I open a SqlConnection.
SqlConnection Conn = new SqlConnection(...);
Conn.Open();
...
Conn.Close();
Conn.Dispose();
//debugger breakpoint
When I look in my debugger at this breakpoint, the Conn.ServerVersion throws a Sql exception:
Connection Closed
Of course I closed the connection, as I should, but is this exception just something to ignore? Or am I supposed to be doing it differently if I wanted to avoid getting this exception, what would I need to do besides keep it open?
My understanding is not to have any exceptions in my code, but I may be wrong. (I am new)
Just avoid examining the connection object after you've disposed of it. ("Doctor, it hurts when I do this..." "Stop doing that then!") The easiest - and most reliable - way to do that is to use a using statement instead:
using (var conn = new SqlConnection(...))
{
conn.Open();
...
}
Then conn will be out of scope after it's been disposed anyway. Note that you don't need to call Close() as well as dispose - and note the more conventional name for the local variable.
You don't actually have a problem.
As you noticed in your question, you can't get the server version from a closed connection.
When you look at that property in the debugger, you will therefore get an exception.
As long as you don't try to access it in actual code from a closed connection, you're perfectly fine.
Related
Am I closing my SQL connection correctly, by placing it inside a "using" block?
This is how I grab a new connection, execute a query, and return the results:
using (SqlConnection objCS = DB.LMSAdminConn())
{
objCS.Open();
SqlCommand objCommand = new SqlCommand("SELECT TOP 1 * FROM users WHERE userid = #userid", objCS);
objCommand.Parameters.Add("#userid", SqlDbType.Int).Value = userid;
SqlDataReader reader = objCommand.ExecuteReader();
while (reader.Read())
{
//do something
}
reader.Close();
}
The connection itself comes from this call:
public static SqlConnection LMSAdminConn()
{
return new SqlConnection(ConfigurationManager.ConnectionStrings["lmsadmin"].ToString());
}
I am opening the connection inside a "using" block, and I thought that the connection would be closed as well, because it is opened inside the "using" block. But since the "new SqlConnection" object is actually generated from an outside class, is my connection still getting appropriately closed? SQL Server shows the connection as still being open, but I'm not sure if that is ADO.NET connection pool recycling / sharing, or if the connection is truly being held open. I don't explicitly call .Close() on the connection inside the "using" block.
Do I need to explicitly close the SqlCommand and SqlDataReader objects as well, or are they disposed when we leave the "using" block as well?
A using block is essentially syntactic sugar for having a try/finally block that calls the Dispose method of the object it is acting on, it doesn't matter where that object was created.
For a SqlConnection object, calling Dispose will close the connection. From the docs:
If the SqlConnection goes out of scope, it won't be closed. Therefore, you must explicitly close the connection by calling Close or Dispose. Close and Dispose are functionally equivalent.
Yes, it will close the connection once it loses scope.
Sample:
using (SqlConnection sqlConn = new SqlConnection("myConnectionString"))
{
sqlConn.Open();
...
}
This code will be converted into the following by the compiler :
try
{
SqlConnection sqlConn = new SqlConnection("myConnectionString");
sqlConn.Open();
...
}
finally
{
sqlConn.Close();
}
As you can see the close() is called in the finally block.
This finally block will force close the connection even if there is an exception during run-time within the using block.
You had an idea about connection pools - this is the right idea!
ADO.NET it is so beautiful that it tries to optimize everything and store it in connection pools. But we still need to always close connections!
I need a to get a bit of understanding in this, When you open a connection to a Database can you leave it open?
How does this connection close?
Is it good practise or bad practice?
Currently I have a request to a database that works no problem
oCON.Open();
oCMD.ExecuteNonQuery();
oCON.Close();
However Some of the examples that I have seen are something like this with no database close.
oCON.Open();
oCMD.ExecuteNonQuery();
How would this connection get closed?
Is this bad practice?
I was looking for a duplicate, as this seems to be a common question. The top answer I found is this one, however, I don't like the answer that was given.
You should always close your connection as soon as you're done with it. The database has a finite number of connections that it allows, and it also takes a lot of resources.
