I am designing a database wrapper for C#.
Below are the two options I have:
Option A:
class DBWrapper:IDisposable
{
private SqlConnection sqlConn;
public DBWrapper()
{
sqlConn = new SqlConnection("my connection string");
sqlConn.Open();
}
public DataTable RunQuery(string Sql)
{
implementation......
}
public Dispose()
{
if(sqlConn != null)
sqlConn.Close();
}
}
Option B:
class DBWrapper
{
public DBWrapper()
{
}
public DataTable RunQuery(string Sql)
{
SqlConnection sqlConn = new SqlConnection("my connection string");
.....implementation......
sqlConn.Close();
}
}
For option A connection is opened when class is instantiated. So no matter how many times the caller calls RunQuery the connection is always ready. But If the application instantiates DBWrapper early in the application, the connection will be just opened and doing nothing until the application is finished. Also, it could have many DBWrapper instantiated during the execution. So, it's kinda wasting resources.
For option B it doesn't have the problem option A has, but the a new connection has to be opened and closed everytime the caller calls RunQuery. I am not sure how much it will hurt the performance.
Please share your expertise. Thank you for reading.
For performance reasons, you'll definitely not want to go with Option B (at least in the cases I experienced.)
Let me suggest Option C:
class DBWrapper:IDisposable {
private SqlConnection sqlConn;
public void EnsureConnectionIsOpen()
{
if (sqlConn == null)
{
sqlConn = new SqlConnection("my connection string");
sqlConn.Open();
}
}
public DataTable RunQuery(string Sql)
{
EnsureConnectionIsOpen();
implementation......
}
public Dispose()
{
if(sqlConn != null)
sqlConn.Close();
}
}
You might consider using the singleton pattern to make sure there is only one instance of your DBWrapper.
A few comments worth considering:
In the approach where you manage a (perhaps) long-lived connection, it is important to check whether the connection is open before running a query. I've run into issues before where NETCF closed unused connections after a while.
In the approach where you open a new connection per-query, ensure that your connection, commands, and (if used) data readers are all properly wrapped in using statements or try/finally+dispose() blocks to free up connections and locks.
Happy coding!
Garbage collector is triggered under rather complex conditions but basically it is invoked when memory exceeds some limit, it is invoked periodically as well but the period is not constant. You never can be sure when exactly garbage collector disposes and consequently (in another run) destroys the object. One thing you can be sure is the fact that garbage collector will never dispose and destroy the object that still has references. For example object that is referenced via static variables on the class neither will be disposed nor destroyed.
Option B is more transactional, which has its advantages. ADO.NET uses implicit connection pooling, so you do not have to worry about creating new instances of SqlConnection frequently.
You should consider whether you are using a connected or disconnected data model; as the second approach lends itself better to a disconnected model.
But as i've said above, connection pooling means that it makes virtually no difference in practical terms.
You could have an option C where the database is opened on request in RunQuery (if it is not open) and closed on disposed (when it was opened). That way the database is only opened when really needed and will be opened only once.
So in pseudo code:
class DBWrapper
{
public DBWrapper()
{
}
SqlConnection sqlConn = null;
public DataTable RunQuery(string Sql)
{
if(sqlConn == null) sqlConn = new SqlConnection("my connection string");
.....implementation......
}
public Dispose()
{
if(sqlConn != null)
sqlConn.Close();
}
}
Also mind that the moment Dispose is called is not always directly after the object is not needed anymore (for example a function variable after the function is used). As far as I know it will be executed when the garbage collector collects the object (which is not directly). But I am not totally sure about this. This behaviour might also differ between web and non web application.
Related
I'm creating an winforms application.
In which one form is made transparent, This form is used to show some popup message boxes, using a timer this form queries database in each seconds.
Currently I'm using database connection inside using method (here postgres Data Base).
Method 1
namespace MyApplication
{
public partial class frmCheckStatus: Form
{
private void timerCheckStatus_Tick(object sender, EventArgs e)
{
using (NpgsqlConnection conn = new NpgsqlConnection("My Connection String"))
{
conn.Open();
//Database queries
//Show popup message
conn.Close();//Forsing to close
}
}
}
}
so in each seconds this connection object is created and disposed.
