Plugin style app question

i am making a plugin style app..but i am facing a dilema...i have the plugins which will implement an interface(or maybe an abstract class i dont really know) so the mother class will know that every plugin have that given functions...but i want interaction between the plugins and the main class(Form)..for example, i want that my plugins give the url to the mother form and that form will do the download and in the end say to the plugin that the download has finished..but how the plugin will know the which function to call? will i have to make a interface that the form will have to implement too, so the plugin knows what are the form's functions?

..
if i wasnt explicit enough say it..i am almost asleep:confused:

i mean, using Cander's way of invoking methods thru reflection i must use interfaces, is there any way of doin that without interfaces? lol

or do u think that with interfaces/abstract class it would be better?

i also realised that by using cander's way i cant use abstract classes instead of interfaces as an abstract class can't be instantiated ....:confused:

It's a back/forth interaction. Have your plugin interface have some kind of init function:

Code:

---

interface IPlugin
{
// ...
  public bool Init(IPluginHost);
}

---

where IPluginHost is an interface that provides methods with which the plugins can request info from your app. It could also expose events if interfaces can do that.
IPluginHost would be implemented by some of your classes (e.g. the main form) and a reference to this class be passed to every plugin as an IPluginHost.

Small sample project.
You have to copy the dll from the Plugin project to the Pluggable's plugin directory every time you recompile.