The "old school" way to ensure the close occurred was with a try/catch/finally block:
SqlConnection connection;
SqlCommand command;
try
{
// Properly fill in all constructor variables.
connection = new SqlConnection();
command = new SqlCommand();
connection.Open();
command.ExecuteNonQuery();
// Parse the results
}
catch (Exception ex)
{
// Do whatever you need with exception
}
finally
{
if (connection != null)
{
connection.Dispose();
}
if (command != null)
{
command.Dispose();
}
}
However, the using statement is the preferred way as it will automatically Dispose of the object.
try
{
using (var connection = new SqlConnection())
using (var command = new SqlCommand())
{
connection.Open();
command.ExecuteNonQuery();
// Do whatever else you need to.
}
}
catch (Exception ex)
{
// Handle any exception.
}
The using statement is special in that even if an exception gets thrown, it still disposes of the objects that get created before the execution of the code stops. It makes your code more concise and easier to read.
As mentioned by christophano in the comments, when your code gets compiled down to IL, it actually gets written as a try/finally block, replicating what is done in the above example.
You want your SqlConnection to be in a using block:
using(var connection = new SqlConnection(connectionString))
{
...
}
That ensures that the SqlConnectionwill be disposed, which also closes it.
From your perspective the connection is closed. Behind the scenes the connection may or may not actually be closed. It takes time and resources to establish a SQL connection, so behind the scenes those connections aren't immediately closed. They're kept open and idle for a while so that they can be reused. It's called connection pooling. So when you open a connection, you might not really be opening a new connection. You might be retrieving one from the connection pool. And when you close it, it doesn't immediately close, it goes back to the pool.
That's all handled behind the scenes and it doesn't change what we explicitly do with our connections. We always "close" them as quickly as possible, and then the .NET Framework determines when they actually get closed. (It's possible to have some control over that behavior but it's rarely necessary.)
Take a look at the Repository Pattern with Unit of Work.
A connection context should be injected into the class which operates commands to the database.
A sql execution class - like a repository class represents - should not create a connection. It is not testable and hurts the paradigm of SRP.
It should accept an IDbConnection object like in the constructor. The repository should not take care if behind the IDbConnection is an instance of SqlConnection, MysqlConnection or OracleConnection.
All of the ADO.NET connection objects are compatible to IDbConnection.
I'm working on an ASP.NET application where, as part of some logic, I want to lock some tables and do work on them. The method runs in a separate thread running as a kind of background task, spawned via a Task. The problem comes in with the error handling...
The code looks more or less like this:
MySqlConnection connection = new MySqlConnection(ConfigurationManager.AppSettings["prDatabase"]);
try
{
connection.Open();
MySqlCommand lock_tables = new MySqlCommand(Queries.lockTables(), connection);
lock_tables.ExecuteNonQuery();
// do a bunch of work here
MySqlCommand unlock_tables = new MySqlCommand(Queries.unlockTables(), connection);
unlock_tables.ExecuteNonQuery();
}
catch (MySqlException mex)
{
// Mostly error logging here
}
finally
{
connection.Close();
}
Pretty simple stuff. Everything works fine and dandy assuming nothing goes wrong. That's a terrible assumption to make, though, so I deliberately set up a situation where things would foul up in the middle and move to the finally block.
The result was that my table locks remained until I closed the app, which I learned by trying to access the tables with a different client once the method completed. Needless to say this isn't my intention, especially since there's another app that's supposed to access those tables once I'm done with them.
I could quickly fix the problem by explicitly releasing the locks before closing the connection, but I'm still left curious about some things. Everything I've read before has sworn that closing a connection should implicitly release the table locks. Obviously in this case it isn't. Why is that? Does connection.Close() not actually completely close the connection? Is there a better way I should be closing my connections?
Try wrapping your Connection and MySqlCommand instance in a using statement. That will release the objects as soon as it leaves the brackets.
using(MySqlConnection conn = new MySqlConnection(connStr))
{
conn.Open();
using(MySqlCommand command = new MySqlCommand("command to execute",conn))
{
//Code here..
}
}
I am having difficulty in reading data from my SQLite database from MonoTouch.
