Develop Reference

DB Connection issue only some times

I have a problem with the below code,There's no any coding error BUT sometimes it throws some exceptions.I just wanted to know any code organizing issue ? & how to fix it.
Sometimes it shows those exceptions
1.ExecuteReader requires an open and available Connection. The connection's current state is closed.
2.Invalid attempt to call FieldCount when reader is closed.
But Sometimes it works without any issue,as expected
My Coding Goes here
[WebMethod, ScriptMethod]
public static List<HomeImageSliders> GetHomeImageSliders()
{
List<HomeImageSliders> HomeImageList = new List<HomeImageSliders>();
try
{
SqlCommand comHomeImage = new SqlCommand("SP_GetHomeImageSliders", conDB);
comHomeImage.CommandType = CommandType.StoredProcedure;
if (conDB.State != ConnectionState.Open)
{
conDB.Open();
}
SqlDataReader rdr = comHomeImage.ExecuteReader();
DataTable dt = new DataTable();
dt.Load(rdr);
foreach (DataRow r in dt.Rows)
{
HomeImageList.Add(new HomeImageSliders
{
Id = (int)r["Id"],
ImagePath = r["ImagePath"].ToString(),
ModifiedDate = Convert.ToDateTime(r["ModifiedDate"]).Date
});
}
}
catch (Exception ee)
{
}
finally
{
conDB.Close();
}
return HomeImageList;
}

You should use the "using" construction:
(using ommand comHomeImage = new SqlCommand("SP_GetHomeImageSliders", conDB) {
(using SqlDataReader rdr = new SqlDataReader) {
//do some things
}
}

I don't know why your connection sometimes is closed when you call
ExecuteReader()
But why don't you use a using block instead like this:
using(SqlConnection conDB = new SqlConnection(connectionString))
{
...
}
this will close your connection to the DB when it loses scope.

try to prevent using the same connection with two or more threads by usinglock 
lock(_conDb)
{
//// your code here
}
and also wrap your conDb in using block as below
using(SqlConnection conDB = new SqlConnection(connectionString))
{
...
}

How to access DB in asp.net on this scenario?

Hello Guys Below is my Code for accessing database. when i try to open site from more than one tab or i open it in debugging mode it gives error!
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
namespace DomeNameSpace
{
public class DAL
{
public static string _ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["xClassConnectionString"].ConnectionString;
public static SqlConnection _Connection = null;
public static SqlConnection Connection
{
get
{
//_Connection.Close();
//private static SqlConnection _Connection = null;
if (_Connection == null)
{
_Connection = new SqlConnection(_ConnectionString);
_Connection.Open();
return _Connection;
}
else if (_Connection.State != System.Data.ConnectionState.Open)
{
_Connection.Open();
return _Connection;
}
else
{
return _Connection;
}
}
}
public static DataSet GetDataSet(string sql)
{
try
{
SqlCommand cmd = new SqlCommand(sql, Connection);
SqlDataAdapter adp = new SqlDataAdapter(cmd);
// Connection.Close();
DataSet ds = new DataSet();
adp.Fill(ds);
return ds;
}
catch (SqlException err)
{
// Replace the error with something less specific.
// You could also log the error now.
throw new ApplicationException("Data error. " + err.Message.ToString());
}
finally
{
Connection.Close();
}
}
public static DataTable GetDataTable(string sql)
{
DataSet ds = GetDataSet(sql);
if (ds.Tables.Count > 0)
return ds.Tables[0];
return null;
}
public static int ExecuteSQL(string sql)
{
try
{
string BegSql = "BEGIN TRY BEGIN TRANSACTION ";
string EndSql = " COMMIT TRANSACTION END TRY BEGIN CATCH ROLLBACK TRANSACTION END CATCH";
string NewSql = BegSql + sql + EndSql;
sql = NewSql;
SqlCommand cmd = new SqlCommand(sql, Connection);
return cmd.ExecuteNonQuery();
}
catch (System.Exception ex)
{
return -1;
}
finally
{
Connection.Close();
}
}
}
}
But i am getting following here error
and output says
what seems to be the problem?

