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.NET Forum / Languages / C# / August 2007DbConnection
Hello, Newsgroupians: I am creating a wrapper for (I)DbConnection. I can connect to a database and place queries; however, I'm having some problems with my wrapped class. In short, I have the following... public class CDB { protected System.Data.IDbConnection m_conn; ... } Now, I've read that I should ALWAYS close my connection when I'm finished. So when my class that wraps the connection is ready for garbage collection, I close the connection in the destructor. It is as follows... ~CDB() { if (this.m_conn.State == System.Data.ConnectionState.Open) { try { this.m_conn.Close(); } catch(Exception e) { Console.WriteLine(e.Message); Console.ReadLine(); // I add this just so I can see the message. } } } The problem is I always receive the exception -- which is an InvalidOperationException -- "Handle is not initialized." Any ideas what I am doing wrong, or how to rectify the situation? Thank you. Should you need more of my code, please do not hesistate to ask. I just thought the other code was irrelevant in this situation. Thank you again. Trecius Which part of the code throws the exception? When you check the connection status or when you try to close the connection? Adrian.  Signature[Please mark my answer if it was helpful to you] > Hello, Newsgroupians: > [quoted text clipped - 38 lines] > > Trecius The method that is creating the exception is this.m_conn.Close(). Trecius > Which part of the code throws the exception? When you check the connection > status or when you try to close the connection? [quoted text clipped - 43 lines] > > > > Trecius Trecius, You shouldn't be cleaning up the database connection like this. Rather, you should be implementing the IDisposable interface on your class, and then calling the Dispose method on the database connection in your implementation of IDisposable. For more information on implementing the IDisposable interface, look at the section of the MSDN documentation titled "Implementing a Dispose Method", located at: http://msdn2.microsoft.com/en-us/library/aa720161(VS.71).aspx Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > Hello, Newsgroupians: > [quoted text clipped - 42 lines] > > Trecius Don't use a finalizer. Make your class implement IDisposable and in Dispose() call Close() on the connection. Any time your custom class contains an instance variable which is IDisposable, your class also needs to be IDisposable. Since the connection is itself a managed object it's improper for you to reference it at all in a finalzer (it may have been finalized already). HTH, Sam ------------------------------------------------------------ We're hiring! B-Line Medical is seeking .NET Developers for exciting positions in medical product development in MD/DC. Work with a variety of technologies in a relaxed team environment. See ads on Dice.com. >Hello, Newsgroupians: > [quoted text clipped - 12 lines] > >Trecius Now, I'm a little confused. So you're all stating that I must ALWAYS call the Dispose() method everytime I use my class? I'm still new to C#, but it's impossible to create a class that can dispose of itself? What about instances where the programmer forgets to call Dispose() on the object they're using. In this case, the Close() method will never be called. I suppose this is the crux of the problem, and it's my question. How can I create my class that will automatically call Close() when the object goes out of scope and needs to be garbage collected. Thank you. Trecius > Don't use a finalizer. Make your class implement IDisposable and in > Dispose() call Close() on the connection. Any time your custom class [quoted text clipped - 31 lines] > > > >Trecius > Now, I'm a little confused. So you're all stating that I must ALWAYS call > the Dispose() method everytime I use my class? I'm still new to C#, but it's [quoted text clipped - 4 lines] > create my class that will automatically call Close() when the object goes out > of scope and needs to be garbage collected. Thank you. using (CDB db = new CDB()) { // Use db here. } The pattern above is used when you instantiate an object that implements IDisposable. When db goes out of scope then the Dispose method is automatically called. This is simply compiler short-hand for this: CDB db; try { db = new CDB(); } finally { db.Dispose(); } Yes, you must somehow get your Dispose (i.e., Close) method to be called or you'll run out of resources (like pooled database connections). If a programmer forgets to use the using statement or the finally statement then they have written incorrect code. Signature-glenn- > Now, I'm a little confused. So you're all stating that I must ALWAYS call > the Dispose() method everytime I use my class? Yes. > I'm still new to C#, but it's > impossible to create a class that can dispose of itself? Yes. The compiler doesn't add any extra code to handle when objects go out of scope. As there is no code to handle this, there is no way of calling a destructor. For fully managed objects there is no need for any destructor, as the garbage collector can free all managed objects by itself. It's only when you class handles unmanaged resources (like a database connection) that you need to dispose the object in a controlled way. > What about > instances where the programmer forgets to call Dispose() on the object > they're using. In this case, the Close() method will never be called. A finalizer is usually used as a backup when implementing IDisposable, so the Close method will be called eventually, but as you have no control over when this happens, you will still experience problems sooner or later if you don't call Dispose properly. > I > suppose this is the crux of the problem, and it's my question. How can I > create my class that will automatically call Close() when the object goes out > of scope and needs to be garbage collected. You can't do that. Nothing happens when an object goes out of scope. SignatureGöran Andersson _____ http://www.guffa.com If you implement the IDisposable interface, then you can using the "using" statement, which will call IDisposable for you. Here's how to implement the IDisposable interface: http://www.codeproject.com/useritems/idisposable.asp and here's an example of using the "using" statement. Remember, connections shouldn't be left open, you should open them, do your task, then close them (release back to the pool) as soon as possible. Code: using (IDbConnection connection = DbProviderFactories.GetFactory(providerName).CreateConnection()) { connection.Open(); using (MyDataAdapter adapter = new MyDataAdapter()) { adapter.Connection = connection; adapter.Fill(myDataTable); } } // IDispose will be called on the connection here, automatically closing it Hope this helps. Ship. On Aug 16, 12:12 am, G?ran Andersson <gu...