I've read that in PHP non-persistent open links are automatically closed at the end of the script's execution. So what's up with asp.net?
Simple: PHP and ASP.NET aren't the same thing. They have different "rules". Don't try to write code in one environment as if it were the other.
In .NET should almost always open a connection as late as you can and close it as early as you can, letting the built-in connection pool handle the "real" connection to the database. Don't leave it to the GC/finalizer to tidy up connections for you.
Yes you need to close open connections to database. ADO.NET has a connection pool, and open connections are reserved to you while it stays opened. When you close it, it will be available to other connection requests.
But if you're using DataAdapter, and you don't open connection manually, you don't need to close it. DataAdapter manages it and closes it's own connection :
SqlConnection nwindConn = new SqlConnection("Data Source=localhost;Integrated Security=SSPI;Initial Catalog=northwind");
SqlCommand selectCMD = new SqlCommand("SELECT CustomerID, CompanyName FROM Customers", nwindConn);
selectCMD.CommandTimeout = 30;
SqlDataAdapter custDA = new SqlDataAdapter();
custDA.SelectCommand = selectCMD;
DataSet custDS = new DataSet();
custDA.Fill(custDS, "Customers");
The easiest way to do this is with a using block:
var cmd = new MySqlCommand("command text here");
using (var con = new MySqlConnection("connection string")) {
cmd.Connection = con;
cmd.Connection.Open();
//execute command here
}
The connection object will be automatically disposed when you leave the scope.
Well, its a good practice to release unmanaged data manually, although the finalizer would take care of it eventually.
ASP.Net is said to be "managed", but that really only refers to memory.
When your page instance goes out of scope your are guaranteed that the memory object which wraps the connection will eventually be collected and cause it to be closed, but you are not guaranteed when that will happen. Since it's much more efficient to release memory in larger batches, your connection may hang around a while, and if you get a lot of these that can cause a big problem.
So, you should make sure your connection is closed.
Since most of the connection objects implement IDisposable and have finalizers you could technically rely on the garbage collector to close the connection for you since the finalizer will call Close() on the connection. Obviously this is a bad idea - wrap your connection objects in using statements or explicitly close them when you are done with them.
Since I require 50 reputation to comment on someones post, I am posting my comment to Adam Lassek as an answer. His approcah is the corrct way to go. The Using satement will clean up the unmanaged code.
Related
I am trying to design an efficient application that connects to a SQL Database and I was wondering what the merits/demerits of creating a SQL connection for each SQL query is,
Eg, Like the code below, making a class with the connection set once so you can call on it when ever you need it in the class.
class SqlQuery
{
SqlConnection Connection = new SqlConnection(#myConnectionString);
public void fillInfoData()
{
SqlCommand updateCommand = new SqlCommand(
#"my sql query", Connection);
Connection.Open();
updateCommand.ExecuteNonQuery();
Connection.Close();
}
}
Or should you create a new connection for each new query?
EDIT: Jon Skeet recommended creating a new connection for each query,
I was wondering why?
EDIT No 2: He stated it in this answer.
You open a connection per query and then when you close it, it goes back to the connection pool and the framework will manage the rest
So you do not have to worry about keeping connection alive, timeouts etc...
MSDN Article
https://msdn.microsoft.com/en-us/library/8xx3tyca(v=vs.110).aspx
I think what he meant was not to keep a global connection object that is open. But you need to create an SqlConnection when you need it, execute all the queries needed at the point (Not a single connection per each query) and then close the connection.
So if you have a bunch of queries that needs to be executed, you don't need a single sql connection for each but one is only enough. But if you're executing a query each time the user clicks a button for example, it's better to open a connection on each click, because that way it is easier to manage closing and disposing the connection.
Just a general question, if I open an OleDbConnection in my program, should I close it at some point? I only ask because I've seen a few tutorials where the presenter doesn't include a statement to close the connection.
In my specific circumstances, I open a connection to access an Excel file, fill a DataTable and grab some values. After that though, there is no reason for me to have the connection open and I'm thinking it would probably cause some issues if I left it open.
Also, is the statement conn.Close(); sufficient to close the connection?
Yes, you should close your connection as soon as you are done with it. If you use your connection in one method and not immediately after it again, close and dispose it, so it can be cleaned up.
You should wrap the creation of a connection in a using statement since that will even close and dispose the connection when an exception occurs.
using (OleDbConnection conn = new OleDbConnection(...))
{
// use the connection inside here
}
You should make use of the using block which will ensure the connection is closed and disposed correctly.
using (OleDbConnection connection = new OleDbConnection(connectionString))
{
connection.Open();
//Do some work
}//The connection is automatically closed when the code exits the using block.
Further reading here
I am working on a project which, up until today, has been fine. However now when I run it and it goes through a few different Stored Procedure calls it is throwing an InvalidOperationException with the message The connection was not closed. The connection's current state is open.
I get that I could put in a check to see if the connection is already open, but this code hasn't changed (it is under Version Control and isn't modified) so I'm looking for other potential explanations.
Could there be some lock in SQL which isn't being released? Is there a process which I should look out for and kill?