A single, static database connection is a famously bad idea. It essentially makes your application single-threaded, which a web application by nature is not.
Don't centralize your connection object like this. Creating a connection object is not a resource-intensive operation. And opening the connection itself isn't particularly resource-intensive, the connection pool takes care of most of the heavy lifting for you and is very well optimized.
Create your database connection objects when you need them, as close to where you use them as possible, and dispose of them as soon as you're done with them. In general, a pattern similar to this:
public void SomeMethodWhichConnectsToDB()
{
using (var connection = new SqlConnection())
using (var command = new SqlCommand())
{
// do something with the connection, execute the command, etc
}
}
You can encapsulate the creation of the connection into a (non-static) method to avoid code duplication and whatnot. But don't re-use the same connection object in memory over and over. Create it, use it, destroy it in as little time as possible.

The error is pretty self-explanatory but the reason you are running into this sort of problem is because the connection object is static, which means, that you are sharing this instance for all the calls you make to the database - all threads executing any kind of data access will use the same connection, which you clearly don't want.
What you should do is create the instance of a SQL Connection inside every method or create a Utility class that returns a new instance for every call.
For example:
public class DBUtility
{
public static DbConnection GetOpenConnection()
{
var conn = new DBConnection(connectionString); //or whatever type
conn.Open();
return conn;
}
}
Now in your methods:
public static int ExecuteSQL(string sql)
{
using (var conn = DBUtility.GetOpenConnection())
{
....
}
}

SQLite Database Locked exception

I am getting Database is locked exception from SQLite for some queries only.
Below is my code:
When I execute any select statement it works fine.
When I am executing any write statement on Jobs Table it also works fine.
This works fine:
ExecuteNonQuery("DELETE FROM Jobs WHERE id=1");
But the same way if I am executing queries for Employees table it is throwing an exception that database is locked.
This throws Exception:
ExecuteNonQuery("DELETE FROM Employees WHERE id=1");
Below are my functions:
public bool OpenConnection()
{
if (Con == null)
{
Con = new SQLiteConnection(ConnectionString);
}
if (Con.State == ConnectionState.Closed)
{
Con.Open();
//Cmd = new SQLiteCommand("PRAGMA FOREIGN_KEYS=ON", Con);
//Cmd.ExecuteNonQuery();
//Cmd.Dispose();
//Cmd=null;
return true;
}
if (IsConnectionBusy())
{
Msg.Log(new Exception("Connection busy"));
}
return false;
}
public Boolean CloseConnection()
{
if (Con != null && Con.State == ConnectionState.Open)
{
if (Cmd != null) Cmd.Dispose();
Cmd = null;
Con.Close();
return true;
}
return false;
}
public Boolean ExecuteNonQuery(string sql)
{
if (sql == null) return false;
try
{
if (!OpenConnection())
return false;
else
{
//Tx = Con.BeginTransaction(IsolationLevel.ReadCommitted);
Cmd = new SQLiteCommand(sql, Con);
Cmd.ExecuteNonQuery();
//Tx.Commit();
return true;
}
}
catch (Exception exception)
{
//Tx.Rollback();
Msg.Log(exception);
return false;
}
finally
{
CloseConnection();
}
}
This is the Exception:
At line 103 : Cmd.ExecuteNonQuery();
Exception Found:
Type: System.Data.SQLite.SQLiteException
Message: database is locked
database is locked
Source: System.Data.SQLite
Stacktrace: at System.Data.SQLite.SQLite3.Step(SQLiteStatement stmt)
at System.Data.SQLite.SQLiteDataReader.NextResult()
at System.Data.SQLite.SQLiteDataReader..ctor(SQLiteCommand cmd, CommandBehavior behave)
at System.Data.SQLite.SQLiteCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.SQLite.SQLiteCommand.ExecuteNonQuery()
at TimeSheet6.DbOp.ExecuteNonQuery(String sql) in d:\Projects\C# Applications\Completed Projects\TimeSheet6\TimeSheet6\DbOp.cs:line 103