@guffa.com> wrote: > > Now, I'm a little confused. So you're all stating that I must ALWAYS call > > the Dispose() method everytime I use my class? [quoted text clipped - 32 lines] > G?ran Andersson > _____http://www.guffa.com >> I'm still new to C#, but it's impossible to create a class that can >> dispose of itself? > > Yes. The compiler doesn't add any extra code to handle when objects go > out of scope. As there is no code to handle this, there is no way of > calling a destructor. Just a clarification. It doesn't call any destructors (because there aren't any), but the compiler treats the using statement specially and does emit code to call an objects Dispose method. You can code the same thing by hand with a try-finally block, but the compiler will do this for you if you can use a using statement. >> What about instances where the programmer forgets to call Dispose() on >> the object they're using. In this case, the Close() method will never [quoted text clipped - 4 lines] > control over when this happens, you will still experience problems > sooner or later if you don't call Dispose properly. Is a finalizer really a backup to Dispose? I've never heard that before and question how it could be used that way. I mean, you have no idea when a finalizer will be called so what can it do of any use since it will not know the state of any unmanaged resources by the time the finalizer runs? I don't think I've ever written a finalizer, especially not one that has anything to do with the IDispose pattern. >> I suppose this is the crux of the problem, and it's my question. How >> can I create my class that will automatically call Close() when the >> object goes out of scope and needs to be garbage collected. > > You can't do that. Nothing happens when an object goes out of scope. I'm not sure I understand what you are saying. In this code: using (CDB db = new CDB()) { ... } When db goes out of scope, the compiler has emitted code to cause the object's Dispose method to be called. So he would implement the IDispose pattern to cause Close (i.e., Dispose) to be called when all the references to the object have gone out of scope. (Of course, as already pointed out, the IDispose pattern need not be implemented unless unmanaged resources are being used by the class.) Or do you mean that the just because all the references to an object have gone out of scope, that doesn't mean anything about when the GC will get around to cleaning up that object? Signature-glenn- >>> I'm still new to C#, but it's impossible to create a class that can >>> dispose of itself? [quoted text clipped - 8 lines] > thing by hand with a try-finally block, but the compiler will do this > for you if you can use a using statement. That is correct. To furter clarify this, it's only the object that you specify in the using clause that is disposed, any other objects that happen to have the using block as scope doesn't get disposed. Example: using (StreamWriter writer = File.CreateText("info.txt)) { String message = "Hello world."; writer.Write(message); } Here the string message has the using block as scope. Still, only the StreamWriter will be disposed, but not the string. (Yes, I know that strings doesn't implement IDisposable, but it's just an example...) >>> What about instances where the programmer forgets to call Dispose() >>> on the object they're using. In this case, the Close() method will [quoted text clipped - 6 lines] > > Is a finalizer really a backup to Dispose? Yes, it's used that way. > I've never heard that before > and question how it could be used that way. I mean, you have no idea > when a finalizer will be called so what can it do of any use since it > will not know the state of any unmanaged resources by the time the > finalizer runs? The Dispose method will call GC.SuppressFinalize, so the finalizer will only run if Dispose was not called. You can check out the example code for the IDisposable interface: http://msdn2.microsoft.com/en-us/library/system.idisposable.aspx > I don't think I've ever written a finalizer, especially > not one that has anything to do with the IDispose pattern. [quoted text clipped - 11 lines] > When db goes out of scope, the compiler has emitted code to cause the > object's Dispose method to be called. The compiler adds the code at the end of the using block. This has nothing at all to do with the scope of the variable. If the scope of the variable is outside the using block, the code is still added at the end of the using block: CDB db = new CDB() using (db) { ... // Dispose is called here } // the db variable is still reachable here > So he would implement the IDispose > pattern to cause Close (i.e., Dispose) to be called when all the > references to the object have gone out of scope. The Dispose method has to be called explicitly (for example by using using). It's not called automatically when the object gets unreachable. > (Of course, as already > pointed out, the IDispose pattern need not be implemented unless [quoted text clipped - 3 lines] > have gone out of scope, that doesn't mean anything about when the GC > will get around to cleaning up that object? I just mean that nothing happens when an object goes out of scope (or to be more accurate, when the reference variable goes out of scope). SignatureGöran Andersson _____ http://www.guffa.com >>>> I'm still new to C#, but it's impossible to create a class that can >>>> dispose of itself? [quoted