I can't really post the code as there is a lot of it, and it is split up into smaller methods which makes it harder to pull out individual items. E.g:
public SqlConnection Connection
{
get
{
this._connection.Open();
return _connection;
}
}
public IDataReader RetrieveRecord(int Id)
{
//SP
SqlCommand cmd = this.Connection.CreateCommand();
cmd.CommandText = "SelectRecord";
cmd.CommandType = CommandType.StoredProcedure;
//Parameters
cmd.Parameters.Add(new SqlParameter("#tID", Id));
//instruct the data reader to close its connection when its Close method is called by passing the CommandBehavior.CloseConnection
return cmd.ExecuteReader(CommandBehavior.CloseConnection);
}
Nothing massively complex, I just don't understand why this connection is now throwing an exception.
The DAL is not stable enough:
You open a Connection that is not closed if something goes wrong in RetrieveRecord.
You should create a new connection inside RetrieveRecord and close it there in a finally block.
Opening a connection is cheap thanks to Connection Pooling.
Problem is with the line
this._connection.Open();
Because you are trying to open an already opened connection.
Try this to check before opening a connection:
if (this._connection.State == ConnectionState.Closed)
this._connection.Open();
Sorted. Two reboots cleared whatever was keeping the connection open.
When you are talking about killing process, I think you know c# well.
You should always use
using()
{
}
Like:
using(SqlConnection con=new SqlConnection("connectionString"))
{
// Do something with con
using(SqlCommand cmd=new SqlCommand("cmdText",con))
{
// Do something with cmd
}
}
You know that SqlCommand and SqlConnection implement IDisposable
So when you put those objects within using, the connection closing and clean up job is automatically done.
No need to close the connection manually in the code, since using will do the work for you.
I'm making a web application in C# with ASP.Net, and I'm using an SQL database for it. In the guides I saw (about SqlConnection and SqlCommand) they open and close the SQL connection each time they want to send a query. But I'm not sure this is the best way to handle the connection, because I also learned some PHP, and as far as I know in PHP you open a connection only once, at start. So what is the best way to handle the connection?
You should generally have one connection, and transaction, for the length of the web request.
If you have two connections, you could potentially have inconsistent data. e.g. in the first query you check the bank balance, then you close it. You then add $5 to the balance and save that away in the next connection. What if someone else added $5 between the two connections? It would go missing.
The easiest thing to do is to open the connection global.asax BeginRequest and save that away into the HttpContext. Whenever you need a connection, pull it from there. Make sure that you .Close the connection in your EndRequest
Also, read up here on connection pooling: http://msdn.microsoft.com/en-us/library/8xx3tyca.aspx
It is not 'expensive' to repeatedly open and close connections, provided the username is the same each time.
Ado.Net has connection pooling already managed for you, so you don't need to worry about reusing connections.
You could use the Using statement to close and dispose connection:
DataTable dt = new DataTable();
using (SqlConnection conn = new SqlConnection("my_connection_string"))
{
using (SqlDataAdapter adapter = new SqlDataAdapter("SELECT * from Table", conn))
{
conn.open();
adapter.Fill(dt);
}
}
Normal practice in .NET is to open a SqlConnection and use a SqlCommand, then dispose of both of them.
using(SqlConnection connection = new SqlConnection("myConnectionString"))
{
using(SqlCommand command = new SqlCommand("dbo.SomeProc"))
{
// Execute the command, when the code leaves the using block, each is disposed
}
}
As a connection to database is an expensive resource it's better to open sql connection as late as possible and close early. So you should open the connection just before executing a command and close it once you have executed it.
On the other hand if you are going to execute many commands in a short interval of time it's better to open the connection once and close after all the commands are executed.
For starters they do not as there is a connection pool that is being used, but SqlCommand has an overload to take a connection object, so you could keep the connection around and pass it.
Generally:
using (var ts = new TransactionScope())
{
using (var connection = new SqlConnection(...))
{
using (var command = new SqlCommand(connection, ...)
{
...
}
using (var command = new SqlCommand(connection, ...)
{
...
}
}
ts.Complete();
}
Of course I would recommend using LINQ-to-SQL or EF at this point.
Try Data Access Application Block in the Enterprise Library if you do not want to worry about opening and closing the connections.
What would happen if you call Close() on a SqlConnection object before you call Close() on a SqlDataReader using that connection?
Actually, what I really want to know is whether or not the order in which you Close them matters. Does calling SqlConnection.Close() completely close the connection, or will it remain open if you do not call Close() on a SqlDataReader using that connection?
Sorry for the multiple questions, but I don't think I really understand how connection closing works.
It'll close the connection (returns it to the pool) and SqlDataReader would throw an exception (System.InvalidOperationException) in case it's used afterward.
Rather than worrying about the order on how to close them, why not wrap them in using statements.
// the following code has parts left out for brevity...
using(var conn = new SqlConnection( ... ))
using(var cmd = new SqlCommand( ... ))
{
conn.Open();
using(var reader = cmd.ExecuteReader())
{
// do whatever with the reader here...
}
}
the using statements ensure that your objects are closed, and in the correct order if you nest them accordingly. See http://msdn.microsoft.com/en-us/library/yh598w02.aspx for more info.
The actual database connection will be closed (returned to the pool) when you close/dispose the SqlConnection object, so the database resources are safe.
However, you should close/dispose the SqlDataReader also, otherwise it will keep a reference to the SqlConnection object, keeping them both in memory. Eventually the finalizer will dispose the SqlDataReader object if you don't do it, and then the SqlConnection object can also be collected, but you have no control over when that happens.
The SqlDataReader can be partially usable after you have closed the connection, but nothing that you really can rely on. It still has some data in it's buffer, so some operations may work, but anything that requires more data from the database will cause an exception.