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extracting a fill method from a form

I have a combobox on a windows form that I fill with a list of names. At the moment I have the following code inside the Form class and it works fine
// This section opens a connection to the database, selects all the portfolio names that have an "in Use" value of 1, and then
// fills Combo Box 2 with the values.
SqlConnection myConnection = new SqlConnection(#"Data Source = (LocalDB)\MSSQLLocalDB; AttachDbFilename = ""C:\Users\Nick\Documents\Investments 4.mdf""; Integrated Security = True; Connect Timeout = 30");
myConnection.Open();
SqlCommand myCommand2 = new SqlCommand();
myCommand2.Connection = myConnection;
myCommand2.CommandText = "SELECT Portfolio_Name FROM Dbo.Name WHERE In_use = 1";
SqlDataReader myReader2 = myCommand2.ExecuteReader();
while (myReader2.Read())
{
comboBox2.Items.Add(myReader2[0]);
}
myConnection.Close();
I would like to be able to extract this into a separate method, and put it into a separate class for general utility methods. However, I'm stuck on a really simple issue. When I put the code into a class, I need to be able to tell it which combox box I want to fill, and I can't figure out how to pass in that information. Sorry if the answer is obvious, but any help would be gratefully received.
Thanks!

Well, if you want to extract, then extract:
// Let's extract a class: it should provide us standard cursors,
// e.g. Protfolio Names
public static class MyData {
// Let's enumerate items returned
public static IEnumerable<string> PortfolioNames() {
// Wrap IDisposable into using
//TODO: move Connection String into a separated method/property
using (SqlConnection con = new SqlConnection(/*connection string here*/)) {
con.Open();
// Make sql readable
//DONE: when presenting something to user, sort it (order by) esp. strings
string sql =
#" select Portfolio_Name
from Dbo.Name
where In_use = 1
order by Portfolio_Name";
// Wrap IDisposable into using
using (SqlCommand q = new SqlCommand(sql, con)) {
// Wrap IDisposable into using
using (var reader = q.ExecuteReader()) {
while (reader.Read())
yield return Convert.ToString(reader[0]);
}
}
}
}
}
And then use
// Adding items in one after one manner is often a bad idea:
// it makes UI repaint each time you add an item and cause blinking.
// Let's fill the ComboBox in one go via AddRange
comboBox2.Items.AddRange(MyData.PortfolioNames().ToArray());

You can use a helper class names Portfolio for data access. The method GetNames does not require a ComboBox instance. This increases the chance that you can reuse the method in another context.
public static class Portfolio
{
public static IList<string> GetNames()
{
SqlConnection myConnection = new SqlConnection(#"Data Source = (LocalDB)\MSSQLLocalDB; AttachDbFilename = ""C:\Users\Nick\Documents\Investments 4.mdf""; Integrated Security = True; Connect Timeout = 30");
myConnection.Open();
SqlCommand myCommand2 = new SqlCommand();
myCommand2.Connection = myConnection;
myCommand2.CommandText = "SELECT Portfolio_Name FROM Dbo.Name WHERE In_use = 1";
SqlDataReader myReader2 = myCommand2.ExecuteReader();
var portfolioNames = new List<string>();
while (myReader2.Read())
{
portfolioNames.Add(myReader2[0]);
}
myConnection.Close();
return portfolioNames;
}
}
Then in your Form you can do something like this:
var names = Portfolio.GetNames();
foreach (var name in names)
{
combobox2.Items.Add(name);
}

It is so simple:
public class MyUtility
{
public static void FillComboBox(System.Windows.Forms.ComboBox comboBox)
{
//comboBox.Items.Clear(); //enable this line if required
using (SqlConnection myConnection = new SqlConnection(#"Data Source = (LocalDB)\MSSQLLocalDB; AttachDbFilename = ""C:\Users\Nick\Documents\Investments 4.mdf""; Integrated Security = True; Connect Timeout = 30"))
{
myConnection.Open();
using (SqlCommand myCommand2 = new SqlCommand())
{
myCommand2.Connection = myConnection;
myCommand2.CommandText = "SELECT Portfolio_Name FROM Dbo.Name WHERE In_use = 1";
using (SqlDataReader myReader2 = myCommand2.ExecuteReader())
{
while (myReader2.Read())
{
comboBox.Items.Add(myReader2[0]);
}
}
}
//myConnection.Close(); //not required inside using block
}
}
}
you may use other methods to get connection string (e.g. from config file).
The usage is so simple, no extra code required:
MyUtility.FillComboBox(comboBox2);