Note : I'm not using this object for any other purpose or inside any forms or methods.
Is it good to create and use a single connection object global to this class, and use inside timer tick function?, and dispose on form close event
Method 2
namespace MyApplication
{
public partial class frmCheckStatus: Form
{
Private NpgsqlConnection conn = new NpgsqlConnection("My Connection String");
private void timerCheckStatus_Tick(object sender, EventArgs e)
{
//Here use conn object for queries.
conn.Open();
//Database queries
//Show popup message
conn.Close();//Forsing to close
}
private void frmCheckStatus_FormClosing(object sender, FormClosingEventArgs e)
{
conn.Dispose();
}
}
}
Which will be better?, considering memory, resource usage, execution time etc. Please give proper reason for your choice of method.
Looking at the documentation for your connection class (Here), it would appear that this supports connection pooling. This will mean that connections to the same endpoint (same connection string) will reuse existing connections rather than incurring the overhead of creating new ones.
Im not familiar with your particular connection, but if the behaviour is anything like SQLConnection class for ADO.net, repeatedly creating a new connection to the same connection string should not be particularly expensive (computationally).
As an aside, i would wrap your connection logic in try / finally to ensure it gets closed in the event of an application exception.
I can't see any advantage to instantiating a new connection every time you run a new query. I know it's done often in code, but there is overhead associated with it, however small. If you're running multiple queries from the start of the program to the end of the program, I think you should re-use the existing connection object.
If your goal is to make the connection "disappear" from the server (which I wouldn't generally worry about if this program runs on one machine -- if it runs on dozens, that's another story -- look up PgBounce), then that should be just as easily accomplished by turning connection pooling off, and then the Close() method would take care of it.
You kind of asked for pros and cons, and while it's not necessarily harmful to instantiate the connection within the loop, I can't imagine how it could be better.
For what it's worth, you may want to consider carrying the connection as a property (preferably outside of the form class, since you may want to eventually use it elsewhere). Something like this:
private NpgsqlConnection _PgConnection;
public NpgsqlConnection PgConnection
{
get
{
if (_PgConnection == null)
{
NpgsqlConnectionStringBuilder sb = new NpgsqlConnectionStringBuilder();
sb.Host = "hostname.whatever.com";
sb.Port = 5432;
sb.UserName = "scott";
sb.Password = "tiger";
sb.Database = "postgres";
sb.Pooling = true;
_PgConnection = new NpgsqlConnection(sb.ToString());
}
if (!_PgConnection.State.Equals(ConnectionState.Open))
_PgConnection.Open();
return _PgConnection;
}
set { _PgConnection = value; }
}
Then, within your form (or wherever you execute your SQL), you can just call the property:
NpgSqlCommand cmd = new NpgSqlCommand("select 1", Database.PgConnection);
...
Database.PgConnection.Close();
And you don't need to worry if the connection is open or closed, or if it's even been created yet.
The only open question would be if you want that connection to actually disappear on the server, which would be changed by altering the Pooled property.
I was using this code:
public class SQLConnection : IConnection
{
private SqlConnection _sqlConnection = null;
//bunch of interface implementations for my project
//the destructor
~SQLConnection()
{
if(_sqlConnection != null)
{
if(_sqlConnection.State == ConnectionState.Open)
{
_sqlConnection.Close();
}
_sqlConnection.Dispose();
}
}
}
This was working well until some time ago, when I started receiving this error: Internal .Net Framework Data Provider error 1
After googling a bit, I found this link (the Caution part) and I think that's what's happening to me.
My class was managing the connection state opening and closing each time, but now that it seems I can't manage it this way, is there any other way that I can do this without having to go to every function that I use the connection and call connection.Close() explicitly?
Always use 'using' for safely disposing the connections.
using(var _sqlConnection = new SqlConnection())
{
//code here
_sqlConnection.Open();
}
//Safely disposed.
