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My app allows plugins, I have a Core class (MarshalByRefObj) that plugins must inherit and this class offers various functionality. Now my question is, when this class is instantiated on main app domain and passed to the plugin in different app domain, what would be the benefit of using delegates in such scenario:

public class Core : MarshalByRefObject
{
    public void DoSomething()
    {
         MyMainApp.Delegate1("SomeMethod", "Test");
    }
}

So as you can see, my core class calls a delegate method on MyMainApp. I could as well just do MyMainApp.SomeMethod("test") instead.

However in many examples online about how remoting and plugin system works, everyone seems to be using delegates. Is there any specific reason for that? Could someone give me a more practical example of why?

Jeroen van Langen
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Alex Maher
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2 Answers2

0

Most of the time the controls in a user interface are created by the main thread unless you intentionally create them in another thread. Here is the important bit: ONLY the thread which created the control can access that control.

If you call DoSomething directly, and code in DoSomething wants to interact with a UI control, it will not be allowed and you will get an exception. MyMainApp.Delegate1("DoSomething" is equivalent to: Please execute the specified method on the main thread. Now it can access UI controls.

There are other reasons too but that is the most important bit to remember. See MSDN for more.

CodingYoshi
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One of the benefits would be, that the information passed to the MyMainApp.Delegate1 is serialized for transport from the plugin appdomain to the main-appdomain. The Delegate1 method will execute the DoSomething in the main domain. They don't share memory (so no directly access to object instances is possible). So you can dynamically run methods on another appdomains. And if it's done via reflection, a plugin might be able to run unlisted methods.

I'd rather not use this type of construction, because there is no compile-time check on calling methods. I'd rather use interfaces that are in satelite assemblies. (to prevent the main-appdomain gets a reference to/loading the plugin assembly, so it can't be unloaded anymore)

The other thing: If you call MyMainApp.SomeMethod("test") directly. This implies that the plugin must know the definition of the plugin loader. Meaning that you get a tight coupling (from the plugin) to the 'parent' application(s version). Which makes the whole plugin structure 'useless'. You could fix that by implementing a ISupportSomeMethod interface on the MyMainApp which is defined in a satelite assembly that is used by both the mainapp en the plugin. If your MyMainApp doesn't implement the ISupportSomeMethod interface, the plugin isn't compatible with that program. This way your MyMainApp can support multiple plugin structures.

In this case you prefer an event structure. Because the child object wants to trigger a method of it's parent. Too bad cross domain event calls are not useful, because your main module will load the assembly and it can't be unloaded. You could write a proxi class for that.

Jeroen van Langen
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