Somewhere along the way a connection is getting left open. Get rid of OpenConnection and CloseConnection and change ExecuteNonQuery to this:
using (SQLiteConnection c = new SQLiteConnection(ConnectionString))
{
c.Open();
using (SQLiteCommand cmd = new SQLiteCommand(sql, c))
{
cmd.ExecuteNonQuery();
}
}
Further, change the way you read data to this:
using (SQLiteConnection c = new SQLiteConnection(ConnectionString))
{
c.Open();
using (SQLiteCommand cmd = new SQLiteCommand(sql, c))
{
using (SQLiteDataReader rdr = cmd.ExecuteReader())
{
...
}
}
}
Do not attempt, to manage connection pooling on your own like you are here. First, it's much more complex than what you have coded, but second, it's handled already inside the SQLiteConnection object. Finally, if you're not leveraging using, you're not disposing these objects properly and you end up with issues like what you're seeing now.

You can use 'using' statement as below, that will make sure connection & command disposed correctly even in exception
private static void ExecuteNonQuery(string queryString)
{
using (var connection = new SQLiteConnection(
ConnectionString))
{
using (var command = new SQLiteCommand(queryString, connection))
{
command.Connection.Open();
command.ExecuteNonQuery();
}
}
}

You should close your DataReader before attempting to write any data to the database. Use:
dr.Close();
after you finish using the DataReader.

In my case it was very stupid of me, I was making changes in SQLite browser and did not click on write changes, which locked the DB to be modified by the services. After I clicked the Write changes button, all the post request worked as expected.

A lot of helpful posts here for folks that may have forgotten to clean up a dangling connection, but there is another way this can happen: SQLite does not support concurrent INSERTs; if you issue two INSERTs at the same time the will be processed in serial. When the INSERTs are quick this is fine, but if an INSERT takes longer than the timeout the second INSERT can fail with this message.
I had this happen when I used a long running transaction to accumulate a bunch of INSERTs into one big commit. Basically I locked the database from any other activity during the transaction. Switching to journal_mode=WAL will allow concurrent writes and reads, but not concurrent writes.
I got rid of the long running transaction and let each INSERT autocommit, and that solved my problem.

Mine was caused by not closing a SqliteDataReader when calling HasRows().
I had this:
using (SQLiteConnection connection = new SQLiteConnection(DbPath))
{
connection.Open();
string sql = $"SELECT * FROM ...";
using (SQLiteCommand command = new SQLiteCommand(sql, connection))
{
return command.ExecuteReader().HasRows;
}
connection.Close();
}
But needed to put a using around the ExecuteReader like so:
using (SQLiteDataReader reader = command.ExecuteReader())
{
return command.ExecuteReader().HasRows;
}
Even though the DbConnection was being disposed and re-created each time the db was still being kept locked by the reader.

I was also getting the same error here:
if (new basics.HindiMessageBox(HMsg, HTitle).ShowDialog()==true)
{
SQLiteConnection m_dbConnection = new SQLiteConnection(MainWindow.con);
m_dbConnection.Open();
sql = "DELETE FROM `users` WHERE `id`=" + SelectedUser.Id;
command = new SQLiteCommand(sql, m_dbConnection);
command.ExecuteNonQuery();
m_dbConnection.Close();
LoadUserDG();
}
but when I just changed SQLiteConnection declaration location
public partial class User : Window
{
SQLiteCommand command;
string sql;
AddUser AddUserObj;
List<basics.users> usersList;
basics.users SelectedUser;
SQLiteConnection m_dbConnection;
// ...
private void DeleteBtn_Click(object sender, RoutedEventArgs e)
{
// ...
if (new basics.HindiMessageBox(HMsg, HTitle).ShowDialog()==true)
{
m_dbConnection = new SQLiteConnection(MainWindow.con);
m_dbConnection.Open();
sql = "DELETE FROM `users` WHERE `id`=" + SelectedUser.Id;
command = new SQLiteCommand(sql, m_dbConnection);
command.ExecuteNonQuery();
m_dbConnection.Close();
LoadUserDG();
}
}
Everything is fine now.
I hope this may work for you, too.
If someone can say how this happened, I would like to know the details to improve my knowledge, please.