text clipped - 23 lines] > StreamWriter will be disposed, but not the string. (Yes, I know that > strings doesn't implement IDisposable, but it's just an example...) Agreed. In that case you'd use this: using (StreamWriter writer = File.CreateText("info.txt)) { using (string message = "Hello world.") { writer.Write(message); } } (Again, as you said, a string is not disposable, but pretend it is.) >> I've never heard that before and question how it could be used that >> way. I mean, you have no idea when a finalizer will be called so what [quoted text clipped - 3 lines] > The Dispose method will call GC.SuppressFinalize, so the finalizer will > only run if Dispose was not called. Ah yes, now that you mention that. I forgot about SuppressFinalize. >>>> I suppose this is the crux of the problem, and it's my question. >>>> How can I create my class that will automatically call Close() when [quoted text clipped - 20 lines] > } > // the db variable is still reachable here The code isn't added to the end of the using block, at least not like that. The compiler automatically emits a finally block. So while "going out of scope" is not technically correct, what I meant was that the db variable is *effectively* out of scope once the finally block runs; that is, the Dispose method has been run on the object once the scope of the using block ends. I didn't mean to say the object referred to by the db variable would be eligible for garbage collection after the using block is finished, just that the Dispose method is guaranteed to have been called at that point. >> So he would implement the IDispose pattern to cause Close (i.e., >> Dispose) to be called when all the references to the object have gone >> out of scope. > > The Dispose method has to be called explicitly (for example by using > using). It's not called automatically when the object gets unreachable. Well, to nit-pick, I would say that using the using statement calls the Dispose method implicitly. Again, I hope I haven't given the impression that Dispose is called somehow once all the references to it go out of scope. >> Or do you mean that the just because all the references to an object >> have gone out of scope, that doesn't mean anything about when the GC >> will get around to cleaning up that object? > > I just mean that nothing happens when an object goes out of scope (or to > be more accurate, when the reference variable goes out of scope). Except that it may make the object referenced by the variable eligible to be garbage collected ("may" because other variables may hold references to the object as well). But as far as Dispose is concerned, agreed, nothing happens when a variable referencing an object goes out of scope. Phew, I think we're saying the same thing! Signature-glenn- > Agreed. In that case you'd use this: > [quoted text clipped - 5 lines] > } > } I prefer: using (StreamWriter writer = File.CreateText("info.txt"), string message = "Hello world.") { } There are some contexts in which nesting using statements may make more sense, but assuming the variables are nominally scoped to the same block logically, the above syntax is preferable to me. Pete >Is a finalizer really a backup to Dispose? I've never heard that before >and question how it could be used that way. I mean, you have no idea >when a finalizer will be called so what can it do of any use since it >will not know the state of any unmanaged resources by the time the >finalizer runs? I don't think I've ever written a finalizer, especially >not one that has anything to do with the IDispose pattern. Yes, a finalizer is very commonly used as a backup of Dispose. Actually the recommended implementation for Dispose is to create a virtual protected member Dispose(bool disposing) and call that with True if called from Dispose and False if called from the Finalizer. The finalizer version of Dispose(false) is only useful when a class contains unmanaged resources which have to be freed. Code paths originating from a finalizer can not call managed objects because the GC may collect them out of order (and their own finalizers should take care of their own cleanup anyways). HTH, Sam ------------------------------------------------------------ We're hiring! B-Line Medical is seeking .NET Developers for exciting positions in medical product development in MD/DC. Work with a variety of technologies in a relaxed team environment. See ads on Dice.com. >> Is a finalizer really a backup to Dispose? I've never heard that before >> and question how it could be used that way. I mean, you have no idea [quoted text clipped - 15 lines] > > HTH, Thanks, Sam. It does help. I have a number of classes that implement the dispose pattern, and use GC.SuppressFinalizer and all that, just had a Senior's Moment, I guess. Signature-glenn- > Hello, Newsgroupians: > [quoted text clipped - 14 lines] > So when my class that wraps the connection is ready for garbage collection, I > close the connection in the destructor. .NET doesn't have destructors. That is a finalizer, and it doesn't work the same as a destructor. The finalizer doesn't run when the object goes out of scope, as a destructor does in a system that uses reference counting. When the garbage collector is about to collect the object, it's placed in a queue of objects that needs finalizing. A background thread is going through the queue and calls the finalizer in each object. The fact that the finalizer is run in a different thread might be the reason for the error that you are getting. A finalizer can not be used to control the life cycle of an object, as you have no control over when the finalizer is run. > It is as follows... > [quoted text clipped - 17 lines] > InvalidOperationException -- "Handle is not initialized." Any ideas what I > am doing wrong, or how to rectify the situation? You should implement the IDisposable interface, so that you can call the Dispose method when you are done with the connection, just as the DbConnection class does. SignatureGöran Andersson _____ http://www.guffa.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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