Also, it is never a good idea to use destructor explicitly in C# code unless you have unmanaged code.
You're probably receiving this error when one code is trying to access SQLConnection that has been already garbage collected. This usually will happen in cases like this one
SqlConnection sqlConn;
using (var sqlConnection = new SqlConnection())
{
sqlConn = sqlConnection;
sqlConnection.Open();
}
sqlConn.Close();
I know this example looks silly as it's overly simplified, but it happens often when programmers tend to share a connection object between Managing Classes. Look for any recent changes in your code. Maybe you're passing an instance of a connection between multiple objects. One of those objects gets Garbage Collected and in turn the connection gets disposed too.
I'm working on an ASP.NET MVC application which uses Linq to SQL to connect to one of about 2000 databases. We've noticed in our profiling tools that the application spends a lot of time making connections to the databases, and I suspect this is partly due to connection pool fragmentation as described here: http://msdn.microsoft.com/en-us/library/8xx3tyca(v=vs.110).aspx
Many Internet service providers host several Web sites on a single
server. They may use a single database to confirm a Forms
authentication login and then open a connection to a specific database
for that user or group of users. The connection to the authentication
database is pooled and used by everyone. However, there is a separate
pool of connections to each database, which increase the number of
connections to the server.
There is a relatively simple way to avoid this
side effect without compromising security when you connect to SQL
Server. Instead of connecting to a separate database for each user or
group, connect to the same database on the server and then execute the
Transact-SQL USE statement to change to the desired database.
I am trying to implement this solution in Linq to Sql so we have fewer open connections, and so there is more likely to be a connection available in the pool when we need one. To do that I need to change the database each time Linq to Sql attempts to run a query. Is there any way to accomplish this without refactoring the entire application? Currently we just create a single data context per request, and that data context may open and close many connections. Each time it opens the connection, I'd need to tell it which database to use.
My current solution is more or less like this one - it wraps a SqlConnection object inside a class that inherits from DbConnection. This allows me to override the Open() method and change the database whenever a connection is opened. It works OK for most scenarios, but in a request that makes many updates, I get this error:
System.InvalidOperationException: Transaction does not match
connection
My thought was that I would then wrap a DbTransaction object in a similar way to what I did with SqlConnection, and ensure that its connection property would point back to the wrapped connection object. That fixed the error above, but introduced a new one where a DbCommand was unable to cast my wrapped connection to a SqlConnection object. So then I wrapped DbCommand too, and now I get new and exciting errors about the transaction of the DbCommand object being uninitialized.
In short, I feel like I'm chasing specific errors rather than really understanding what's going on in-depth. Am I on the right track with this wrapping strategy, or is there a better solution I'm missing?
Here are the more interesting parts of my three wrapper classes:
public class ScaledSqlConnection : DbConnection
{
private string _dbName;
private SqlConnection _sc;
public override void Open()
{
//open the connection, change the database to the one that was passed in
_sc.Open();
if (this._dbName != null)
this.ChangeDatabase(this._dbName);
}
protected override DbTransaction BeginDbTransaction(IsolationLevel isolationLevel)
{
return new ScaledSqlTransaction(this, _sc.BeginTransaction(isolationLevel));
}
protected override DbCommand CreateDbCommand()
{
return new ScaledSqlCommand(_sc.CreateCommand(), this);
}
}
public class ScaledSqlTransaction : DbTransaction
{
private SqlTransaction _sqlTransaction = null;
private ScaledSqlConnection _conn = null;
protected override DbConnection DbConnection
{
get { return _conn; }
}
}
public class ScaledSqlCommand : DbCommand
{
private SqlCommand _cmd;
private ScaledSqlConnection _conn;
private ScaledSqlTransaction _transaction;
public ScaledSqlCommand(SqlCommand cmd, ScaledSqlConnection conn)
{
this._cmd = cmd;
this._conn = conn;
}
protected override DbConnection DbConnection
{
get
{
return _conn;
}
set
{
if (value is ScaledSqlConnection)
_conn = (ScaledSqlConnection)value;
else
throw new Exception("Only ScaledSqlConnections can be connections here.");
}
}
protected override DbTransaction DbTransaction
{
get
{
if (_transaction == null && _cmd.Transaction != null)
_transaction = new ScaledSqlTransaction(this._conn, _cmd.Transaction);
return _transaction;
}
set
{
if (value == null)
{
_transaction = null;
_cmd.Transaction = null;
}
else
{
if (value is ScaledSqlTransaction)
_transaction = (ScaledSqlTransaction)value;
else
throw new Exception("Don't set the transaction of a ScaledDbCommand with " + value.ToString());
}
}
}
}
}
I don't think that's going to work off a single shared connection.
