Re: modular application

From: Saso Zagoranski (saso.zagoranski_at_guest.arnes.si)
Date: 02/06/04 

Date: Fri, 6 Feb 2004 22:19:45 +0100

Thanks Sami!
You really helped me a lot!

Just one more thing :)
How do you "cast" an object if you have the Type of the object stored in
some variable t?
So you can't use:
myGpsObject = (Gps) myObject;

but instead:
myGpsObject = ??? t.GetType() ???

Again,
thanks a lot for your help

"Sami Vaaraniemi" <samivawantsnospam@jippii.fi> wrote in message
news:c004o1$ocr$1@phys-news1.kolumbus.fi...
>
> "Saso Zagoranski" <saso.zagoranski@guest.arnes.si> wrote in message
> news:c000in$nks$1@planja.arnes.si...
> > That's the approach I was thinking about...
> > Just a few more questions...
> > Do your plugins inherit from IPlugin or do the implement it? I mean do
you
> > have this:
> > class MyPlugin : System.Object, IPlugin ? (or MyPlugin:
> > System.Windows.Forms, IPlugin or ...)
>
> My plugins inherit from IPlugin - which is the same as saying they
implement
> IPlugin. They also happen to be Forms, so they look like this (the second
> option above):
>
> class MyPlugin : System.Windows.Forms, IPlugin
>
> Sometimes the plugins implement a plugin-specific interface, e.g.,
>
> interface ISpecialPlugin : IPlugin {...}
>
> class MySpecialPlugin : System.Windows.Forms, ISpecialPlugin
>
> All these interfaces (IPlugin, ISpecialPlugin, IHost) are defined in an
> assembly which is referenced by the plugins and by the shell.
>
> >
> > In this case, isn't the Type of MyPlugin actually MyPlugin? To get the
> > interface
> > you would then have to use MyPlugin.GetType().GetInterface("IPlugin")
and
> to
> > call
> > the methods of this interface you would then use MethodInfo.Invoke(...).
> > And most importantly... You can't cast an object of type MyPlugin to
> > IPlugin, can you?
>
> Yes, the actual type of MyPlugin is MyPlugin. Due to the fact that
MyPlugin
> and all other plugins inherit from IPlugin, the shell can easily access
the
> IPlugin interface when it loads the plugin:
>
> Assembly asm = Assembly.LoadFrom(<url>); // this string comes from

> e.g., a config file
> IPlugin plugin = (IPlugin)asm.CreateInstance("MyPlugin"); // the
string
> "MyPlugin" string comes from e.g., a config file
>
> This way the shell can call the methods of IPlugin directly and there is
no
> need to invoke them through reflection. The shell only uses the interface,
> it never uses the actual type MyPlugin.
>
> >
> > Here's what I do so far:
> > I load all the types in an assembly. I go through all the types and
search
> > if those types implement
> > IModule. I create an instance of the object which implements the IModule
> > interface with:
> > object myObject = Activator.CreateInstance(typeofObject);
> > I then use the MethodInfo.Invoke and PropertyInfo.GetValue, to start the
> > OnConnect method of
> > the plugin (OnConnect is part of IModule) and to get the list of
Services
> > (also part of IPlugin) that the plugin provides (if any).
>
> This sounds fine to me except that you don't necessarily have to use
> MethodInfo.Invoke. Instead, simply cast your object to IModule:
>
> object myObject = Activator.CreateInstance(typeofObject);
> IModule module = (IModule)myObject;
>
> In order for this to work though, you need to define the interface IModule
> in a separate assembly and have your loader application reference that
> assembly.
>
> > I then add "myObject", it's services and it's type into an array at the
> > host. So when some plugin would call:
> > Host.FindPlugin("GPS");
> > The Host would find that the GpsPlugin provides the GPS service and it
> would
> > provide the
> > object reference and Type to the plugin which requires the question.
> > The main problem is:
> > How do I cast the GpsPlugin (which is stored at the host and received as
> > System.Object) to the type
> > of GpsPlugin (note that the host stored the Type information)?
>
> My earlier comments already answer this. Define the interface IGpsPlugin
in
> a shared assembly. Have the plugin reference that assembly. Then in the
> plugin you can simply write:
>
> IGpsPlugin gpsPlugin =
(IGpsPlugin)Host.FindPlugin(typeof(IGpsPlugin));
>
> You will not need to use reflection if you define the interfaces in a
> separate assembly, and have the host and the plugins reference this
> assembly. I know that some developers prefer the flexibility of reflection
> over having interfaces. My personal preference (in this case) is type
safety
> through interfaces.
>
> > I wouldn't want to hardcode the casting... Is this even possible?
> >
> > Thanks,
> > saso
>
> Sure,
> Sami
>
>
>

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