WebMethod failure

I have the following webmethod on a asmx page that will not connect to my database. Can anyone shed any light on where I have gone wrong.
[WebMethod]
public int[] getEmployeeIDs()
{
Connection conn = new Connection();
Recordset rs = new Recordset();
conn.ConnectionString = "Data Source=MyWebBasedServer;Initial Catalog=MyDatabase;Persist Security Info=False;User ID=MyLogin;Password=MyPassword";
rs.Open("SELECT ID from MyTable", conn, CursorTypeEnum.adOpenStatic);
int[] ID = new int[rs.RecordCount];
int i = 0;
while (!rs.EOF)
{
ID[i++] = rs.Fields["ID"].Value;
rs.MoveNext();
}
rs.Close();
conn.Close();
return ID;
}
Error message I get is
The connection cannot be used to perform this operation. It is either
closed or invalid in this context. (Pointing to "int[] ID = new
int[rs.RecordCount];")
Thanks.

The code below shows two common ways to retrieve a result set from a SELECT statement in ADO.Net.
There's a website called ConnectionStrings.com that shows all the various ways to connect to SQL Server, along with many other kinds of databases.
If you're new to C# programming, the using statement is a great way to avoid resource leaks when handling objects that implement IDisposable.
Returning complex types from a WebMethod might result in an error. The method's underlying XML serializer might not know how to handle certain types. In that case, XmlIncludeAttribute can be used to provide explicit type information. Here's an MSDN thread discussing how to go about that.
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Web.Services;
namespace ConsoleApplication19
{
public class Program
{
public static void Main(String[] args)
{
var connectionString = "Data Source=MyWebBasedServer;Initial Catalog=MyDatabase;Persist Security Info=False;User ID=MyLogin;Password=MyPassword;";
var a = GetEmployeeIDs_Version1(connectionString);
var b = GetEmployeeIDs_Version2(connectionString);
}
/* Version 1
Use a "while" loop to fill a WebData list and return it as an array. */
[WebMethod]
private static WebData[] GetEmployeeIDs_Version1(String connectionString)
{
using (var connection = new SqlConnection(connectionString))
{
connection.Open();
var commandText = "SELECT ID, SurName from MyTable";
using (var command = new SqlCommand() { Connection = connection, CommandType = CommandType.Text, CommandText = commandText })
{
using (var reader = command.ExecuteReader(CommandBehavior.CloseConnection))
{
var result = new List<WebData>();
while (reader.Read())
result.Add(new WebData() { ID = Convert.ToInt32(reader["ID"]), Surname = reader["Surname"].ToString() });
return result.ToArray();
}
}
}
}
/* Version 2
Fill a DataSet with the result set.
Because there's only one SELECT statement, ADO.Net will
populate a DataTable with that result set and put the
DataTable in the dataset's Tables collection.
Use LINQ to convert that table into a WebData array. */
[WebMethod]
private static WebData[] GetEmployeeIDs_Version2(String connectionString)
{
using (var connection = new SqlConnection(connectionString))
{
connection.Open();
var commandText = "SELECT ID, SurName from MyTable";
using (var command = new SqlCommand() { Connection = connection, CommandType = CommandType.Text, CommandText = commandText })
{
using (var adapter = new SqlDataAdapter())
{
var dataSet = new DataSet();
adapter.SelectCommand = command;
adapter.Fill(dataSet);
return
dataSet
// There should only be one table in the dataSet's Table's collection.
.Tables[0]
.Rows
// DataTable isn't LINQ-aware. An explicit cast is needed
// to allow the use of LINQ methods on the DataTable.Rows collection.
.Cast<DataRow>()
// The rows in a DataTable filled by an SqlDataAdapter
// aren't strongly typed. All of a row's columns are
// just plain old System.Object. Explicit casts are necessary.
.Select(row => new WebData() { ID = Convert.ToInt32(row["ID"]), Surname = row["Surname"].ToString() })
// Use LINQ to convert the IEnumerable<WebData> returned by
// the .Select() method to an WebData[].
.ToArray();
}
}
}
}
}
public class WebData
{
public Int32 ID { get; set; }
public String Surname { get; set; }
}
}