I had the same issue when loading a lot of data to different tables from multiple threads.
When trying to do the inserts I was getting database locked because the program was doing too many insert too fast and SQLite didn't have time to complete each transaction before another one came.
The insert are done through threading because I didn't want the interface to be locked and wait for the insert to be done.
My solution is to use BlockingCollection with ThreadPool.QueueUserWorkItem.
This allows me to free the interface while doing the inserts.
All the insert are queued and executed in FIFO (First In First Out) order.
Now the database is never locked while doing any SQL transaction from any thread.
public class DatabaseQueueBus
{
private BlockingCollection<TransportBean> _dbQueueBus = new BlockingCollection<TransportBean>(new ConcurrentQueue<TransportBean>());
private CancellationTokenSource __dbQueueBusCancelToken;
public CancellationTokenSource _dbQueueBusCancelToken { get => __dbQueueBusCancelToken; set => __dbQueueBusCancelToken = value; }
public DatabaseQueueBus()
{
_dbQueueBusCancelToken = new CancellationTokenSource();
DatabaseQueue();
}
public void AddJob(TransportBean dto)
{
_dbQueueBus.Add(dto);
}
private void DatabaseQueue()
{
ThreadPool.QueueUserWorkItem((param) =>
{
try
{
do
{
string job = "";
TransportBean dto = _dbQueueBus.Take(_dbQueueBusCancelToken.Token);
try
{
job = (string)dto.DictionaryTransBean["job"];
switch (job)
{
case "SaveClasse":
//Save to table here
break;
case "SaveRegistrant":
//Save Registrant here
break;
}
}
catch (Exception ex)
{//TODO: Handle this exception or not
}
} while (_dbQueueBusCancelToken.Token.IsCancellationRequested != true);
}
catch (OperationCanceledException)
{
}
catch (Exception ex)
{
}
});
}
}
The inserts are done this way, but without the queuing I was still getting the lock issue.
using (SQLiteConnection c = new SQLiteConnection(BaseDal.SQLiteCon))
{
c.Open();
using (SQLiteCommand cmd = new SQLiteCommand(sql, c))
{
cmd.ExecuteNonQuery();
}
c.Close();
}

behaviour of try catch and finally with return statement workflow in c#

I have little doubt about this try, catch and finally with return statement workflow...
This function is used to retrieve employee leave information for supervisor view. It works very well, but if there data found for if statement it will be return otherwise else block will be return. Even if the both get returns, its going to finally statement.
I don't know why?
Code snippet here:
List<Leave> ILeaveData.GetLeaveForSupervisorView(int userID)
{
SqlConnection con = new SqlConnection(_connectionString);
SqlCommand cmd = new SqlCommand("Storeprocedurename", con);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.Add(new SqlParameter("#Id", SqlDbType.Int));
cmd.Parameters["#Id"].Value = userID;
// Get employee leave information
try
{
con.Open();
DataSet ds = new DataSet();
SqlDataAdapter adapter = new SqlDataAdapter();
adapter.SelectCommand = cmd;
adapter.Fill(ds);
if (ds.Tables[0].Rows.Count > 0)
{
List<Leave> leave = new List<Leave>();
for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
{
leave.Add(CreateLeaveForAdminViewFromDataRow(ds.Tables[0].Rows[i]));
}
return leave; // if data found then statement return here
}
else
{
return null; // otherwise return here
// throw new Exception("Data Error");
}
}
catch (SqlException err)
{
IErrorLog elog = new ErrorLog(_connectionString);
elog.LogSystemError(err);
throw new ApplicationException("Data Error", (Exception)err);
}
finally
{
if (con != null)
{
con.Close();
}
}
}
regards
Sarva

Finally statements are always executed, even if no exception occurs.
http://msdn.microsoft.com/en-gb/library/zwc8s4fz(v=vs.110).aspx:
Typically, the statements of a finally block run when control leaves a try statement. The transfer of control can occur as a result of normal execution, of execution of a break, continue, goto, or return statement, or of propagation of an exception out of the try statement.

finally block is designed to always execute, regardless of whether exception has been thrown or not.

