棱镜MEF——导入服务

Question

我正在尝试弄清楚如何正确地将服务导入到我的 ViewModel 中...这是我的相关代码（我省略了不重要的内容）：

ClientBootstrapper.cs:

public sealed class ClientBootstrapper : MefBootstrapper
{
    protected override void ConfigureAggregateCatalog()
    {
        base.ConfigureAggregateCatalog();

        //Add the executing assembly to the catalog.
        AggregateCatalog.Catalogs.Add(new AssemblyCatalog(Assembly.GetExecutingAssembly()));
    }

    protected override DependencyObject CreateShell()
    {
        return Container.GetExportedValue<ClientShell>();
    }

    protected override void InitializeShell()
    {
        base.InitializeShell();

        Application.Current.MainWindow = (Window)Shell;
        Application.Current.MainWindow.Show();
    }
}

ClientShell.xaml .cs：

[Export()]
public partial class ClientShell : Window
{
    [Import()]
    public ClientViewModel ViewModel
    {
        get
        {
            return DataContext as ClientViewModel;
        }
        private set
        {
            DataContext = value;
        }
    }

    public ClientShell()
    {
        InitializeComponent();
    }
}

ClientViewModel.cs：

[Export()]
public class ClientViewModel : NotificationObject, IPartImportsSatisfiedNotification
{
    [Import()]
    private static RandomService Random { get; set; }

    public Int32 RandomNumber
    {
        get { return Random.Next(); } //(2) Then this throws a Null Exception!
    }

    public void OnImportsSatisfied()
    {
        Console.WriteLine("{0}: IMPORTS SATISFIED", this.ToString()); //(1)This shows up
    }
}

RandomService.cs：

[Export()]
public sealed class RandomService
{
    private static Random _random = new Random(DateTime.Now.Millisecond);

    public Int32 Next()
    {
        return _random.Next(0, 1000);
    }
}

---

I do get the notification that all the import parts have been satisfied (1), but then I get a NullReferenceException (2) on return Random.Next(); inside of the ClientViewModel. Not sure why I would get a NullReferenceException after I'm told that all Imports are satisfied...

Answer 1

MEF 无法满足静态属性的导入。使随机服务成为实例属性。

Answer 2

您可以使用[ImportingConstructor]并在构造函数中设置静态属性。

private static RandomService Random { get; set; }

[ImportingConstructor]
public ClientViewModel(RandomService random)
{
   Random = random;
}

只是不要将其设置为静态字段。