Loading a DLL dynamically

[ExcalibursZone]

If you don't know the name of the function at runtime that you need then the dll is pretty much useless to you. Though there is a way to find out all the functions in a dll (load it up in a hex editor and look towards the end of the dll).

If you want a plug-in type archetecture like netscape or ie, you need to design an interface (a blank class with all the functions you will need a dll to support compiled into a dll) that any new dll will use (implements <interface name>). This way, if you need the dll to run code that modifies something, you pass the data to a known function that is available to any dll you load.

To load a dll that you don't know about until runtime, keep an ini file, a registry setting, or some other way to store the dll names in and do something along these lines:

Dim myDLL As Object

Set myDLL = CreateObject(myDLLName & "." & myDLLObject)

I'm fully unsure if this will work, but you can give it a try

[Matthew Draper]

Yes, I do know the name of the function at runtime, just not design time, and yes, it is for a plug-in like system. What would myDLLName be? The DLL filename w/o extension? What about paths? Can I include a path in that?

I think I understand what you mean with the interface... there isn't a way of running a function w/o knowing it's name at design time, though?

Also, when using myDLLName, would the DLL have to be registered with REGSVR32 before being referenced? If so, is there a way of doing this through API rather than a DOS command?

Thanks for your help; I'll look further into the CreateObject command, too.

[ExcalibursZone]

If you do not know the name of the function at design time, how can you use it?

the DLLName is actually the application name (not the file name) of the .dll you're grabbing the object (class) from. For instance:

Set F = CreateObject("Scripting.FileSystemObject")
Set F = CreateObject("Excel.Sheet1")

it's the internal application name of the dll.

I believe it must be registered, yes.

You can use the Shell command to run regsvr32 to register the dll. I'm unsure if there is a way via API, there probably is, but I don't know what it is.

[Matthew Draper]

I'm constructing a scripting language of sorts.. and I want to be able to "plug-in" extra commands (from DLLs) at runtime. The function name is unique for each command, ideally, so you can't actually know what they are at design time.

In C/C++, you would do this using something to do with function pointers, probably.

How do I set the application name for a DLL I create, do you know?

Thanks again for all your help

[ExcalibursZone]

Well, an idea would be to do this:

Within your command, add the following code:

Public Function Do_Command()
End Function

Or something very similar to this. Make the project name equal to your command name. Then all you have to do is run that function whenever you need to execute your command.
This way, you know the function name right off the bat, no matter what command is added, and if you make it an interface that each command implements, you are sure to have the correct function each time. It won't matter what code is in that function in your command, just that the function exists is enough. That's the cool thing about OOP

[Matthew Draper]

So... is the "internal application name" the project name, or some well-hidden project setting?

Thanks (yet again ) for your help... I'll have to try this later today, when I can be bothered creating & compiling some test projects

[ExcalibursZone]

it is the project name (or possibly the application name setting in compile/options).

[Matthew Draper]

Okay, thanks

[jim mcnamara]

For what it's worth -

MicroBasic and I had a go around on whether or not you can Enum methods and properties in an unknown COFF object file. I found documentation about code that does this for the Object Browser, and it isn't just part of VB, it's part of SCM, and other OS components. I believe it's one of those deliberately obfuscated features that is a major security loophole.

[Matthew Draper]

Sounds interesting, Jim. Could you give me a few more details?

[Matthew Draper]

I can't get this to compile, let alone actually communicate

Can you see any probs in this code (attached) ?

[gaffa]

Assuming you are attempting to plug-in COM (ActiveX) DLL's, you can use the TypeLibInfo library to query the interface at runtime and find out what methods/properties etc it supports.

Are you writing a plugin system for plugins that support a standard interface (so it would be early-binding), or a generic plugin system for any interface (late-binding)?

I've posted a couple of things on this site before about creating plug-in systems, but I haven't got the links handy. They do come up in a search though (under "plugin" I think)

The CreateObject uses the ProgrammaticID of the control, which is the name of the project (ie: the project name in the properties window in VB), followed by the name of the class that is being created.

Alternatively, you can use the GUID, and extract the ProgID from the GUID using the TypeLibInfo library, or even better, load the file by file name, register the file using the API and then extract the ProgID from that file (which is handy if you want to have a plug-in directory where all you plugins go, without necessarily having to register those DLL's before the application loads.)

The attached project demostrates a basic plugin system (it just loads language DLLs from a particular directory at runtime). This is the original post where there is a better run down of how it works: http://forums.vb-world.com/showthrea...threadid=53873

- gaffa

[Matthew Draper]

Thanks gaffa, that app is a real help .

Now I just have to work out if the DLLs can also contain a reference to an Interface, which is passed to it's function from the app, and this interface would supply various I/O functions for the DLL. Do you know if this is / is not possible?

I'll try later today, and will probably post whether or not I've been sucessful.

Thanks again

[Matthew Draper]

Yes! I got it working to do a callback to the main application, and I've now come to the conslusion that this is *perfect* for what I want.

    For Each Day In Eternity
        Call BeGratefulTo("gaffa")
    Next Day

    Dim intCount As Integer
    For intCount = 0 To (1 / 0) ' Infinity
        Call SendMessageTo("gaffa", "Thank you!")
    Next intCount

And all similar code snippets, as appropriate

[gaffa]

Glad to be of help.

That sample isn't fully featured though. I think there is a bug when registering the control - if I recall I put some REALLY bad code in there in the form of error handlers to get round a couple of problems.

But apart from that, it seems to work OK. On the link I posted to the original post where that code was avaialbe, there was another "bit" to it - I think it was a single DLL exposing two languages - Australian English and US English. There's not a lot of difference, just a loop through the interfaces collection.

Cheers

- gaffa