Final block will be executed in all cases.

Getting a connection to database lost error

every once and a while my application throws a connection to database lost error.
The database class I got from a tutorial site and is below, it works great except for the above error sometimes, im guessing its timing out, like if the person using it goes for a smoke break and comes back and tries to continue where they left off.
And of coarse being the normal end-user they close then error message THEN come get me to tell me they got an error.
But until the error comes up again i thought i would ask what part of this code could be changed to prevent that error
this is a firebird db server and a c# application
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Windows.Forms;
using FirebirdSql.Data.FirebirdClient;
namespace _0912111
{
class DatabaseConnection
{
private FbConnection conn;
private FbCommand sqlCommand;
private FbDataAdapter DB;
private DataSet DS = new DataSet();
public DatabaseConnection()
{
conn = new FbConnection("User=myuser;" + "Password=mypw;" + "Database=dbpath;" + "DataSource=serverip;" + "Port=dbport;" + "Dialect=3;" + "Charset=UTF8;");
}
public void showDbError(string theError)
{
MessageBox.Show("Could not connect to database\n\nError Details:\n" + theError);
}
public FbConnection Openconn()
{
if (conn.State == ConnectionState.Closed || conn.State == ConnectionState.Broken)
{
try
{
conn.Open();
}
catch (Exception e)
{
showDbError(e.Message.ToString());
}
}
return conn;
}
public FbConnection Closeconn()
{
if (conn.State == ConnectionState.Open)
{
conn.Close();
}
return conn;
}
public void nonQuery(string txtQuery)
{
FbCommand cmd = new FbCommand(txtQuery);
try
{
cmd.Connection = Openconn();
cmd.ExecuteNonQuery();
}
catch (Exception Ex)
{
showDbError(Ex.Message.ToString());
throw Ex;
}
finally
{
cmd = null;
}
}
public FbDataReader returnDataReader(string txtQuery)
{
FbCommand cmd = new FbCommand();
try
{
cmd.Connection = Openconn();
cmd.CommandText = txtQuery;
FbDataReader rd;
rd = cmd.ExecuteReader();
return rd;
}
catch (Exception Ex)
{
showDbError(Ex.Message.ToString());
throw Ex;
}
finally
{
cmd = null;
}
}
}
}
I would think that the code in her that says
if (conn.State == ConnectionState.Closed || conn.State == ConnectionState.Broken)
{
would prevent it??

Look, I suggest you to use using and rewrite your methods in a cleaner way, no reason to have another method to open and another to close connection, I'm not doing it anymore since longer than 5 years :D
also, no reason to do a ToString() on Ex.Message and also, notice, in C# you should throw exceptions with only throw not throw exc.
one of your methods would become this for example:
public void nonQuery(string txtQuery)
{
using(var conn = new FbConnection(GetMyConnectionString(...parameters...)))
{
using(var cmd = new FbCommand(txtQuery))
{
try
{
cmd.Connection = conn;
conn.Open();
cmd.ExecuteNonQuery();
}
catch (Exception ex)
{
showDbError(ex.Message);
throw;
}
}
}
}

I'm 100% with Davide Piras on this. (upvoted him)
Delete the "Openconn" and "Closeconn" methods from your DatabaseConnection class. Then change your queries to have using statements for the connection open and command execution.
The database drivers already know how to perform connection pooling. Maintaining an open connection in code is not just a waste of time, but a potential cause of issues like the one you are experiencing. Other issues it can cause are leaked memory and the ability to open further connections with the database server.
So, rewrite your code to use best practices for database access and the problem will go away.

I'll leave this example that speaks of the connection and some examples that I hope will help.
http://code.msdn.microsoft.com/Esempio-applicazione-dati-494c129a
Regards.