Using MEF for Plug-in Design: Part 1

MEF stands for Managed Extensibility Framework. It is now included in .Net 4.0.

Here I show you a simplest example to create a plugin to load modules dynamically.

Step 1. Design your interface. For example:
<br />public interface IMathFunc<br />{<br /> string Name { get; set; }<br /> double FuncX(double x);<br />}<br />
Step 2. Implement the interface, and "export" the class.
a. Add reference to System.ComponentModel.Composition
b. Add using System.ComponentModel.Composition;
c. Create a class that implement the interface and add Export attribute. For example:
<br />[Export(typeof(IMathFunc))]<br />public class SinFunc : IMathFunc<br />{<br /> public SinFunc()<br /> {<br /> Name = "sin(x)";<br /> }<br /> <br /> public string Name { get; set; }<br /><br /> public double FuncX(double x)<br /> {<br /> return Math.Sin(x * Math.PI / 180.0);<br /> }<br />}<br />
Step 3. "Import" the class.
a. Create a Catalog. Since plug-ins usually implemented as separate DLLs, let's use DirectoryCatalog for this case.
b. Create the CompositionContainer with the catalog.
c. Use ImportMany to import a list of the classes.
d. Do ComposeParts() for the class you want to import the plugins.
e. In case, some user dropped in a new DLL, you can do Catalog.Refresh() to refresh the list of plugins.
For example:
<br />class Program<br />{<br /> [ImportMany(AllowRecomposition=true)]<br /> List<IMathFunc> MathFuncs { get; set; }<br /> DirectoryCatalog Catalog { get; set; }<br /> CompositionContainer Container { get; set; }<br /><br /> static void Main(string[] args)<br /> {<br /> Program p = new Program();<br /> string pluginDir = p.GetPluginDir();<br /> p.Compose();<br /> p.Run();<br /> }<br /><br /> private void Compose()<br /> {<br /> Catalog = new DirectoryCatalog(GetPluginDir());<br /> Container = new CompositionContainer(Catalog);<br /> Container.ComposeParts(this);<br /> }<br /><br /> private void Run()<br /> {<br /> double x = 0.0;<br /> while (true)<br /> {<br /> foreach (IMathFunc fx in MathFuncs)<br /> {<br /> Console.WriteLine("Func Name = {0}", fx.Name);<br /> Console.WriteLine("Func({0}) = {1}", x, fx.FuncX(x));<br /> }<br /> x += 15.0;<br /> Console.ReadLine();<br /> Console.WriteLine("Refreshing Catalog...");<br /> Catalog.Refresh();<br /> }<br /> }<br /> . . .<br /> . . .<br /> }<br />
It's simple. Isn't it?