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Posted 26 Jun 2005
Licenced CPOL

Plugin-Ready Application Development

, 26 Jun 2005
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How to design an app that can easily use plugins.


Days ago I was thinking about creating some plugins for an application that I had developed. How to do that? The idea: distribute plugins as .NET DLLs that expose specific interfaces.


Each plugin must have at least one main method attending to a specific duty. In a simple plugin system a single method is enough to draw the architecture.

Design of the interface

Supposing the plugin computes a value from a parameter, the plugin’s interface is:

interface IPlugin {
   static int MainMethod(object o);

All the plugins must implement this interface.

Linking to the plugin

Now it is necessary to explain how to use the plugin at runtime. The plugins are saved in the ‘\Plugins’ directory inside the startup path of the application. The code for the usage of plugins is very simple: (Please note the using clause:)

using System.Reflection;

private string pluginDir = Application.StartupPath + "\\Plugins";

private IPlugin[] plugins;

private void loadPlugins() {

   string[] p = Directory.GetFiles(pluginDir);
   // Assuming there are no files different that plugins’ DLLs
   plugins = new IPlugin[p.Length];
   for(int i = 0; i < 0; i++) {
      Assembly asm = Assembly.LoadFrom(p[i]);
      // Must be a full-qualified name (MyNamespace.IPlugin)
      Type type = asm.GetType("IPlugin", true);
      plugins[i] = (IPlugin)Activator.CreateInstance(type);

Using plugins

After invoking loadPlugins(), all the plugins can be used as follows:

myResult = plugins[myIndex].MainMethod(myParam);

Now, it is enough to smartly manage the myIndex value…


This technique is very 'general purpose'. For your application you may define different interfaces to implement many useful functions. You can use a main interface that tells the plugin-user app which type of plugin to expect and invoke the correct methods. Another ‘support’ interface may expose the menu entries and toolbar buttons.


This article is an abbreviation (and a generalization) of a real project that is currently working. Once you know how to design the architecture, you can do your job without much problems, but it is not always very easy, particularly while developing certain types of plugin, where you have to modify the interfaces and update all the old plugins…


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Dario Solera
Italy Italy
Software Development Manager working on IaaS cloud computing. Cloud believer, (former) entrepreneur, F1 addict.

Follow me at or on Twitter.

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Comments and Discussions

GeneralUnLoad PlugIns Pin
FredyAlfredo26-Jun-05 11:00
memberFredyAlfredo26-Jun-05 11:00 
GeneralRe: UnLoad PlugIns Pin
Dario Solera26-Jun-05 21:02
memberDario Solera26-Jun-05 21:02 
GeneralRe: UnLoad PlugIns Pin
Thomas Lykke Petersen26-Jun-05 21:09
memberThomas Lykke Petersen26-Jun-05 21:09 
GeneralRe: UnLoad PlugIns Pin
Dario Solera26-Jun-05 21:16
memberDario Solera26-Jun-05 21:16 
GeneralRe: UnLoad PlugIns Pin
Thomas Lykke Petersen27-Jun-05 2:15
memberThomas Lykke Petersen27-Jun-05 2:15 
GeneralRe: UnLoad PlugIns Pin
mav.northwind26-Jun-05 23:15
membermav.northwind26-Jun-05 23:15 
Assemblies cannot be unloaded separately and calling Finalize will not unload the assembly.

They are unloaded when the AppDomain they've been loaded into is unloaded. In the case of most .NET applications 1 Application equals 1 AppDomain, so any assemblies loaded at runtime are released when the application is terminated.

In order to be able to remove plugins at runtime you'll have to load them into a separate AppDomain (but then you'll have to marshal each call between the 2 AppDomains). I think I remember several other articles on plugins here that also deal with this topic.

GeneralRe: UnLoad PlugIns Pin
Anonymous18-Aug-05 4:06
memberAnonymous18-Aug-05 4:06 

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