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.

keeping connection string and connection object centrally in c# application

Im a beginner to C# (.net of course) and for my final year project im developing a payroll system. Now I have some issues regarding ado.net sql connection object.
To keep the connection string centrally I have used a separate class call db. Taking another step to this centralization thinking, I've initialized the connection object also centrally in this db class as follows.
class db
{
string connectionString = ("connection string will be here...");
public SqlConnection GetConn()
{
SqlConnection NewConn = new SqlConnection(connectionString);
return NewConn;
}
}
Now Im using this connection object as follows in my application...
I just want to know whether I would face issues in future because of this practice and also appreciate if one of experts could explain me what is the best practice in this regard.
Thanks in advance
class client
{
db NewDB = new db(); // db class is instantiated...
SqlConnection newCon; // object referece newConn is created...
//Method to insert new clients to 'client' table
public void addNewClient(DateTime entDate, client NewClient)
{
try
{
newCon = NewDB.GetConn(); // connection object is assigned to newCon... but this is optional and I put this for the clarity
string CommandString = "INSERT INTO client(Client_Name, C_Add, Contact_Person, C_Mob_No, C_Tel_No, Remarks, Ent_Date)" +
" VALUES (#CName, #CAdd, #CPerson, #CMob, #CTel, #Remarks, #entDate)";
SqlCommand SqlCom = new SqlCommand();
SqlCom.CommandText = CommandString;
SqlCom.Parameters.Add("#CName", SqlDbType.VarChar).Value = NewClient.CName;
SqlCom.Parameters.Add("#CAdd", SqlDbType.VarChar).Value = NewClient.CAdd;
SqlCom.Parameters.Add("#CPerson", SqlDbType.VarChar).Value = NewClient.CPerson;
SqlCom.Parameters.Add("#CMob", SqlDbType.Char).Value = NewClient.CMob;
SqlCom.Parameters.Add("#CTel", SqlDbType.Char).Value = NewClient.CTel;
SqlCom.Parameters.Add("#Remarks", SqlDbType.VarChar).Value = NewClient.Remarks;
SqlCom.Parameters.Add("#entDate", SqlDbType.Date).Value = entDate;
SqlCom.Connection = newCon;
newCon.Open();
SqlCom.ExecuteNonQuery();
}
catch
{
throw;
}
finally
{
newCon.Close(); // newCon object is global to entire class so can call its close method.
}
}
}

You don't need to use global connection object. Your db connections are stored in connection pool. So you won't run out of connections. Read more about connections pooling.

Looking at your client class it is bad practice to write raw SQl in the code. It is better practice to write a stored procedure and call it from the code passing the parameters.
public void addNewClient(DateTime entDate, client NewClient)
{
try
{
newCon = NewDB.GetConn(); // create connection
conn.Open(); //open connection
// create a command object identifying the stored procedure
SqlCommand SqlCom = new SqlCommand("Store Procedure name", newConn);
// add parameter to sql command, which is passed to the stored procedure
SqlCom .Parameters.Add(new SqlParameter("#CName", NewClient.CName));
// Rest of parameters
// execute the command
cmd.ExecuteReader();
}
}
Creating a class to store the connection is good practice. However you could expand on this further.
public struct slqParameters
{
public object ParamData { get; set; }
public string ParamKey { get; set; }
public SqlDbType ParamDatabaseType { get; set; }
public int ParamSize { get; set; }
}
class db
{
private string connectionString = ("connection string will be here...");
public static void ExecuteStoreProcedure(string ProcedureName, ref slqParameters[] CommandParameters)
{
string str_ConnectionString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
try
{
using (SqlConnection sqlConnection = new SqlConnection(str_ConnectionString))
{
using (SqlCommand sqlCommand = new SqlCommand(sProcedureName, sqlConnection) { CommandType = CommandType.StoredProcedure })
{
// Add all the parameters to the sql command.
foreach (slqParametersParameter in CommandParameters)
{
// Add a parameter
sqlCommand.Parameters.Add(new SqlParameter(Parameter.ParamKey, Parameter._ParamDatabaseType , Parameter._ParamSize ) { Value = Parameter.ParamData });
}
sqlConnection.Open();
DataTable dtTable = new DataTable();
sqlCommand.ExecuteReader())
}
}
}
catch (Exception error)
{
throw error;
}
}
}
This is only a ruff guide as i have not tested it yet but it should work
To use it on your page
Public SomeMethod()
{
slqParameters[] parameters = new Parameters[1]
{
new sqlParameters{ ParamData = , paramKey = "#CName", ParamDatabaseType = NewClient.CName}
};
db.ExecuteStoreProcedure("Store Procedure name", parameters);
}

