and tried returning just the return cmd.ExecuteReader();
However an error is thrown on the while (reader.Read()) - Invalid attempt to call read when the reader is closed. I am guessing this is because the SqlConnection property is disposed (and hence closed) after returning the IDataReader.
I'm aware that I can wrap the GetRs method with a new sql connection but this means rewriting a lot of my code, and I was hoping that I would be able to dispose the reader AND the connection with my using(var reader = GetRs()) method.
Is there any way I can still use these methods without loading the whole dataset into memory?

You can inject in your code with a Action<DataTableReader> parameter to you GetRs call.
Try this:
public static void GetRs(string sql, Action<DataTableReader> consumer)
{
using (var con = NewSqlConnection())
{
con.Open();
GetRs(sql, con, consumer);
}
}
public static void GetRs(string sql, SqlConnection dbconn, Action<DataTableReader> consumer)
{
using (var cmd = new SqlCommand(sql, dbconn))
{
if (dbconn.State == ConnectionState.Closed)
{
dbconn.Open();
}
DataTable myTable = new DataTable();
var reader = cmd.ExecuteReader();
myTable.Load(reader);
consumer(myTable.CreateDataReader());
}
}
(I removed your try/catch code for clarity.)
Then you call it like this:
public static void Test()
{
GetRs("SELECT Col FROM TABLE", reader =>
{
while(reader.Read())
{
// do stuff with data here e.g. var value = reader[0];
}
});
}

Your connection object was closed by the following line
using (var con = NewSqlConnection())
{
con.Open();
return GetRs(sql, con);
}//Connection object gets released here
But still, you are returning the reader object which was created by SqlCommand object using the above connection object. Hence the reader object is closed.

C# Mysql Connection must be valid and open

First of all: I got my code running without using oop. I declared all my variables inside the same class and opened/closed the connection right before and after passing the query to the db. That worked! Now with some new experiences I tried to split my code into different classes. Now it wont work anymore.
It tells me "Connection must be valid and open". Enough text, here's my current code:
Services.cs
public static MySqlConnection conn // Returns the connection itself
{
get
{
MySqlConnection conn = new MySqlConnection(Services.ServerConnection);
return conn;
}
}
public static string ServerConnection // Returns the connectin-string
{
get
{
return String.Format("Server={0};Port=XXXX;Database=xxx;Uid=xxx;password=xxXxxXxXxxXxxXX;", key);
}
}
public static void DB_Select(string s, params List<string>[] lists)
{
try
{
MySqlCommand cmd = conn.CreateCommand();
cmd.CommandType = CommandType.Text;
string command = s;
cmd.CommandText = command;
MySqlDataReader sqlreader = cmd.ExecuteReader();
while (sqlreader.Read())
{
if (sqlreader[0].ToString().Length > 0)
{
for (int i = 0; i < lists.Count(); i++)
{
lists[i].Add(sqlreader[i].ToString());
}
}
else
{
foreach (List<string> save in lists)
{
save.Add("/");
}
}
}
sqlreader.Close();
}
catch (Exception ex)
{
MessageBox.Show("Error while selecting data from database!\nDetails: " + ex);
}
}
LoginForm.cs
private void checkUser(string username, string password)
{
using (Services.conn)
{
Services.conn.Open();
Services.DB_Select("..a short select statement..");
Services.conn.Close();
}
I guess this is all we need. I have shortened my code to get a focus on the problem.
I created Services.cs to get a global way to access the db from all forms without copy&pasting the connection info. Now when I reach my LoginForm.cs it throws an error "Connection must be valid and open". I've already debugged my code. It's all time closed. Even when passing conn.Open() it stays closed. Why?
Another try: I've also tried placing conn.Open() and conn.Close() inside Services.DB_Select(..) at the beginning and end. Same error here.
I have to say: The code worked before and I've used the same connection-string. So the string itself is surely valid.
I appreciate any help given here!

