Workshop: Importing Multiple Parts

If you have multiple classes that export the IPersonPart interface that we created in the previous workshop, MEF won't know which part to import into the MyImportedPersonPart property, and will throw an exception when you call CompositionInitializer.SatisfyImports. However, by turning the MyImportedPersonPart property into an array/collection, and using the ImportMany attribute instead of Import, multiple parts can be imported into the MyImportedPersonPart property.

In this workshop, we'll modify what we have from the previous two workshops, and make multiple parts available to the MainPage class.

1. Add a new class to the Chapter14Sample project, named AnotherPersonPart.cs, and add the following code to it:
   using System.ComponentModel.Composition;

namespace Chapter14Sample
{
    [Export(typeof(IPersonPart))]
    public class AnotherPersonPart : IPersonPart
    {
        public string Name { get; set; }

        public AnotherPersonPart()
        {
            Name = "Marge Simpson";
        }
    }
}

   As you can see, it's simply another implementation of the IPersonPart interface, essentially identical to the PersonPart class, but with a different value assigned to its Name property in its constructor.

2. If you try and run the application now, an exception will be raised because MEF doesn't know which class (PersonPart or AnotherPersonPart) it should import into the MyImportedPersonPart property on the MainPage class. We can make the MyImportedPersonPart property accept multiple parts by turning it into an array, and decorating it with the ImportMany attribute instead of the Import attribute, like so:
   [ImportMany]
public IPersonPart[] MyImportedPersonParts { get; set; }
3. In the first workshop, we created a TextBlock to display the value of the imported part's Name property. We now need to modify this code to display the value of the Name property for all of the imported parts. Instead of the TextBlock, let's display the names in a ListBox control. The full code for the MainPage class follows, with the required changes in bold:
   using System.ComponentModel.Composition;
using System.Windows.Controls;

namespace Chapter14Sample
{
    public partial class MainPage : UserControl
    {
        [ImportMany]
        public IPersonPart[] MyImportedPersonParts { get; set; }

        public MainPage()
        {
            InitializeComponent();

            CompositionInitializer.SatisfyImports(this);

            var list = new ListBox();
            list.ItemsSource = MyImportedPersonParts;
            list.DisplayMemberPath = "Name";
            LayoutRoot.Children.Add(list);
        }
    }
}
4. Run your application. Both parts will show up in the ListBox that has been added to the view. Go ahead and add additional classes to your project that implement the IPersonPart interface and export themselves. They too will automatically appear in the list.