LINQ to SQL works best with Unit of Work type connections - create your connection, do your atomically grouped work and close the connection as quickly as possible and reopen for the next task. If you do that then passing in a connection string (or using custom constructor that only passes a tablename) is pretty straight forward.
If factoring your application is a problem, you could use a getter to manipulate the cached DataContext 'instance' and instead create a new instance each time you request it instead of using the cached/shared instance and inject the connection string in the Getter.
But - I'm pretty sure this will not help with your pooling issue though. The SQL Server driver caches connections based on different connection string values - since the values are still changing you're right back to having lots of connections active in the connection string cache, which likely will result in lots of cache misses and therefore slow connections.
I think I figured out a solution that works for my situation. Rather than wrapping SqlConnection and overriding Open() to change databases, I'm passing the DBContext a new SqlConnection and subscribing to the connection's StateChanged event. When the state changes, I check to see if the connection has just been opened. If so, I call SqlConnection.ChangeDatabase() to point it to the correct database. I tested this solution and it seems to work - I see only one connection pool for all the databases rather than one pool for each db that has been accessed.
I realize this isn't the ideal solution in an ideal application, but for how this application is structured I think it should make a decent improvement for relatively little cost.
I think, that the best way is making UnitOfWork pattern with Repository pattern to work with Entity Framework. Entity Framework has FirstAsync, FirstOrDefaultAsync, this helped me to fix the same bug.
https://msdn.microsoft.com/en-us/data/jj819165.aspx
Please help!
Background info
I have a WPF application which accesses a SQL Server 2005 database. The database is running locally on the machine the application is running on.
Everywhere I use the Linq DataContext I use a using { } statement, and pass in a result of a function which returns a SqlConnection object which has been opened and had an SqlCommand executed using it before returning to the DataContext constructor.. I.e.
// In the application code
using (DataContext db = new DataContext(GetConnection()))
{
... Code
}
where getConnection looks like this (I've stripped out the 'fluff' from the function to make it more readable, but there is no additional functionality that is missing).
// Function which gets an opened connection which is given back to the DataContext constructor
public static System.Data.SqlClient.SqlConnection GetConnection()
{
System.Data.SqlClient.SqlConnection Conn = new System.Data.SqlClient.SqlConnection(/* The connection string */);
if ( Conn != null )
{
try
{
Conn.Open();
}
catch (System.Data.SqlClient.SqlException SDSCSEx)
{
/* Error Handling */
}
using (System.Data.SqlClient.SqlCommand SetCmd = new System.Data.SqlClient.SqlCommand())
{
SetCmd.Connection = Conn;
SetCmd.CommandType = System.Data.CommandType.Text;
string CurrentUserID = System.String.Empty;
SetCmd.CommandText = "DECLARE #B VARBINARY(36); SET #B = CAST('" + CurrentUserID + "' AS VARBINARY(36)); SET CONTEXT_INFO #B";
try
{
SetCmd.ExecuteNonQuery();
}
catch (System.Exception)
{
/* Error Handling */
}
}
return Conn;
}
I do not think that the application being a WPF one has any bearing on the issue I am having.
The issue I am having
Despite the SqlConnection being disposed along with the DataContext in Sql Server Management studio I can still see loads of open connections with :
status : 'Sleeping'
command : 'AWAITING COMMAND'
last SQL Transact Command Batch : DECLARE #B VARBINARY(36); SET #B = CAST('GUID' AS VARBINARY(36)); SET CONTEXT_INFO #B
Eventually the connection pool gets used up and the application can't continue.
