Extremely basic MDI plugin system

[Blupig]

Hi

I've got an MDI form with a menustrip that already detects all .dll files in a folder and adds them to the menu to show the user which plugins are available. The thing is that I don't know what to do from here on out...

I'd like to be able to update my program without having to stifle through all my code (it's a very big program) and be able to create basic new features through external DLLs. It's extremely useful for users to make their own features too

Anyway, what a plugin would look like is most likely just a form with controls on it that the user makes themselves in VB. I'd like the plugin to activate through an init() sub within the plugin by clicking on the menu item shown in my MDI form.

I kind of started with my own code but it didn't really get anywhere:

        ListFilesInMenuStrip(PluginsToolStripMenuItem, App_Path() & "Plugins\")
        For Each c As Control In Me.Controls
            If TypeOf c Is MenuStrip Then
                For Each child In c.Controls
                    If TypeOf child Is ToolStripMenuItem Then
                        Dim m As ToolStripMenuItem
                        m = child

                    End If
                Next
            End If
        Next

...Where ListFilesInMenuStrip() is my function to add all the DLLs to the menu and App_Path is just a shortcut function which serves the same purpose as VB6's App.Path() class.

I've also done research but only came across all their weird complex CodeDom and other weird things...This seems so simple but I just can't wrap my mind around it :I

[jmcilhinney]

Search the web for plugin architecture .net or the like and you'll find various examples. There's likely some on this very site. Basically, you create a DLL that contains an interface and all plugins must implement, often named IPlugin. You will then need a reference to that DLL in your application project and you develop to that. Each plugin author will also need a reference to that DLL in there own project and they then implement your interface. At run time, you find the DLL and use Reflection to interrogate it to see if it contains a type that implements IPlugin. If it does you load it and interact with it via the IPlugin interface.

The .NET Framework also has an add-in framework built in, in the System.AddIn namespace, but I have no experience with that.

[Blupig]

Search the web for plugin architecture .net or the like and you'll find various examples. There's likely some on this very site. Basically, you create a DLL that contains an interface and all plugins must implement, often named IPlugin. You will then need a reference to that DLL in your application project and you develop to that. Each plugin author will also need a reference to that DLL in there own project and they then implement your interface. At run time, you find the DLL and use Reflection to interrogate it to see if it contains a type that implements IPlugin. If it does you load it and interact with it via the IPlugin interface.

The .NET Framework also has an add-in framework built in, in the System.AddIn namespace, but I have no experience with that.

Is there nothing anymore simple that can achieve the same thing? I was thinking simply importing the selected DLL and calling on a method common within each plugin which would be Init() or something similar...Possible?

[jmcilhinney]

The way I have explained is not difficult, so you don't need an easier way.

[NickThissen]

Is there nothing anymore simple that can achieve the same thing? I was thinking simply importing the selected DLL and calling on a method common within each plugin which would be Init() or something similar...Possible?

That's exactly what do, but you need an interface for that. If the plugins that other users write do not contain an Init method, how could you call it? The plugin interface could be as simple as:

Public Interface IPlugin
   Sub Initialize()
End Interface

Now, people that want to write a plugin implement your IPlugin interface and are therefore required to have an Initialize method. Now, you do know for certain that their classes will have the Initialize method, and you can call it by creating an instance of their class as an IPlugin, something like

Dim type As Type = 'Get the plugin type from the DLL using reflection
Dim plugin As IPlugin = DirectCast(Activator.CreateInstance(type), IPlugin)
plugin.Initialize()