C# Invalid attempt to call Read when reader is closed

I am having Invalid attempt to call Read when reader is closed error when I am doing 3 tier project in C# language.
What I am trying to do is retrieve address data column by joining two tables together and display in a drop down list.
Here is my data access layer:
public List<Distribution> getDistributionAll()
{
List<Distribution> distributionAll = new List<Distribution>();
string address;
SqlDataReader dr = FoodBankDB.executeReader("SELECT b.addressLineOne FROM dbo.Beneficiaries b INNER JOIN dbo.Distributions d ON d.beneficiary = b.id");
while (dr.Read())
{
address = dr["addressLineOne"].ToString();
distributionAll.Add(new Distribution(address));
}
return distributionAll;
}
And this is my FoodBankDB class:
public class FoodBankDB
{
public static string connectionString = Properties.Settings.Default.connectionString;
public static SqlDataReader executeReader(string query)
{
SqlDataReader result = null;
System.Diagnostics.Debug.WriteLine("FoodBankDB executeReader: " + query);
SqlConnection connection = new SqlConnection(connectionString);
SqlCommand command = new SqlCommand(query, connection);
connection.Open();
result = command.ExecuteReader();
connection.Close();
return result;
}
}
I separated the these into two class so that whenever my connection string is changed, I can amend the whole project easily by changing the FoodBankDB class.
And this is my business logic layer:
public List<Distribution> getAllScheduledDistribution()
{
List<Distribution> allDistribution = new List<Distribution>();
Distribution distributionDAL = new Distribution();
allDistribution = distributionDAL.getDistributionAll();
return allDistribution;
}
And last but not least, my presentation layer:
List<Distribution> scheduledList = new List<Distribution>();
scheduledList = packBLL.getAllScheduledDistribution();
ddlScheduleList.DataSource = scheduledList;
ddlScheduleList.DataTextField = "address";
ddlScheduleList.DataValueField = "address";
ddlScheduleList.DataBind();
It was working well if I didn't split the data access layer and connection string class. Does anybody know how to solve this error?
Thanks in advance.
Updated portion
public static string GetConnectionString()
{
return connectionString;
}

It doesn't work because you close the connection before returning the reader. Reader works only when the connection is open:
result = command.ExecuteReader();
connection.Close();
return result; // here the reader is not valid
Generally speaking, you should not be returning a reader to a business layer. Reader should be used only in the data access layer. It should be used and then it and the connection should be closed.
You should rather return an object that can work after the connection is closed, e.g. a DataSet or DataTable or alternatively a collection of DTO's. For example:
public List<Distribution> getDistributionAll()
{
List<Distribution> distributionAll = new List<Distribution>();
using (var connection = new SqlConnection(FoodBankDB.GetConnectionString())) // get your connection string from the other class here
{
SqlCommand command = new SqlCommand("SELECT b.addressLineOne FROM dbo.Beneficiaries b INNER JOIN dbo.Distributions d ON d.beneficiary = b.id", connection);
connection.Open();
using (var dr = command.ExecuteReader())
{
while (dr.Read())
{
string address = dr["addressLineOne"].ToString();
distributionAll.Add(new Distribution(address));
}
}
}
return distributionAll;
}

Previous one is a good example ... But you can also accomplish it by below code which is automatically close a connection instance when datareader.close() method called ...
reader = Sqlcmd.ExecuteReader(CommandBehavior.CloseConnection);