In two articles on the Developer Code Samples Gallery is described how to create a simple plugin mechanism. The sample code is written in C# and VB.NET.
The first article introduces into Creating a simple plugin mechanism (part 1) by reflections.
The second article introduces into Creating a simple plugin mechanism (part 2) by the Managed Extensibility Framework (MEF).
On the TechNet Wiki both articles are outlined in Creating a simple plugin mechanism.
To solve this problem, a simple plugin mechanism is implemented that search and loads plugins from a predefined location. The created plugins are projects in form of built assemblies (DLLs).
First we need to define an Interface that all plugins must implement. This Interface is often included in an own project, so other developers only need the assembly of this project to write their own plugins. The members of the Interface depend on what your application is intended to do. In this sample we have one property that is returning a name and one method that is doing something.
To provide a plugin, you have to create a new project and add a reference to PluginContracts. Then you have to implement IPlugin.
Next we need to implement the framework in our main application that knows how to find and how to handle the plugins.
Searching for plugins
First of all we have to know where to search for plugins. Usually we will specify a folder in that all plugins are put in. In this folder we search for all assemblies.
Loading the plugins
Next we have to load the assemblies. Therefore we are using Reflections (System.Reflection ).
Searching for plugin implementations
Now we have loaded all assemblies from our predefined location, we can search for all types that implement our Interface IPlugin.
Instantiate plugin implementations
Last we create instances from our found types using Reflections.
In the main application we can use the implemented properties and methods of our plugins. We create a button for each loaded plugin and connect the content and the click event of the button to the property and the method of the plugin to demonstrate that.
We are using a Dictionary with the name of the plugin as key to memorize, which button content belongs to which plugin. So we can execute the correct plugin method on certain button click.
Creating a plugin mechanism with MEF differs not in all parts from creating a plugin mechanism from scratch. So only the differences are described in the next sections. Furthermore with the MEF, the plugin framework exists already and must not be implemented.
In the source of the plugins that we want to provide, we have to make our first changes. We have to add the reference to System.ComponentModel.Composition and using System.ComponentModel.Composition. Now we can mark the class with the Export attribute. So the MEF will later find and process this class. Furthermore the type is explicit stated. Hence we want to use the interface rather than the concrete class in the main project.
Next we need to implement the mechanism in our main application that knows how to find and how to handle the plugins. For that we are using methods provided by MEF.
So that we can use the Export marked parts, we have to mark properties with the Import attribute. In our case we want to use several exported parts, so we are using the ImportMany attribute.Last we need to search for the exported and imported parts and compose them. Therefore we specify a folder in that all plugins are put in.
So far we have searched and initialized our plugins, so we can use the implemented properties and methods of our plugins. To access our plugins we use the implemented MEFPluginLoader.
So that we do not need to implement for each new solution its own plugin loader, we use Generics. This allows us to remove the dependence from a certain plugin interface (here IPlugin).
With that we could also use the same plugin loader to resolve several types of plugins in the same project. So we could have an ICalculationPlugin, IExporterPlugin, ISomethingElsePlugin. All these plugin interfaces can have their own properties and methods. They can be accessed from different places in different cases.