I have a VB6 DLL embedded in some ASP pages. The DLL hits a Codebase database, an obsure and obsolete database engine (a dialect/variation on dBase) that virtually no-one has even heard of. It takes Codebase nearly a second to initialise a new connection, which is unacceptably slow and so I've created a connection pool, managed by a VB class. The single instance of this class is created at the start of a VB module, i.e.:
Private m_codebaseManager As New CodebaseManager
My problem is that periodically the class initialization method is called again completely wrecking my pooling class and I've no idea why. Terminate does not fire and there's no sign of any crash occurring, so why on earth is initialize called? My understanding is that data in non-class modules persists for the lifetime of the DLL. Is that correct and if not, under what circumstances does a module 'restart'?
I would recommend removing the "New" from the variable declaration. Declaring a variable "As New" causes it to be checked every time it is referenced, and if set to Nothing a new instance of your CodebaseManager will be created.
A better solution would be to declare your variable like this:
Private m_codebaseManager As CodebaseManager
and then explicitly set it to a new instance when your application starts:
Set m_codebaseManager = New CodebaseManager
This means you can be sure you won't be creating any unintentional new instances of CodebaseManager. You'll probably then still have a bug but at least it will be an "Object or with block variable not set" error which you should be able to easily fix.
My understanding is that data in non-class modules persists for the lifetime of the DLL. Is that correct and if not, under what circumstances does a module 'restart'?
Sort of. Global state (module public/private vars) are apartment specific and is stored in TLS slots. VB6 supports apartment threading only, so each thread gets a "fresh" copy of the global state. Because ASP environment is multi-threaded so each thread gets a separate DB connection "pool".
If you need a real global state you have to use Application object to store it. If you put apartment threaded objects there (like VB6 ones) these can serialize you multi-threaded ASP environment and degrade performance. Use ADO objects or Dictionary objects or anything you are certain is free threaded.
Btw, you can let COM+ do the object/connection pooling for you. If OLEDB provider is a better one it can do connection pooling internally too (SQLOLEDB for MSSQL is an example).
But it's a DLL and not an ActiveX exe, so there's no Main() function and nowhere for the 'Set m_codebaseManager = New CodebaseManager' line to go except in global scope.
Related
This is more of varification, but i want to be sure before i start altering some old code to clean it up.
If you have private varibales declared inside a module but outside a subroutine, when are these actually created. For example, this is how a module is set up:
'Local objects.....
'Function Main.....
'Subroutines.......
Private Constants..
Private variables..
More Subroutines...
If those variables are only used in one subroutine, should they be declared inside that subroutine or in the local objects or right outside the subroutine as they are now?
Thanks!
The CLR has no support for modules or module variables so modules become static classes and module variables become static fields.
As a rule, variables should be declared as close to the point they are used as possible. Their scope should also be as constrained as possible.
Turning a variable into a field is a pretty bad coding practice for several reasons:
It is extremely easy to make a mistake and reuse the same field in another part of your module, creating unexpected conditions.
You increase the lifetime of the objects in the variable significantly. Typically, once you exit the method, the variable is available for garbage collection. By turning it into a static field, the object will stay alive until it's replaced or the application terminates
Multiple threads will be able to see and access the same static field, potentially creating race conditions. Given how many things work asynchrously nowadays, this can be a significant problem
Either I don't understand COM objects, either it's all that confusing.
I frequently 'create' , 'dispatch' COM objects - either in Python, either in VB ( Obj = win32com.client.Dispatch('Visum.Visum') or Obj = CreateObject("Excel.Application") respectively).
That's easy, obvious and everything's fine.
But how can I:
a) connect to COM object which is already running
b) get list of running processes which are COM objects
in VB what is the reference, and additional info on: CreateObject command. In Python it's part of well defined, comprehensive library of win32com whereas in VB it's just single method without reference: http://msdn.microsoft.com/en-us/library/7t9k08y5(v=vs.80).aspx
Highly confusing (COM object,server? connection, dispatching, registering??)
Thanks for claryfying
Rafal
i2
First of all, I would suggest reading a good introduction to COM. Personally, i think that 'Essential COM' by Don Box is the best book about COM. Spending a few hours with it will save you many hours later.
