so my question is relatively simple, can I create VB6 application that references a class in a dll, and then substitute that dll for another at runtime?
Now my intial guess is... no chance in VB6.
So my thoughts turned to a VB.net interop dll. Could I do it in here, and then call the interop dll from the VB?
Again, my guess would be no.... but I'd be happy if someone knew differently.
The only thing that I think would actually work would be DI in .Net, but I'm limited to .net 2, or 3.5 at a big push, so I dont know if that is possible.
So for the background....
I have a dll that a specific site uses, but we dont want to ship that out to everyone. Instead, we want to build a clone dll which just has the interfaces setup so that the VB6 build will complete.
When it gets to the site that needs it, they want to replace the dummy dll, and drop in their version instead.
Note: We do use RegFreeCOM when its gets installed, so I do have the manifest files that I could play around with if needed.
Any ideas would be much appreciated.
Nick
Its a COM dll so its not statically linked to the VB6 exe, so long as the clsids and interface ids are the same in the type library for both DLLs, you can swap them around as you see fit. (If its a VB6 dll this is trivial to do with the 'binary compatibility' build option)
You could also use late binding instead and instead of making a reference directly in your VB6 code, you would create an object and then set that object to an instance.
Examples and information:
MVPS
Microsoft
I want to create an ActiveX DLL from Visual Basic 6 from which I would like to call some public functions. I will call this DLL only from VB6. However, it seems that only classes get exported. Is there any workaround?
I know there is a way to create DLLs from VB6 with standard WINAPI functions. This is not what I want, because I would have to type thousands of Declare instructions, and I would lose the dynamic linking so I don't need to recompile applications when changing the DLL.
I will state my problem just in case anyone has a better idea. I've got a bunch of relatively big projects, each with its own code, and then I have a lot of "Generic" code which is used in several projects. It's an annoyance to add every file to each new project, and having to recompile all of them for each minor change. So I thought of creating a DLL, so I would just "Add reference" when I begin a new project, and don't have to worry anymore about recompiling (at least for minor changes) but I raged when discovered that only classes got exported.
I wouldn't mind to reorganize the code in classes, but it's an overwhelming task: there are some 10 years of 3-4 people code, so it's not something I can do overnight.
Yes, it's easy.
Put all the utility routines in special classes in the DLL.
Set the Instancing property of those classes as GlobalMultiUse.
Build the DLL.
In your client project (with a reference to the DLL) you will now be able to call the functions and subroutines as if they were in a module in that project. You won't need to create any objects.
You can read more in the VB6 manual.
I recently ran across a DLL installed on my system that Dependancy Walker (and every other utility I tried) says has zero exports by name or ordinal, yet the file is approximately 4mb in size. I thought the sole purpose of a DLL was to export functions for use by other code so what would be the purpose of a dll with no visible exports?
One way to think of a DLL is as a container for functions. Exporting a function from a DLL makes those functions visible to callers outside of the DLL. While exporting functions from a DLL is perhaps the most common way to provide access to them, many platforms provide other ways to access functions which have not been exported such as reflection in the .NET Framework and Java and (I think) LoadLibtary / GetProcAddress in Win32
Reasons for doing this are varied, often it is because it is beneficial to the developer to have functions in a library but undesirable for those functions to be called from external applications
Resource-only DLL, maybe? Those are used quite often for localization purposes, for example.
EDIT: it's also possible to have a DLL with code that does something in DllMain() to somehow make its functionality available. The DLL can register itself with some global dispatcher, for example, or create named kernel objects...
I've been working non-stop for the last three days on a completely managed interface to Erlang. At this point, I've decided that there simply must be an easier way. I've got a little over 3000 lines and it's not even in a compilable state yet. To be honest, I'm getting lost in my own code.
So, I then remembered that Erlang has a C library called erl_interface. Unfortunately, it only comes as a .LIB file, which isn't usable via P/Invoke. I'm now investigating ways to expose the static library through a DLL.
I'd like to stay away from Visual C++, mostly because I'm not a C/C++ programmer by nature and I find it really difficult to configure. TinyC is my compiler of choice when working with anything in C.
How can I go about this?
I know I can link erl_interface to a DLL, but how can I expose the functions? Do I have to essentially wrap each and every one of them in my own exports? That probably won't be a problem, since I could write a script to generate the code from the header file. But is there an easier way that I just don't know about?
Also, please don't recommend OTP.NET. It's a nice library, but I'm looking to use this is a large project, so I'd like to keep it in-house.
So, your problem is one of turning a static lib into a dynamic one.
The least-effort solution would be to write a thin shim file in 'C', that just delegates to the files in the .lib e.g.
ReturnType my_method1(args...) {
return real_method1(args...);
}
...
and build a DLL from that and the static lib.
Afterthought -- There is another approach you could take -- which is build the .lib into a C++/CLI assembly and do the transition/wrapping in that. It's what C++/CLi is there for, after all.
If you want some help with interfacing to Erlang with C, have a look at "EPAPI" (Erlang Port API) link text. You can of course browse the source code since it is hosted on Google Code. A DEBIAN repository is also available.
Windows still use DLLs and Mac programs seem to not use DLL at all. Are there benefits or disadvantages of using either technique?
If a program installation includes all the DLL it requires so that it will work 100% well, will it be the same as statically linking all the libraries?
MacOS X, like other flavours of Unix, use shared libraries, which are just another form of DLL.
And yes both are advantageous as the DLL or shared library code can be shared between multiple processes. It does this by the OS loading the DLL or shared library and mapping it into the virtual address space of the processes that use it.
On Windows, you have to use dynamically-loaded libraries because GDI and USER libraries are avaliable as a DLL only. You can't link either of those in or talk to them using a protocol that doesn't involve dynamic loading.
