该类型不能用作泛型类型或方法中的类型参数“TTo”。 没有隐式引用转换

Question

I'm currently trying to use Unity to inject ViewModels into my WPF Application.

I tend to use a ViewModelLocator class to store all of my ViewModels in one area, for quick navigation between them

I've split up my app into four projects:

KeystonePP.Models - Holds my EF Model

KeystonePP.Startup - Main startup logic

KeystonePP.ViewModels - ViewModels and their interfaces

KeystonePP.Views - Views

I have an IViewModelLocator interface in a separate project: KeystonePP.ViewModels

public interface IViewModelLocator
{
    // No code here. Just a contract
}

This is implemented by my ViewModelLocator class

public class ViewModelLocator : ObservableObject, IViewModelLocator
{
    // Lots of code here
}

In my KeystonePP.Startup project, I have adjusted my MainWindow.xaml.cs constructor as such:

public partial class MainWindow : Window
{
    public MainWindow (IViewModelLocator viewModelLocator)
    {
        InitializeComponent();
        DataContext = viewModelLocator;
    }
}

However, when I attempt the following code in App.xaml.cs , I'm getting an error warning:

public partial class App : Application
{
    /// <summary>
    /// Startup Logic for App
    /// </summary>
    /// <param name="e"></param>
    protected override void OnStartup (StartupEventArgs e)
    {
        base.OnStartup(e);

        IUnityContainer container = new UnityContainer();
        container.RegisterType<IViewModelLocator, MainWindow>();
    }
}

I get the following error:

The type 'KeystonePP.Startup.MainWindow' cannot be used as type parameter 'TTo' in the generic type or method 'UnityContainerExtensions.RegisterType(IUnityContainer, params InjectionMember[])'. There is no implicit reference conversion from 'KeystonePP.Startup.MainWindow' to 'KeystonePP.ViewModels.Utility.Interfaces.IViewModelLocator'.

Have I gone too complicated, or am I missing something simple setting up Unity?

Answer 1

You want to register abstractions with their implementations.

IUnityContainer container = new UnityContainer();
container.RegisterType<IViewModelLocator, ViewModelLocator>();
container.RegisterType<MainWindow>();

that way when you call to resolve MainWindow

var mainWindow = container.Resolve<MainWindow>();
mainWindow.Show();

the container will know how to inject the implementation based on the abstraction when resolving.