The problem is that you don't store the connection that was returned from your factory property. But don't use a property like a method. Instead use it in this way:
using (var con = Services.conn)
{
Services.conn.Open();
Services.DB_Select("..a short select statement..", con ));
//Services.conn.Close(); unnecessary with using
}
So use the same connection in the using that was returned from the property(or better created in the using) and pass it to the method which uses it. By the way, using a property as factory method is not best practise.
But in my opinion it's much better to create the connection where you use it, best place is in the using statement. And throw the con property to the garbage can, it is pointless and a source for nasty errors.
public static void DB_Select(string s, params List<string>[] lists)
{
try
{
using(var conn = new MySqlConnection(Services.ServerConnection))
{
conn.Open();
MySqlCommand cmd = conn.CreateCommand();
cmd.CommandText = s;
using( var sqlreader = cmd.ExecuteReader())
while (sqlreader.Read())
{
if (sqlreader[0].ToString().Length > 0)
{
for (int i = 0; i < lists.Count(); i++)
{
lists[i].Add(sqlreader[i].ToString());
}
}
else
{
foreach (List<string> save in lists)
{
save.Add("/");
}
}
} // unnecessary to close the connection
} // or the reader with the using-stetement
}
catch (Exception ex)
{
MessageBox.Show("Error while selecting data from database!\nDetails: " + ex);
}
}

Try to restructure your Services class as follows
public static MySqlConnection conn // Returns the connection itself
{
get
{
MySqlConnection conn = new MySqlConnection(Services.ServerConnection);
return conn;
}
}
private static string ServerConnection // Returns the connectin-string - PRIVATE [Improved security]
{
get
{
return String.Format("Server={0};Port=XXXX;Database=xxx;Uid=xxx;password=xxXxxXxXxxXxxXX;", key);
}
}
// Rather than executing result here, return the result to LoginForm - Future improvement
public static void DB_Select(MySqlConnection conn ,string s, params List<string>[] lists)
{
try
{
MySqlCommand cmd = conn.CreateCommand();
cmd.CommandType = CommandType.Text;
string command = s;
cmd.CommandText = command;
MySqlDataReader sqlreader = cmd.ExecuteReader();
while (sqlreader.Read())
{
if (sqlreader[0].ToString().Length > 0)
{
for (int i = 0; i < lists.Count(); i++)
{
lists[i].Add(sqlreader[i].ToString());
}
}
else
{
foreach (List<string> save in lists)
{
save.Add("/");
}
}
}
sqlreader.Close();
}
catch (Exception ex)
{
MessageBox.Show("Error while selecting data from database!\nDetails: " + ex);
}
}
In LoginForm.cs use returning connection and store it there. When you need to execute query, use
MySqlConnection conn=Services.conn(); // Get a new connection
Services.DB_Select(conn,"..a short select statement.."); // Executing requirement
Services.conn.Close();
Additional - I suggest you need to return MySqlDataReader to LoginForm and handle results there

private MySqlConnection _conn;
public MySqlConnection conn // Returns the connection itself
{
get
{
if(_conn == null)
_conn = new MySqlConnection(Services.ServerConnection);
return _conn;
}
}

How to copy MySqlDataReader into an array and then loop throught the array?

I am new to C# so yes this should be a faily easy question but I can't seem to find the answer to it.
I have a method that query a database.
What I am trying to do here is handle the loop though the data outside the method.
public MySqlDataReader getDataSet(string query)
{
MySqlDataReader dataset = null;
MySqlConnection conn = new MySqlConnection(conn_string);
if (startConnection(conn) == true)
{
MySqlCommand cmd = new MySqlCommand(query, conn);
dataset = cmd.ExecuteReader();
closeConnection(conn);
}
return dataset;
}
what I could do is write a while loop just before the closeConnection(conn); line and handle the data. But, I don't want to do it inside this method and I want to do it somewhere else in my code.
In one of my forms I want to read the database on the load so here is what I tried to do
public newDepartment()
{
InitializeComponent();
inputDepartmentName.Text = "Hi";
dbConnetion db = new dbConnetion();
MySqlDataReader ds = db.getDataSet("SELECT name FROM test;");
while (ds.Read())
{
//Do Something
}
}
The problem that I am having is that I get an error Invalid attempt to Read when reader is closed
Which I belive I get this issue because I close the connection and then I am trying to read it. so What I need to do is read the data from the query and put it in an array and then loop through the array and deal with the data in a different form.
How can I workaround this issue? if my idea is good then how can I copy the data into an array and how do I loop though the array?
Here is the full class
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using MySql.Data.MySqlClient;
using System.Windows.Forms;
namespace POS
{
public class dbConnetion
{
//private OdbcConnection conn;
private readonly string mServer;
private readonly string mDatabase;
private readonly string mUid;
private readonly string mPassword;
private readonly string mPort;
private readonly string conn_string;
public dbConnetion()
{
mServer = "localhost";
mDatabase = "pos";
mUid = "root";
mPassword = "";
mPort = "3306";
conn_string = String.Format("server={0};user={1};database={2};port={3};password={4};", mServer, mUid, mDatabase, mPort, mPassword);
}
//Start connection to database
private bool startConnection(MySqlConnection mConnection)
{
try
{
mConnection.Open();
return true;
}
catch (MySqlException ex)
{
MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK);
return false;
}
}
//Close connection
private bool closeConnection(MySqlConnection mConnection)
{
try
{
mConnection.Close();
return true;
}
catch (MySqlException ex)
{
MessageBox.Show(ex.Message);
return false;
}
}
public MySqlDataReader getDataSet(string query)
{
MySqlDataReader dataset = null;
MySqlConnection conn = new MySqlConnection(conn_string);
if (startConnection(conn) == true)
{
MySqlCommand cmd = new MySqlCommand(query, conn);
dataset = cmd.ExecuteReader();
closeConnection(conn);
}
return dataset;
}
public void processQuery(string strSQL, List<MySqlParameter> pars)
{
MySqlConnection conn = new MySqlConnection(conn_string);
if (startConnection(conn) == true)
{
MySqlCommand cmd = new MySqlCommand(strSQL, conn);
foreach (MySqlParameter param in pars)
{
cmd.Parameters.Add(param);
}
cmd.ExecuteNonQuery();
closeConnection(conn);
}
}
}
}