That being said, let's move to your questions.
a) In order to retrieve an existing COM object from VB, you need to call GetObject function. This will only work for COM objects which are registered with Running Object Table. Excel does this, so there shouldn't be any problems with that.
b) as Hans Passant correctly noted, this question doesn't make sense. Processes and COM objects are completely unrelated things, except for the fact that processes host the COM objects. It is similar to asking 'which processes are instances of class X'. However, a process can host a COM object (or many of them). These object can be alive only temporarily, so what would you expect there? Besides, many COM classes are implemented within dlls, but created instances obviously are hosted within a process - so what would you expect there?
COM technology is something I didn't not use for a while, so sorry if some info might be wrong (anyway, I've got my book "Inside COM+ base services" with me!)
Briefly speaking, let's give a simplified definition of a COM object: it nothing else than a standard DLL with classes exposing a standard interface IUnkown with the following methods:
AddRef: to register a client consuming the COM object
Release: to unregister a client (usually unused objects stop running, but other might remain resident)
QueryInterface: to get the address of a function
QueryInterface is used to dynamically retrieve the address of a function (late-binding) which is powerful (you don't have to know the DLL at compile time) but time consuming. On the other hand, you can directly reference the addresses of the functions at compile-time because those DLL are registered in the system (all the informations are stored in the registry)
Creating a new object or getting the address of a running object is done through RPC (remote procedure call) which will load and/or play the role of a proxy
So to create a new instance of a COM object, you will call CreateObject (in VB) (Co)CreateInstance in VC++ (or even c#?), while to get an executing instance, you might want to call GetObject().
Getting the list of COM objects is not that easy (I guess not possible at all) because, as told earlier, a COM object is nothing else but a DLL: this means that the library will mainly load in the address space of each process which access is private. Even for out-process COM objects (opposite to in-process) which are object shared between many processes (i.e.: Excel is an out-of-process COM object: you don't load the DLL in your application's address space), they are loaded by a host (rpc.exe or a proprietary host)
Hope this helps
Serge
I'm trying to split some prior crafted code into a DLL. It's a simple logger system.
There are a few things that need to be shared with the main form in the project, so I set them up as a shared variable, but I don't use shared stuff often, and I worry it will cause variable conflicts regarding scope. I figured I would make a post here about it and see if someone can explain what I don't fully understand.
Since this is a logger it will be used a couple of places. Other DLLs that need logging may reference it through a instanced object and project reference. My main form will also have an instanced object and a reference to the logger libary.
Since one of my properties is a connection string and it's shared, does this mean that a instance of my logger class inside a DLL will have the same shared values as a instance on my main UI form? Or will the fact that the instance is inside of a DLL provide the scope boundary I need? I'm hoping it does..
I mainly worry that I might want to log using two different connection strings down the road.
(I hope my question makes sense. If it doesn't, post comments and I'll try to clarify.)
No, the fact that the instance is in a DLL does not provide the scope boundary you need. If the class or members in the DLL were declared static they would be shared and you could run into problems. So, just don't declare them static and be sure to create new instances of the object(s) when you consume them and you should be ok.
I have written some code in VB that verifies that a particular port in the Windows Firewall is open, and opens one otherwise. The code uses references to three COM DLLs. I wrote a WindowsFirewall class, which Imports the primary namespace defined by the DLLs. Within members of the WindowsFirewall class I construct some of the types defined by the DLLs referenced. The following code isn't the entire class, but demonstrates what I am doing.
Imports NetFwTypeLib
Public Class WindowsFirewall
Public Shared Function IsFirewallEnabled as Boolean
Dim icfMgr As INetFwMgr
icfMgr = CType(System.Activator.CreateInstance(Type.GetTypeFromProgID("HNetCfg.FwMgr")), INetFwMgr)
Dim profile As INetFwProfile
profile = icfMgr.LocalPolicy.CurrentProfile
Dim fIsFirewallEnabled as Boolean
fIsFirewallEnabled = profile.FirewallEnabled
return fIsFirewallEnabled
End Function
End Class
I do not reference COM DLLs very often. I have read that unmanaged code may not be cleaned up by the garbage collector and I would like to know how to make sure that I have not introduced any memory leaks. Please tell me (a) if I have introduced a memory leak, and (b) how I may clean it up.
(My theory is that the icfMgr and profile objects do allocate memory that remains unreleased until after the application closes. I am hopeful that setting their references equal to nothing will mark them for garbage collection, since I can find no other way to dispose of them. Neither one implements IDisposable, and neither contains a Finalize method. I suspect they may not even be relevant here, and that both of those methods of releasing memory only apply to .Net types.)