On other OSes, you want to use dynamic loading anyway for complex apps, otherwise your binary would bloat for no good reason, and it increases the probably that your app would be incompatible with the system in the long run (However, in short run static linking can somewhat shield you from tiny breaking changes in libraries). And you can't link in proprietary libraries on OSes which rely on them.
Windows still use DLLs and Mac
programs seem to not use DLL at all.
Are they benefits or disadvantages of
using either technique?
Any kind of modularization is good since it makes updating the software easier, i.e. you do not have to update the whole program binary if a bug is fixed in the program. If the bug appears in some dll, only the dll needs to be updated.
The only downside with it imo, is that you introduce another complexity into the development of the program, e.g. if a dll is a c or c++ dll, different calling conventions etc.
If a program installation includes all
the DLL it requires, will it be the
same as statically linking all the
libraries?
More or less yes. Depends on if you are calling functions in a dll which you assume static linkage with. The dll could just as well be a "free standing" dynamic library, that you only can access via LoadLibrary() and GetProcAddress() etc.
One big advantage of shared libraries (DLLs on Windows or .so on Unix) is that you can rebuild the library and its consumers separately while with static libraries you have to rebuild the library and then relink all the consumers which is very slow on Unix systems and not very fast on Windows.
MacOS software uses "dll's" as well, they are just named differently (shared libraries).
Dll's make sense if you have code you want to reuse in different components of your software. Mostly this makes sense in big software projects.
Static linking makes sense for small single-component applications, when there is no need for code reuse. It simplifies distribution since your component has no external dependencies.
Besides memory/disk space usage, another important advantage of using shared libraries is that updates to the library will be automatically picked up by all programs on the system which use the library.
When there was a security vulnerability in the InfoZIP ZIP libraries, an update to the DLL/.so automatically made all software safe which used these. Software that was linked statically had to be recompiled.
Windows still use DLLs and Mac programs seem to not use DLL at all. Are they benefits or disadvantages of using either technique?
Both use shared libraries, they just use a different name.
If a program installation includes all the DLL it requires so that it will work 100% well, will it be the same as statically linking all the libraries?
Somewhat. When you statically link libraries to a program, you will get a single, very big file, with DLLs, you will have many files.
The statically linked file won't need the "resolve shared libraries" step (which happens while the program loads). A long time ago, loading a static program meant that the whole program was first loaded into RAM and then, the "resolve shared libraries" step happened. Today, only the parts of the program, which are actually executed, are loaded on demand. So with a static program, you don't need to resolve the DLLs. With DLLs, you don't need to load them all at once. So performance wise, they should be on par.
Which leaves the "DLL Hell". Many programs on Windows bring all DLLs they need and they write them into the Windows directory. The net effect is that the last installed programs works and everything else might be broken. But there is a simple workaround: Install the DLLs into the same directory as the EXE. Windows will search the current directory first and then the various Windows paths. This way, you'll waste a bit of disk space but your program will work and, more importantly, you won't break anything else.
One might argue that you shouldn't install DLLs which already exist (with the same version) in the Windows directory but then, you're again vulnerable to some bad app which overwrites the version you need with something that breaks your neck. The drawback is that you must distribute security fixes for your app yourself; you can't rely on Windows Update or similar things to secure your code. This is a tight spot; crackers are making lots of money from security issues and people will not like you when someone steals their banking data because you didn't issue security fixes soon enough.
If you plan to support your application very tightly for many, say, 20 years, installing all DLLs in the program directory is for you. If not, then write code which checks that suitable versions of all DLLs are installed and tell the user about it, so they know why your app suddenly starts to crash.
Yes, see this text :
Dynamic linking has the following
advantages: Saves memory and
reduces swapping. Many processes can
use a single DLL simultaneously,
sharing a single copy of the DLL in
memory. In contrast, Windows must load
a copy of the library code into memory
for each application that is built
with a static link library. Saves
disk space. Many applications can
share a single copy of the DLL on
disk. In contrast, each application
built with a static link library has
the library code linked into its
executable image as a separate
copy. Upgrades to the DLL are
easier. When the functions in a DLL
change, the applications that use them
do not need to be recompiled or
relinked as long as the function
arguments and return values do not
change. In contrast, statically linked
object code requires that the
application be relinked when the
functions change. Provides
after-market support. For example, a
display driver DLL can be modified to
support a display that was not
available when the application was
shipped. Supports multilanguage
programs. Programs written in
different programming languages can
call the same DLL function as long as
the programs follow the function's
calling convention. The programs and
the DLL function must be compatible in
the following ways: the order in which
the function expects its arguments to
be pushed onto the stack, whether the
function or the application is
responsible for cleaning up the stack,
and whether any arguments are passed
in registers. Provides a mechanism
to extend the MFC library classes. You
can derive classes from the existing
MFC classes and place them in an MFC
extension DLL for use by MFC
applications. Eases the creation
of international versions. By placing
resources in a DLL, it is much easier
to create international versions of an
application. You can place the strings
for each language version of your
application in a separate resource DLL
and have the different language
versions load the appropriate
resources. A potential
disadvantage to using DLLs is that the
application is not self-contained; it
depends on the existence of a separate
DLL module.
From my point of view an shared component has some advantages that are somtimes realized as disadvantages.
shared component defines interfaces in your process. So you are forced to decide which components/interfaces are visible outside and which are hidden. This automatically defines which interface has to be stable and which does not have to be stable and can be refactored without affecting any code outside the component..
Memory administration in case of C++ and Windows must be well thought. So normally you should not handle memory outside of an dll that isn't freed in the same dll. If you do so your component may fail if: different runtimes or compiler version are used.
So I think that using shared coponents will help the software to get better organized.