Putting the records into an array would destroy the best feature of a using a datareader: that you only need to allocate memory for one record at a time. Try doing something like this:
public IEnumerable<T> getData<T>(string query, Func<IDataRecord, T> transform)
{
using (var conn = new MySqlConnection(conn_string))
using (var cmd = new MySqlCommand(query, conn))
{
conn.Open();
using (var rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
yield return transform(rdr);
}
}
}
}
While I'm here, there's a very serious security flaw with this code and the original. A method like this that only accepts a query string, with no separate mechanism for parameters, forces you to write code that will be horribly horribly vulnerable to sql injection attacks. The processQuery() method already accounts for this, so let's extend getDataset() to avoid that security issue as well:
public IEnumerable<T> getData<T>(string query, List<MySqlParameter> pars, Func<IDataRecord, T> transform)
{
using (var conn = new MySqlConnection(conn_string))
using (var cmd = new MySqlCommand(query, conn))
{
if (pars != null)
{
foreach(MySqlParameter p in pars) cmd.Parameters.Add(p);
}
conn.Open();
using (var rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
yield return transform(rdr);
}
}
}
}
Much better. Now we don't have to write code that's just asking to get hacked anymore. Here's how your newDepartment() method will look now:
public newDepartment()
{
InitializeComponent();
inputDepartmentName.Text = "Hi";
dbConnetion db = new dbConnetion();
foreach(string name in db.getDataSet("SELECT name FROM test;", null, r => r["name"].ToString() ))
{
//Do Something
}
}
One thing about this code is that is uses a delegate to have you provide a method to create a strongly-typed object. It does this because of the way the datareaders work: if you don't create a new object at each iteration, you're working on the same object, which can have undesirable results. In this case, I don't know what kind of object you're working with, so I just used a string based on what your SELECT query was doing.
Based on a separate discussion, here's an example of calling this for a more complicated result set:
foreach(var item in db.getDataSet(" long query here ", null, r =>
new columnClass()
{
firstname = r["firstname"].ToString(),
lastname = r["lastname"].ToString(),
//...
}
) )
{
//Do something
}

Since you are new to .Net I thought I point out that there are two layers of database access in ADO.Net. There are the data reader way that you are using and all of that is online only forward reading of queries. This is the lowest level access and will give you the best performance but it is more work. For most connection types you can only execute one command or have one active data reader per connection (And you can't close the connection before you have read the query as you are doing).
The other form is the offline data adapter and requires just a little bit different code, but is generally easier to use.
public DataTable getDataSet(string query)
{
MySqlConnection conn = new MySqlConnection(conn_string);
if (startConnection(conn) == true)
{
MySqlDataAdapter adapter = new MySqlDataAdapter(query, conn);
DataTable table = new DataTable();
adapter.Fill(table);
closeConnection(conn);
return table;
}
return null;
}
This will result in you getting a DataTable with columns and rows corresponding to the result of your query (Also look into command builders if you want to post changes back to the database later on from it, but for that you will need to keep the connection open).
One nice thing with using the data adapter is that it will figure out what the correct data types should be so you don't have to worry about invalid cast exceptions while reading the data from the data reader.
As somebody pointed out though you will need to read all the data into memory which could be a problem if you are dealing with a lot of memory. Also the DataTable class is really slow when you start dealing with a lot of records. Finally DataTable and DataSet classes also generally hook well into UI components in .Net so that their contents can easily be displayed to users.