So I can only conclude that somehow running the SQLCommand to set the Context_Info is meaning that the connection doesn't get disposed of when the DataContext gets disposed.
Can anyone spot anything obvious that would be stopping the connections from being closed and disposed of when the DataContext they are used by are disposed?
From MSDN (DataContext Constructor (IDbConnection)):
If you provide an open connection, the
DataContext will not close it.
Therefore, do not instantiate a
DataContext with an open connection
unless you have a good reason to do
this.
So basically, it looks like your connections are waiting for GC to finalize them before they will be released. If you have lots of code that does this, one approach might be to overide Dispose() in the data-context's partial class, and close the connection - just be sure to document that the data-context assumes ownership of the connection!
protected override void Dispose(bool disposing)
{
if(disposing && this.Connection != null && this.Connection.State == ConnectionState.Open)
{
this.Connection.Close();
this.Connection.Dispose();
}
base.Dispose(disposing);
}
Personally, I would happily give it (regular data-context, w/o the hack above) an open connection as long as I was "using" the connection (allowing me to perform multiple operations) - i.e.
using(var conn = GetConnection())
{
// snip: some stuff involving conn
using(var ctx = new FooContext(conn))
{
// snip: some stuff involving ctx
}
// snip: some more stuff involving conn
}
The SqlProvider used by the LINQ DataContext only closes the SQL connection (through SqlConnectionManager.DisposeConnection) if it was the one to open it. If you give an already-open SqlConnection object to the DataContext constructor, it will not close it for you. Thus, you should write:
using (SqlConnection conn = GetConnection())
using (DataContext db = new DataContext(conn))
{
... Code
}
I experienced the same issue using the Entity Framework. My ObjectContext was wrapped around a using block.
A connection was established when I called SaveChanges(), but after the using statement was out of scope, I noticed that SQL Management Studio still had a "AWAITING COMMAND" for the .NET SQL Client.
It looks like this has to do with the behavior of the ADO.NET provider which has connection pooling turned on by default.
From "Using Connection Pooling with SQL Server" on MSDN (emphasis mine):
Connection pooling reduces the number of times that new connections need to be opened. The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks to see if there is an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of actually closing it. Once the connection is returned to the pool, it is ready to be reused on the next Open call.
Also ClearAllPools and ClearPool seems useful to explicitly close all pooled connections if needed.
I think the connection, while no longer referenced, is waiting for the GC to dispose of it fully.
Solution:
Create your own DataContext class which derives from the auto-generated one. (rename the base one so you don't have to change any other code).
In your derived DataContext - add a Dispose() function. In that - dispose the inner connection.
Well thanks for the help chaps, it has been solved now..
Essentially I took elements of most of the answers above and implemented the DataContext constructor as above (I already had overloaded the constructors so it wasn't a big change).
// Variable for storing the connection passed to the constructor
private System.Data.SqlClient.SqlConnection _Connection;
public DataContext(System.Data.SqlClient.SqlConnection Connection) : base(Connection)
{
// Only set the reference if the connection is Valid and Open during construction
if (Connection != null)
{
if (Connection.State == System.Data.ConnectionState.Open)
{
_Connection = Connection;
}
}
}
protected override void Dispose(bool disposing)
{
// Only try closing the connection if it was opened during construction
if (_Connection!= null)
{
_Connection.Close();
_Connection.Dispose();
}
base.Dispose(disposing);
}
The reason for doing this rather than some of the suggestions above is that accessing this.Connection in the dispose method throws a ObjectDisposedException.
And the above works as well as I was hoping!
The Dispose should close the connections, as MSDN points out:
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. If the
connection pooling value Pooling is
set to true or yes, the underlying
connection is returned back to the
connection pool. On the other hand, if
Pooling is set to false or no, the
underlying connection to the server is
closed.
My guess would be that your problem has something to do with GetContext().