Not sure what to recommend here. There is most definitely no memory leak here, the garbage collector releases COM reference counts. COM objects are not disposable but you can release them early with Marshal.ReleaseComObject(). The trouble with doing this explicitly is that it is normally very hard to track interface references.
In your code snippet for example, calling ReleaseComObject on the icfMgr won't have any effect. There's a hidden reference through the LocalPolicy member that will keep the interface reference alive. You'd have to call ReleaseComObject on that hidden reference as well.
I would not recommend making this a practice at all. Getting it wrong produces hard to diagnose failure, you're essentially back to the bad old days of explicit memory management. But it is somewhat manageable in your specific example.
You are exactly right: unmanaged code cannot be managed and thus needs to be managed by hand: disposed of. However, this greatly depends on what you are doing, but in many cases, it is sufficient to wrap the object instantiation around a Using-block. This only works if you use an object that implements IDisposable.
However, the way you currently create an instance of a COM object, you will not have the possibility to clean up easily. It depends on the object. When it doesn't need cleaning up (check the destructor of FwMgr), it doesn't need disposing either. However, most COM objects do need disposal.
So, how to add the IDisposable interface to a COM object that doesn't natively support it? It's a bit of work to do so manually, but you should create a wrapper .NET assembly. Luckily, the work has been taken out of our hands and Microsoft has created some tools and guidelines.
Some of this information is covered here too. You may want to also look up WeakReference as an alternative.
Note that COM and .NET do not talk well together, but they do talk. An excellent reference is .NET and COM The Complete Interoperability Guide by Don Box, SAMS Publishing.
EDIT:
In answer to your "memory leak" question: it is impossible to tell whether you introduced a memory leak, and how big it is. It depends on how often you call your COM object. Call it once per running process? Don't worry too much. Call it hundredths of times in an inner loop? Be very careful. Want to know for sure? Lookup the original documentation or source: if it releases handles, memory or other resources when it is destructed, then yes, you introduced a leak.
Is there possible to create a COM-instance in it's own, dedicated, host-process?
I guess some background is needed.
We have an end-user client which has it's central logical components inside an singleton-COM object. (Not propper singleton, but it uses global variables internally, so it would fail.) So that there should be only one instance per exe-file. Convenient while making the client.
However, I should now make a "client-simulator" to test the server-side. I therefore which to make 20 instances of the client-component.
If I could make each instance instanciate in its own exe-host, then the singleton-issue would be handled.
Regards
Leif
I have been struggling with this problem for a few days. I finally found a solution that works. My COM object is written using ATL, so my code snippet will be geared toward that, but the technical solution should be clear. It all hinges on how the class objects are registered. The REGCLS_SINGLEUSE flag is the key. I now have separate processes for each object instance.
In the ATL module, override the RegisterClassObjects() function as follows:
HRESULT RegisterClassObjects(DWORD dwClsContext, DWORD dwFlags) throw()
{
return base::RegisterClassObjects(CLSCTX_LOCAL_SERVER, REGCLS_SUSPENDED | REGCLS_SINGLEUSE);
}
From MSDN regarding REGCLS_SINGLEUSE:
REGCLS_SINGLEUSE
After an application is connected to a class object with
CoGetClassObject, the class object is removed from public view so that
no other applications can connect to it. This value is commonly used
for single document interface (SDI) applications. Specifying this
value does not affect the responsibility of the object application to
call CoRevokeClassObject; it must always call CoRevokeClassObject when
it is finished with an object class.
My theory is that because the registration was removed from public view, it causes a new process to be created for the subsequent instantiations.
This other question mentioned a description of how to use DLLHost as a surrogate process:
http://support.microsoft.com/kb/198891
I've never tried this myself, and I don't know off-hand if you can specify flags for the factories (which control if surrogates can be reused for multiple objects), but maybe you can tweak that via DCOMCNFG or OLEVIEW.
My COM days are long gone, but as far as I remember, there's no built-in way to do that.
It might be easier to rewrite your code so it supports multiple instances than to go the one-process-per-instance route with COM, but here's what you could do:
Use thread-local storage for your global variables and write another CoClass, where each instance owns its own thread through which accesses to the class with the global variables are marshaled. This would at least allow you to avoid the performance impact of DCOM.
Write your own out-of-process exe server (similar to windows' DllHost.exe) to host your COM instances. This requires IPC (Inter-Process Communication), so you either have to code something yourself that marshals calls to the external process or use DCOM (presuming your COM object implements IDispatch)