passing a string to a class

I have a problem with my SQL datareader, i want to make an external class and make the code in my xaml.cs as short as possible, because there are a lot of sqldatareaders needed in my program. for this I want to pass following two strings to the datareader class:
public void refreshcombobox()
{
cbGebruiker.Items.Clear();
database = new DataBase();
string sqlrdr = "(rdr.GetString(1).ToString().Trim())";
List<string> reader = database.ReaderRdr("Select * from Gebruikers", ref sqlrdr);
foreach (String str in reader)
{
cbGebruiker.Items.Add(str);
}
}
however, when I do this this is the result in my program instead of the actual results that are stored in the database:
http://i58.tinypic.com/301j2vo.jpg (I can't post images)
can somebody help me with this? I've searched everywhere...
I don't know how to pass the rdr.GetString(1).ToString().Trim() to make it actually look stuff up in the db. Instead of just copying the string directly into the list.
This is the class:
namespace ClassLib
{
public class DataBase
{
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["kassadatabase"].ConnectionString);
public object ScalarObject(string sql)
{
object value = null;
try
{
conn.Open();
SqlCommand cmd = new SqlCommand(sql, conn);
value = cmd.ExecuteScalar();
}
catch (SqlException ex)
{
MessageBox.Show(ex.Message);
}
finally
{
if (conn != null) conn.Close();
}
return value;
}
public List<string> ReaderRdr(string sql)
{
SqlDataReader rdr = null;
List<string> reader = new List<string>();
try
{
conn.Open();
SqlCommand cmd = new SqlCommand(sql, conn);
rdr = cmd.ExecuteReader();
while (rdr.Read())
{
//reader.Add(rdr.GetString(1).ToString().Trim());
}
}
catch (SqlException ex)
{
MessageBox.Show(ex.Message);
}
finally
{
if (rdr != null) rdr.Close();
if (conn != null) conn.Close();
}
return reader;
}
//public List<string> ReaderRdr(string sql, ref string str)
//{
// SqlDataReader rdr = null;
// List<string> reader = new List<string>();
// try
// {
// conn.Open();
// SqlCommand cmd = new SqlCommand(sql, conn);
// rdr = cmd.ExecuteReader();
// while (rdr.Read())
// {
// //MessageBox.Show(str.ToString());
// //var strRdr = str;
// //MessageBox.Show(strRdr.ToString());
// //reader.Add(rdr.GetString(1).ToString().Trim());
// reader.Add(str);
// Console.WriteLine(String.Format("{0}", rdr[0]));
// }
// }
// catch (SqlException ex)
// {
// MessageBox.Show(ex.Message);
// }
// finally
// {
// if (rdr != null) rdr.Close();
// if (conn != null) conn.Close();
// }
// return reader;
//}
public void ExecuteNQuery(string insertString)
{
try
{
conn.Open();
SqlCommand cmd2 = new SqlCommand(insertString, conn);
cmd2.ExecuteNonQuery();
}
catch (SqlException ex)
{
MessageBox.Show(ex.Message);
}
finally
{
if (conn != null) conn.Close();
}
}
}
}
public List<string> ReaderRdr(string sql)
{
SqlDataReader rdr = null;
List<string> reader = new List<string>();
try
{
conn.Open();
SqlCommand cmd = new SqlCommand(sql, conn);
rdr = cmd.ExecuteReader();
while (rdr.Read())
{
//reader.Add(rdr.GetString(1).ToString().Trim());
}
.....
now in the methode public list i want to replace the //reader.Add(rdr.GetString(1).ToString().Trim()); part(wich works fine)
with a string that is passed to the method.
public List<string> ReaderRdr(string sql, string strRdr)
{
SqlDataReader rdr = null;
List<string> reader = new List<string>();
try
{
conn.Open();
SqlCommand cmd = new SqlCommand(sql, conn);
rdr = cmd.ExecuteReader();
while (rdr.Read())
{
//reader.Add(strRdr);
}