I use a pattern that looks something like this often. I'm wondering if this is alright or if there is a best practice that I am not applying here.
Specifically I'm wondering; in the case that an exception is thrown is the code that I have in the finally block enough to ensure that the connection is closed appropriately?
public class SomeDataClass : IDisposable
{
private SqlConnection _conn;
//constructors and methods
private DoSomethingWithTheSqlConnection()
{
//some code excluded for brevity
try
{
using (SqlCommand cmd = new SqlCommand(SqlQuery.CountSomething, _SqlConnection))
{
_SqlConnection.Open();
countOfSomething = Convert.ToInt32(cmd.ExecuteScalar());
}
}
finally
{
//is this the best way?
if (_SqlConnection.State == ConnectionState.Closed)
_SqlConnection.Close();
}
//some code excluded for brevity
}
public Dispose()
{
_conn.Dispose();
}
}
Wrap your database handling code inside a "using"
using (SqlConnection conn = new SqlConnection (...))
{
// Whatever happens in here, the connection is
// disposed of (closed) at the end.
}
The .Net Framework mantains a connection pool for a reason. Trust it! :)
You don't have to write so much code just to connect to the database and release the connection.
You can just use the 'using' statement and rest assured that 'IDBConnection.Release()' will close the connection for you.
Highly elaborate 'solutions' tend to result in buggy code. Simple is better.
MSDN Docs make this pretty clear...
The Close method rolls back any pending transactions. It then releases the connection to the connection pool, or closes the connection if connection pooling is disabled.
You probably haven't (and don't want to) disable connection pooling, so the pool ultimately manages the state of the connection after you call "Close". This could be important as you may be confused looking from the database server side at all the open connections.
An application can call Close more than one time. No exception is generated.
So why bother testing for Closed? Just call Close().
Close and Dispose are functionally equivalent.
This is why a using block results in a closed connection. using calls Dispose for you.
Do not call Close or Dispose on a Connection, a DataReader, or any other managed object in the Finalize method of your class.
Important safety tip. Thanks, Egon.
I'm guessing that by _SqlConnection.State == ConnectionState.Closed you meant !=
This will certainly work. I think it is more customary to contain the connection object itself inside a using statement, but what you have is good if you want to reuse the same connection object for some reason.
One thing that you should definitely change, though, is the Dispose() method. You should not reference the connection object in dispose, because it may have already been finalized at that point. You should follow the recommended Dispose pattern instead.
Since you're using IDisposables anyway. You can use the 'using' keyword, which is basically equivalent to calling dispose in a finally block, but it looks better.
See this question for the answer:
Close and Dispose - which to call?
If your connection lifetime is a single method call, use the using feature of the language to ensure the proper clean-up of the connection. While a try/finally block is functionally the same, it requires more code and IMO is less readable. There is no need to check the state of the connection, you can call Dispose regardless and it will handle cleaning-up the connection.
If your connection lifetime corresponds to the lifetime of a containing class, then implement IDisposable and clean-up the connection in Dispose.
Put the connection close code inside a "Finally" block like you show. Finally blocks are executed before the exception is thrown. Using a "using" block works just as well, but I find the explicit "Finally" method more clear.
Using statements are old hat to many developers, but younger developers might not know that off hand.
no need for a try..finally around a "using", the using IS a try..finally
Might I suggest this:
class SqlOpener : IDisposable
{
SqlConnection _connection;
public SqlOpener(SqlConnection connection)
{
_connection = connection;
_connection.Open();
}
void IDisposable.Dispose()
{
_connection.Close();
}
}
public class SomeDataClass : IDisposable
{
private SqlConnection _conn;
//constructors and methods
private void DoSomethingWithTheSqlConnection()
{
//some code excluded for brevity
using (SqlCommand cmd = new SqlCommand("some sql query", _conn))
using(new SqlOpener(_conn))
{
int countOfSomething = Convert.ToInt32(cmd.ExecuteScalar());
}
//some code excluded for brevity
}
public void Dispose()
{
_conn.Dispose();
}
}
Hope that helps :)