Using the Managed Extensibility Framework (MEF) to implement a third-party plugin solution. Part 2–Plugin Support in the Application

Now that I have a feature-complete application from my perspective, I am ready to add plugin support so that other developers can extend the functionality of my application.  I don’t want to make my application open source, so I want to just expose enough information about my application to them that they are able to create plugins.

In order to give my application access to MEF, I need to add a reference to the appropriate namespace within my windows forms application.  The namespace I need to reference is System.ComponentModel.Composition.  Once I’ve done that, I need to add a new project to my solution of type “Class Library.”  This project will be my “Plugin API” and will compile to a .dll file that I can distribute to developers as a part of my SDK so that they can create plugins that adhere to my standard.  After creating my new project, I need add a reference to that project from within my Windows Forms project.

At this point, my application solution looks like this:

Solution Explorer

Here are the contents of IPlugin.cs in the MyPluginApp.Plugins project:

using System;

namespace MyPluginApp.Plugins
    //This interface defines what we expect a plugin developer to impliment in order for our application to make use of their plugin
    public interface IPlugin
        //This method's signature matches the MyButtonClickedEventHandler in our application
        void HandleNumbers(decimal num1, decimal num2);

    //MEF allows for metadata to be exported along with the object itself, we need a separate interface to receive any metatdata that we expect
    // plugin developers to include as well.
    public interface IPluginMetaData
        string Name { get;}

Next I need to update my UI so that it is able to display the list of available plugins and allow the user enable/disable them at will.  I’ve opted to use a checked list box to achieve that goal.  The plugins will load initially disabled, and the user simply needs to check the box next to the plugin in order to enable it, similarly unchecking the box will cause the plugin to be disabled.

Here is my New UI


And now the code for the form, including the code to enable plugin support.  New lines of code are highlighted for your convenience.

using System;
using System.Windows.Forms;

using System.Collections.Generic;
using MyPluginApp.Plugins;  //Our plugin namespace
using System.ComponentModel.Composition.Hosting; //MEF namespace

namespace MyPluginApp
    public partial class Form1 : Form
        private delegate void MyButtonClickedEventHandler(decimal num1, decimal num2);
        private event MyButtonClickedEventHandler MyButtuttonClicked;

        private readonly Dictionary<string, IPlugin> _plugins = new Dictionary<string, IPlugin>();

        public Form1()


        //the Leave event handler for both text boxes
        private void textNum_Leave(object sender, EventArgs e)
            decimal d;
            var textNum = (TextBox) sender;

            if (!decimal.TryParse(textNum.Text, out d))
                textNum.Text = "5";

        //The KeyDown event handler for both text boxes
        private void txtNum_KeyDown(object sender, KeyEventArgs e)
            if (e.KeyData == Keys.Enter || e.KeyData == Keys.Return)
                btnMyButton.Focus(); //fires the Leave event for the active text box

        //OnClick event handler for the button
        private void btnMyButton_Click(object sender, EventArgs e)
            decimal num1;
            decimal num2;

            MessageBox.Show(string.Format("You entered {0} and {1}", txtNum1.Text, txtNum2.Text));

            if (decimal.TryParse(txtNum1.Text, out num1) && decimal.TryParse(txtNum2.Text, out num2))
                //If our new event has any subscribers, invoke the event.
                if(this.MyButtuttonClicked != null)
                    this.MyButtuttonClicked.Invoke(num1, num2);

        private void LoadPlugins()
            var pluginDir = System.IO.Path.GetDirectoryName(Application.ExecutablePath) + @"\Plugins\";

            //Create plugin directory if it doesn't exist

            //DirectoryCatalog gets a list of all .dll files in our plugin directory
            var catalog = new DirectoryCatalog(pluginDir, "*.dll");

            //CompositionContainer parses all of the dlls in the catalog and identifies their exports
            var container = new CompositionContainer(catalog);

            //Iterate through each export that matches our IPlugin interface and add it to our plugin collection
            foreach (var plugin in container.GetExports<IPlugin, IPluginMetaData>())
                this._plugins.Add(plugin.Metadata.Name, plugin.Value);
                this.clbPlugins.Items.Add(plugin.Metadata.Name, false);

            //Best practice per MS is to dispose the CompositionContainer as soon as we are finished with it.

        //ItemCheck event for clbPlugins (checked list box)
        private void clbPlugins_ItemCheck(object sender, ItemCheckEventArgs e)
            var item = clbPlugins.Items[e.Index].ToString();

            if (e.NewValue == CheckState.Checked)
                //If the user checked the box for the plugin, subscribe the plugin's handler to the event
                this.MyButtuttonClicked += this._plugins[item].HandleNumbers;
                //If the user unchecked the box for the plugin, unsubscribe the plugin's handler to the event
                this.MyButtuttonClicked -= this._plugins[item].HandleNumbers;

This concludes post #2.  In Post #3 I will create a few example plugins in their own solution without access to any of the code contained in Form1.cs, just as I would expect a 3rd party developer to be able to do.

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