Ben d'état

Ben Scott


~/Setting up MEF

29 Apr 2012

The source code for the example application created while writing this post is available on GitHub.

I’ve been working on adding support for plugins to MarkPad. Initially this was going to use NuGet to drag in the plugins, and MEF to dynamically load the . Because of problems like how to manage updates with new versions and the general awkwardness of the build system, I’ve decided to drop NuGet for now and just link the plugins statically, still using MEF to handle basic dependency injection and loading up the plugins.

MEF seems tragically difficult to debug, especially when setting it up in an existing application like MarkPad, with Autofac already set up and doing magic, so I’ve set up a test application to get a feel of the general layout. This is a bit of a walkthrough of the result.

Projects / assemblies

There are three assemblies in my test application:

Both the Consumer and Plugins assemblies reference the Contracts assembly. In a real-world application, each plugin would reside in its own assembly and would be loaded from an Extensions folder or similar, but in this example I’ll just make the Consumer assembly reference the Plugins assembly. This is just to get the Plugins dll into the output directory, as later on MEF just imports Plugins.dll by name anyway. I could probably drop the reference and copy Plugins.dll to the output directory in the build, but what I’ve got is a bit simpler in the short term.

Spec for the example

The application

The application will consist of just two buttons and a text box. The first button will call all of the HelloWorld plugins, and the second will call all of the GoodbyeCruelWorld plugins. The value of the text box (labeled ‘Name’) will be made available to the plugins.

HelloWorld plugins will implement IHelloWorldPlugin, GoodbyeCruelWorld plugins will implement IGoodbyeCruelWorldPlugin, and the application will implement IGetName to expose the entered name.

I’ve got both HelloWorld and GoodbyeCruelWorld plugins to demonstrate different plugins providing different functionality to the consuming application, while using the same API.

Plugins

I’m going to have three plugins:

Initial setup

I won’t write the MEF decorators or management code yet, instead I’ll get the implementation code out of the way.

Contracts

The contracts / plugin API is very simple. I want a base plugin type (IPlugin), two plugin types (IHelloWorldPlugin and IGoodbyeCruelWorldPlugin) and a way to expose what is in the ‘name’ text box (IGetName).

public interface IPlugin { }

public interface IHelloWorldPlugin : IPlugin
{
	string GetHelloMessage();
}

public interface IGoodbyeCruelWorldPlugin
{
	string GetGoodbyeMessage();
}

public interface IGetName
{
	string GetName();
}

Plugins

The actual implementation of the plugins is also very simple.

public class BasicHelloWorldPlugin : IHelloWorldPlugin
{
	public string GetHelloMessage() { return "Hello, world!"; }
}

public class HelloNamePlugin : IHelloWorldPlugin
{
	IGetName _getName;
	public string GetHelloMessage() { return string.Format("Hello, {0}!", _getName.Name); }
}

public class GoodbyeCruelWorldPlugin : IGoodbyeCruelWorldPlugin
{
	public string GetGoodbyeMessage() { return "Noooooooo!!!!"; }
}

The IGetName _getName instance will be injected by MEF, I’ll add the decorator later on.

Consumer

First I’ll just sketch out how I want the consuming code to see and execute the plugins.

The application in the Consumer assembly needs to implement IGetName. There are other ways of doing this but this is rather straightforward. More complex examples might need to use singletons or have a bootstrap procedure that allows the plugin to connect to the consumer.

public partial class Form1 : Form, IGetName
{
	...
	public string GetName() { return GetNameTextBox.Text; }
}	

The plugins are injected into two sets of collections. MEF uses the exported interface to import the implementation into the correct places, so these will get the correct plugins assigned. Then the plugins are called in the appropriate places.

IEnumerable<IHelloWorldPlugin> _helloWorldPlugins;
IEnumerable<IGoodbyeCruelWorldPlugin> _goodbyeCruelWorldPlugins;

private void HelloWorld_Click(object sender, EventArgs e)
{
	foreach (var plugin in _helloWorldPlugins)
	{
		MessageBox.Show(plugin.GetHelloWorldMessage());
	}
}

private void GoodbyeCruelWorld_Click(object sender, EventArgs e)
{
	foreach (var plugin in _goodbyeCruelWorldPlugins)
	{
		MessageBox.Show(plugin.GetGoodbyeMessage());
	}
}

Hooking up MEF

Decorating the contract interfaces and importing fields

All the assemblies need to reference System.ComponentModel.Composition. As an aside, have you installed the PowerCommands extension for Visual Studio 2010? It lets you copy and paste project references within the Solution Explorer. Groovy.

All the contract endpoints (the interfaces that will be imported using MEF) need to have the InheritedExport attribute added to the interface. In this example, that means all of the interfaces in Contracts. Here is what IHelloWorldPlugin should look like with the InheritedExport attribute:

[InheritedExport]
public interface IHelloWorldPlugin : IPlugin
{
	string GetHelloMessage();
}

Any fields that need to be imported by MEF also need to be decorated. The _getName field in HelloNamePlugin should look like this:

[Import]
IGetName _getName;

And the consumer’s plugin collections should look like this:

[ImportMany]
IEnumerable<IHelloWorldPlugin> _helloWorldPlugins;
[ImportMany]
IEnumerable<IGoodbyeCruelWorldPlugin> _goodbyeCruelWorldPlugins;

If you run the application at this point, it will crash and burn because the consumer’s fields are all still null. I still need to configure MEF and get it to compose the parts of the application.

MEF configuration

The following goes in the consuming application’s constructor, after InitializeComponent() since this is a WinForms app:

var catalog = new AggregateCatalog();
catalog.Catalogs.Add(new AssemblyCatalog("Plugins.dll"));
var container = new CompositionContainer(catalog);
container.ComposeParts(this);

This creates an aggregate catalog, which brings together all the locations (catalogs) where the exported implementations are found. In this case it is just Plugins.dll. There are a couple of other XxxCatalog implementations that can be used, for more information the catalogs page in the MEF documentation touches on them.

The catalog is then used to create a composition container. Each class in the consumer that needs to import classes using MEF will need this catalog, so in more complex applications the composition container will need to be passed around. The ComposeParts() method call is what performs the actual dependency injection, finding all the fields decorated with [Import] and [ImportMany] and assigning them with the exported implementations that were found in the aggregate catalog.

Shake your groove thang

With everything wired up and MEF properly configured, pressing the ‘Say Hello’ button should show 'Hello, World' and 'Hello, <whatever is in the name text box>' . Pressing the ‘Say Goodbye’ button should just show a pitiful cry of despair.

In case you missed it at the top, the entire project is on GitHub.


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