In my previous post I used MEF to decouple my unit tests from runtime data by using the Repository Design Pattern to implement a repository meant for “test time” consumption. In this post, I’ll take the Repository Design Pattern one step further and couple it with MEF to provide design time data.

I’ll start by adding a new class library project named Customer.Repositories.Design to the solution (I don’t think we need a picture for this). I’ll then delete the default Class1.cs file that gets created. Next I’ll add references to the following:

• The Customer.Contracts project
• System.ComponentModel.Composition

Next I’ll implement the ICustomer and ICustomerRepository interfaces. Here’s what I came up with:

Nothing too fancy. They’re virtually identical to the classes we created for unit testing. I do want to show the implementation of the DesignCustomerRepository:

As with the test repository, I placed the [Export] attribute on the DesignCustomerRepostiory to make it available to be consumed by MEF. Also pay attention to the first name (“Design”) of each customer in the repository. This will be important to keep in mind when we finally get the runtime repository implemented.

Now we need to consume this repository. To do so, I’ll start by creating a WPF application called Customer.Wpf. I’ll then add a reference to the Customer.Core project. I’m then going to add a class to the project called ViewModelLocator. This class will determine which repository the view model will used based on whether we’re in the designer or if we’re running the application. (This class is based on the Laurent Bugnion’s MVVMLight Toolkit.) There are a couple of things I ‘d like to highlight in the class. The first is how we determine if were in design mode:

The implementation for Silverlight is a little bit different, but I’ll cover that in a future post. The next is the constructor for the class:

Here I’m passing in the path to the assembly for the AssemblyCatalog to use in the view model. When we’re ready for runtime data we’ll plug in an else statement here.

Next I’ll want to make the ViewModelLocator class a resource that I can use in databinding. To do this, I’ll update App.xaml to look like the following:

I’ll then update MainWindow.xaml to bind to the CustomerListViewModel property of the ViewModelLocator class:

If we take a peek at the design surface…

…we’ll see our “design” customers displayed!

You can download the source code from this post here. In my next post I’ll use MEF to get our runtime customer data.