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.
Related
I am doing windows forms application with connection to SQL Server (disconnected mode). Every action I handle in my application needs the connection. So I don't know where to close the connection.
I am asking what if i don't close the connection. I know that there is no runtime error that appears. But is there any other error that could appear?
There is a risk that if you are using the same connection for each method, from a private field or GetConnection() method etc.; you could end up trying to read/write when the connection is still open.
This could then throw errors and you could end up in a whole heap of it, if using a live SQL database.
If you can, create a new instance of a connection in a using statement each time you want to access the DB.
Like this:
var connectionString = "YourConnectionStringToTheDB";
var queryString ="YourSQLQueryHere"; // FROM * IN tbl SELECT * etc...
using (SqlConnection connection = new SqlConnection(
connectionString))
{
using (SqlCommand command = new SqlCommand(queryString, connection))
{
command.Connection.Open();
command.ExecuteNonQuery();
}
}
This way it is closed and disposed of when the code runs outside of the using statement's scope.
Notice that the SqlCommand is also in a using statement, as it too can be disposed of.
This is cleaner code and marks the objects for disposal by the Garbage Collector. (Google is your friend) - BIG topic; a lot of stuff to read.
Here is a similar question which gives you some more detail: The C# using statement, SQL, and SqlConnection
EDIT
Better still, you could wrap your SQL code in a try { } catch { } block to handle any errors at runtime.
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 have the code below to query records from a stored procedure but am concerned I may not be disposing what I need to or am disposing when the object would be cleared by the Garbage Collector shortly afterward anyway.
Do I need to dispose the SqlDataReader since it is within the try catch block?
Do I need to run both cmd.Dispose and cmd.Connection.Close or does one infer the other?
Would the garbage collector eventually dispose of all these objects anyway (maybe not timely enough) or do these objects implictly require dispose possibly because of using unmanaged code?
public void GetData(string studentID)
{
SqlCommand cmd = new SqlCommand("sp_stored_proc",
new SqlConnection(Settings.Default.connectionString))
{ CommandType = CommandType.StoredProcedure };
try
{
cmd.Connection.Open();
cmd.Parameters.AddWithValue("#student_id", studentID);
SqlDataReader dr = cmd.ExecuteReader();
//do something with the data
if (dr != null)
dr.Dispose();
}
catch
{
//error handling
}
finally
{
if (cmd != null)
{
cmd.Dispose();
cmd.Connection.Close();
}
}
}
You should dispose the data reader, and the command. No need to separately close the connection if you dispose the command. You should ideally do both using a using block:
using (SqlCommand cmd = new...)
{
// do stuff
using (SqlDataReader dr = cmd.ExecuteReader())
{
// do stuff
}
}
If you need exception handling do that separately either inside or around the using blocks - no need for the finally for the Dispose calls though with using.
If you use something like this:
public void GetData(string studentID)
{
using (SqlConnection connection = new SqlConnection(Settings.Default.connectionString))
{
connection.Open();
using (SqlCommand command = connection.CreateCommand())
{
command.CommandType = CommandType.StoredProcedure;
command.CommandText = "sp_stored_proc";
command.Parameters.AddWithValue("#student_id", studentID);
using (SqlDataReader dataReader = command.ExecuteReader())
{
// do something with the data
}
}
}
}
then all of your Disposable objects will get disposed of correctly. Calling Dispose() on the SqlConnection, SqlCommand and SqlDataReader objects (which is what the using block does when it exits) closes them correctly.
Additionally, this approach keeps all of your variables scoped to where they are used.
The downside to this approach is that if you need error handling using a try/catch, you have to either wrap it around the whole method body, or use several of them to handle connection errors differently from reading errors, etc...
Do I need to dispose the SqlDataReader
since it is within the try catch
block?
-- Yes, as being inside of the try catch will not call the dispose method.
Do I need to run both cmd.Dispose and cmd.Connection.Close or does one infer the other?
-- Yes, you need to run both. Calling Cmd.dispose does not close the connection.
The dispose method is meant to be used by the programmer to clean up resources which either aren't directly managed by the garbage collector, or the need to be cleared out after the program is done using them to free up space. Technically, one could set up the program so the GC would handle it's disposal, but that's an assumption I wouldn't make, especially since the programmer writing the class exposed the dispose method for you. Putting the Command in a using statement is probably the easiest route, because you know it will get disposed when the code leaves the declaration space.
using (var connection = new Connection ())
{
using (var cmd = new Command())
{
}
}
Personally if something has a dispose method then it is worth using it anyway as they will prevent protential memory leaks.
To make a long story short; if it implements IDisposable, you should call Dispose.
Even if you use Reflector to figure out that Dispose in one object invokes Dispose on another object, I would still recommend to call Dispose on both, since this is internal implementation details that may change in some future release, so you should not rely on that always being true.
So, Dispose anything that is IDisposable.
you should open Connection firstly by Connection.Open();
then use the methods such as SqlDataReader to read
after all,close SqlDataReader firstly and then close connection
you can use keyword "using" to dispose it, but it is not a good idea
in fact the keyword "using" is to dispose the object automatically.
in other word,the object should achieve dispose method
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.