I can read and write without any difficulty for the first few screens and then suddenly I am unable to create any further connections with the error:
Mono.Data.Sqlite.SqliteException: Unable to open the database file
at Mono.Data.Sqlite.SQLite3.Open (System.String strFilename, SQLiteOpenFlagsEnum flags, Int32 maxPoolSize, Boolean usePool) [0x0007e] in /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLite3.cs:136
at Mono.Data.Sqlite.SqliteConnection.Open () [0x002aa] in /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLiteConnection.cs:888
I ensure that i dispose and close every connection each time i use it but still i have this problem. For example:
var mySqlConn = new SqliteConnection(GlobalVars.connectionString);
mySqlConn.Open();
SqliteCommand mySqlCommand = new SqliteCommand(SQL, mySqlConn);
mySqlCommand.ExecuteNonQuery();
mySqlConn.Close();
mySqlCommand.Dispose();
mySqlConn.Dispose();
I'm guessing that I'm not closing the connections correctly. Any help would be greatly appreciated.
I'm pretty sure you guess is right. However it's pretty hard to guess what went wrong (e.g. what's defined in your connectionString will affect how Sqlite is initialized and will work).
From your example you seem to be disposing the SqliteConnection correctly but things could still go wrong. E.g. if some code throws an exception (and you catch them somewhere) then the Dispose call might never be called. It would be safer to do something like:
using (var mySqlConn = new SqliteConnection(GlobalVars.connectionString) {
mySqlConn.Open();
using (SqliteCommand mySqlCommand = new SqliteCommand(SQL, mySqlConn)) {
mySqlCommand.ExecuteNonQuery();
// work with the data
}
mySqlConn.Close();
}
That would ensure that the automagically finally clauses will dispose of the instance you create.
Also you might want to consider reusing your (first) connection instance, e.g. opening it once and re-use it everywhere in your application. OTOH you need to be aware of threading in this case (by default, you can change it, each connection is only safe to use on the thread that has created it).
Reusing could help your app performance but it also does not really fix your issue (but it might hide it). So I suggest you try to debug this first:
Using MonoDevelop you can set a breakpoint on line #136 on the /Developer/MonoTouch/Source/mono/mcs/class/Mono.Data.Sqlite/Mono.Data.Sqlite_2.0/SQLite3.cs file (which is included with your MonoTouch installation) to see the actual n error code (before it gets translated to a string).
You can also set breakpoints on the dispose code to ensure it gets executed (and does not return errors). The number of connection creations and disposals should match. If not then use the Call Stack to see who's opening without closing.
I would suggest using the "using" block..That will make sure that everything is disposed off correctly and also that you are not closing connections when it is already closed..
using (SqliteConnection conn = new SqliteConnection(GlobalVars.connectionString))
{
conn.Open ();
SqliteCommand command = new SqliteCommand (conn);
.............
}
OK - i've got it working now by moving the close and dispose into a "finally".
var mySqlConn = new SqliteConnection (GlobalVars.connectionString);
mySqlConn.Open ();
try {
// CODE HERE
} finally {
mySqlConn.Close();
mySqlConn.Dispose();
}
I am doing my database access methods to SQL Server like this
using (SqlConnection con = new SqlConnection(//connection string)
{
using (SqlCommand cmd = new SqlCommand(storedProcname, con))
{
try{
con.open();
//data reader code
}
catch
{
}
}
}
Do I need to be closing or disposing of SqlCommand, or will the using statement take care of that for me? I just don't want connection hanging open
Thanks
The using will take care of it for you. Under the hood, SqlConnection.Dispose() calls the SqlConnection.Close() method, and SqlCommand.Dispose() calls SqlCommand.Close().
As additional background, a using statement is syntactic sugar for a try ... finally that disposes the IDisposable object in the finally.
As an aside, you can make the code more concise and readable as follows:
using (SqlConnection con = new SqlConnection(/*connection string*/))
using (SqlCommand cmd = new SqlCommand(storedProcname, con))
{
//...
}
As Phil said, the using clause will take care of it for you. When compiled down it wraps the connection create in a try .. finally and places the connection disposal call inside the finally.
For more information you can see the using statement article at msdn.
Yes your code will close the connection, however that typcally means release back to the connection pool to be truely closed later.
If you execute this snippet of code, and then do an sp_who and observe that your connection is still there, that would be why.
If you absolutely need the connection truely closed (an edge case to be sure) then use the ClearAllPools
static method of ths SqlConnection
Using keyword will automatically close the connection for you
so you don't need to worry about calling connection.close() at the end every time.
when the scope
using (SqlConnection con = new SqlConnection(//connection string)
{
}
will over , connection will automatically be disposed by runtime. so don't worry
I think "using" was not required for SqlCommand. "Using" for SqlConnection would have done the job for you alone.
In fact you connection is submitted to Connection pool.