I'm not 100% sure what you're trying to do, but I can tell you right now that this is what you're doing wrong - the string (str) you're passing to ReaderRdr is just a string literal of C# code. There's super hacky (super inadvisable) things you can do to mimic what exists in other languages as eval(), but there's no built-in way to do that in C#. Nothing (sensible) you do to "(rdr.GetString(1).ToString().Trim())" is ever going to get a string, or cast it to string, or trim anything.
Within your ReaderRdr function, all you're accomplishing is just to add the string str to your List<string> reader. This accomplishes nothing and has no bearing whatsoever on the results you get from your database query in your SqlDataReader rdr. If you want to store the data you actually get from your database, use rdr, not the (useless) string argument str.
Also, I feel like you must have left something out of your code - you're instantiating your SqlCommand cmd with conn as your second argument, but I don't see that defined anywhere within your ReaderRdr method, and it's not an argument passed to ReaderRdr. You don't have an SqlConnection object as a field or property within your class, do you?
As far as what you should maybe do, despite lacking much of any context in terms of your actual aims - if you want to get any given column of the result for each row returned by your SqlDataReader:
rdr = cmd.ExecuteReader();
while (rdr.Read())
{
var yourDataCell = rdr[yourColumnIndex];
// or:
var yourDataCellOtherWay = rdr["YourColumnName"];
}
Alternately, you can just iterate through each of the cells in any given row produced by your SqlDataReader like so:
for(int i = 0 ; i < numberOfColumns; i++) {
// do something with rdr[i] here
}
I'm not sure if there's anything you can do establish numberOfColumns based on the state of your SqlDataReader, but others might know better.

You actually adding the String that you passed to a function to your reader reader.Add(str); You get the response from SQL I your rdr.
This item will show you something from your database:
Console.WriteLine(String.Format("{0}", rdr[0]));

C# Mysql multiple queries

Im trying to build up a little status-tool. I need to get results of multiple queries (about 4-5). The general connection-setup and 'how-to-read-data' is already done but I cant figure out how the another query executed.
Everything I found while searching for it is for the SqlClient. Im totally overcharged with this.
Here is my code so far (be patient, im a newbie to this):
private void button1_Click(object sender, EventArgs e)
{
if(listView1.Items.Count > 1)
{
listView1.Items.Clear();
}
var listMember = new List<string>{};
var listOnline = new List<string>{};
// SQL PART //
string connString = "Server=10*****;Port=3306;Database=e***;Uid=e***;password=********************;";
MySqlConnection conn = new MySqlConnection(connString);
MySqlCommand command = conn.CreateCommand();
command.CommandText = "SELECT fullname,online FROM member WHERE active = '1' ORDER BY online DESC";
try
{
conn.Open();
}
catch (Exception ex)
{
listView1.Items.Add("Error: " + ex);
}
MySqlDataReader reader = command.ExecuteReader();
while(reader.Read())
{
listMember.Add(reader["fullname"].ToString());
listOnline.Add(reader["online"].ToString());
}
conn.Close();
// SQL ENDING //
// SET ENTRIES TO LISTVIEW //
int counter = 0;
foreach(string member in listMember)
{
ListViewItem item = new ListViewItem(new[] { member, listOnline.ElementAt(counter) });
item.ForeColor = Color.Green;
listView1.Items.Add(item);
counter++;
}
}
Im not really sure how the design/layout will look like in the end, so I would like to just append the results to lists in the sql-part to process the data later out of the lists.
Do I really have to setup a complete new connection after conn.Close()? Or is there any other way? I can just imagine: 5 queries with their own connection,try,catch and 2 loops... this will get about 100-200 lines just for getting the results out of 5 queries. Isnt that a bit too much for such an easy thing?
Hope for some help.
Greetings.
According to the new comments my latest code:
Top:
public partial class Form1 : Form
{
public static string connString = "Server=10****;Port=3306;Database=e****;Uid=e****;password=****;";
public Form1()
{
InitializeComponent();
MySqlConnection conn = new MySqlConnection(connString); // Error gone!
}
Body part:
public void QueryTwoFields(string s, List<string> S1, List<string> S2)
{
try
{
MySqlCommand cmd = conn.CreateCommand(); // ERROR: conn does not exist in the current context.
cmd.CommandType = CommandType.Text;
string command = s;
cmd.CommandText = command;
MySqlDataReader sqlreader = cmd.ExecuteReader();
while (sqlreader.Read())
{
S1.Add(sqlreader[0].ToString());
S2.Add(sqlreader[1].ToString());
}
sqlreader.Close();
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString());
}
}
private void button1_Click(object sender, EventArgs e)
{
if(listView1.Items.Count > 1)
{
listView1.Items.Clear();
}
var listMember = new List<string>{};
var listOnline = new List<string>{};
using (conn) // ERROR: conn does not exist in the current context.
{
conn.Open();
///...1st Query
QueryTwoFields("SELECT fullname,online FROM member WHERE active = '1' ORDER BY online DESC",listMember,listOnline);
//...2nd query
//QueryTwoFields("your new Select Statement", otherList, otherList);
}
}

You don't have to close connection every time you execute one query rarher than close the sqlreader assigned to that connection. Finally when all of your queries have been executed you close the connection. Consider also the use of using:
You cal also define a method for execution your Query in order for your code not to be repetive:
public void QueryTwoFields(string s, List<string> S1, List<string> S2)
///Select into List S1 and List S2 from Database (2 fields)
{
try
{
MySqlCommand cmd = conn.CreateCommand();
cmd.CommandType = CommandType.Text;
string command = s;
cmd.CommandText = command;
MySqlDataReader sqlreader = cmd.ExecuteReader();
while (sqlreader.Read())
{
S1.Add(sqlreader[0].ToString());
S2.Add(sqlreader[1].ToString());
}
sqlreader.Close();
}
catch (Exception ex)
{
MessageBox.Show(ex.ToString());
}
}
private void button1_Click(object sender, EventArgs e)
{
if(listView1.Items.Count > 1)
{
listView1.Items.Clear();
}
var listMember = new List<string>{};
var listOnline = new List<string>{};
// SQL PART //
using (conn)
{
conn.Open();
///...1st Query
QueryTwoFields("SELECT fullname,online FROM member WHERE active = '1' ORDER BY online DESC",listmember,listonline)
//...2nd query
QueryTwoFields("your new Select Statement",myOtherList1,myOtherlist2)
....
}
}
EDIT :
Take in mind you cant define QueryTwoFields method inside button handler. You must define it outside (see code above).
Also Define your connection data in the start of the programm:
namespace MyProject
{
/// <summary>
/// Defiine your connectionstring and connection
/// </summary>
///
public partial class Form1 : Form
{ public static string connString = "Server=10*****;Port=3306;Database=e***;Uid=e***;password=********************;";
MySqlConnection conn = new MySqlConnection(connString);
.........

Datatables are fantastic
Using a data table is a nice way to do both read and write. And it comes with the luxury of eveything you can do with a datatable - like asssigning it directly to a datagrid control, sorting, selecting and deleting while disconnected.
The sample below assumes a MySqlConnection conection property managed by calls to your own OpenConnection() and CloseConnection() methods not shown.
Simple datatable read demo:
public DataTable Select(string query = "")
{
//Typical sql: "SELECT * FROM motorparameter"
DataTable dt = new DataTable();
//Open connection
if (this.OpenConnection() == true)
{
//Create Command
MySqlCommand cmd = new MySqlCommand(query, connection);
//Create a data reader and Execute the command
MySqlDataReader dataReader = cmd.ExecuteReader();
dt.Load(dataReader);
//close Data Reader
dataReader.Close();
//close Connection
this.CloseConnection();
//return data table
return dt;
}
else
{
return dt;
}
}
In case of writing back the datatable to the database - supply the SQL you used in the read (or would have used to read to the data table):
public void Save(DataTable dt, string DataTableSqlSelect)
{
//Typically "SELECT * FROM motorparameter"
string query = DataTableSqlSelect;
//Open connection
if (this.OpenConnection() == true)
{
//Create Command
MySqlCommand mySqlCmd = new MySqlCommand(query, connection);
MySqlDataAdapter adapter = new MySqlDataAdapter(mySqlCmd);
MySqlCommandBuilder myCB = new MySqlCommandBuilder(adapter);
adapter.UpdateCommand = myCB.GetUpdateCommand();
adapter.Update(dt);
//close Connection
this.CloseConnection();
}
else
{
}
}
The neat thing the datatable is extremely flexible. You can run your own selects against the table once it contains data and before writing back you can set or reset what rows needs updating and by default the datatable keeps track of what rows you update in the table. Do not forget primary key column(s) for all tables in the db.
For multiple queries consider if possible using a join between the database tables or same table if data related or use a UNION sql syntax if column count and type of data is the same. You can allways "create" your extra column in the select to differ what data comes from what part of the UNION.
Also consider using CASE WHEN sql syntax to conditionally select data from different sources.