Object reference exeption - Dependency Injection with Entity Framework WPF

Question

I tried to implement connection to database using Entity Framework and Dependency Injection.
I want to create Host in App.xaml.cs.

public partial class App : Application
{
  
    public static IHost? AppHost { get; private set; }
    public App()
    {
        AppHost = Host.CreateDefaultBuilder()
             .ConfigureServices((hostContext, services) =>
             {
                 services.AddSingleton<LoginWindow>();
                 services.AddSingleton<LoginViewModel>();

                 services.AddDbContext<KnitterNotebookContext>(
                     options =>
                     {
                         string appSettingsPath = Path.Combine(ProjectDirectory.ProjectDirectoryFullPath, "appsettings.json");
                         string appSettingsString = File.ReadAllText(appSettingsPath);
                         AppSettings AppSettings = JsonConvert.DeserializeObject<AppSettings>(appSettingsString)!;
                         options.UseSqlServer(AppSettings.KnitterNotebookConnectionString);
                     });
             })
             .Build();
    }

    protected override async void OnStartup(StartupEventArgs e)
    {
        await AppHost!.StartAsync();
        var startupWindow = AppHost.Services.GetRequiredService<LoginWindow>();
        startupWindow.Show();
        base.OnStartup(e);
    }

    protected override async void OnExit(ExitEventArgs e)
    {
        await AppHost!.StopAsync();
        base.OnExit(e);
    }

I want to pass DbContext as parameter to ViewModel, but when I do, it throws exception.

 public class LoginViewModel : BaseViewModel
{
    public LoginViewModel(KnitterNotebookContext knitterNotebookContext)
   //public LoginViewModel()
    {
        KnitterNotebookContext = knitterNotebookContext;
        ShowRegistrationWindowCommand = new RelayCommand(ShowRegisterWindow);
        LogInCommandAsync = new AsyncRelayCommand(LogIn);
    }

    private KnitterNotebookContext KnitterNotebookContext { get; set; } 
 }

There is no problem if I use parameterless constructor of LoginViewModel and create new instances of KnitterNotebookContext with new(), but I want to pass it as a parameter. How to solve it?

You'll need to include the full stack trace. As text, not an image. We don't need to see visual studio, we need to copy / paste your code and errors. — Jeremy Lakeman
– Jeremy Lakeman, Commented Feb 10, 2023 at 0:11
You're showing us this failing on loginwindow but somehow loginviewmnodel and it's parameterless constructor come into this. How so? — Andy
– Andy, Commented Feb 10, 2023 at 9:16

Rusmirnator · Accepted Answer · 2023-02-10 09:34:53Z

I happened to encounter the same problem at one time - i'm guessing that you are setting DataContext for MainWindow in xaml, like this:

<Window.DataContext>
    <local:LoginViewModel/>
<Window.DataContext>

Doing it this way, when your LoginWindow is created, tries to create new instance of its DataContext (your LoginViewModel), but it does not know how to resolve constructor - since you injected it with service, it's not parameterless anymore.

You have to provide a way, to enforce DataContext to be instantiated by ServiceProvider (a container for all injected services).

You could achieve this through use of a helper class, which is usable in xaml (inherits from MarkupExtension), like this one below.

public class ServiceDispatcher : MarkupExtension
{
    public static Func<Type, object> Resolver { get; set; }
    public Type Type { get; set; }
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        return Resolver?.Invoke(Type);
    }
}

And assigning it's service dispatching Resolver delegate in your App.xaml.cs

public App()
{
    AppHost = Host.CreateDefaultBuilder()
         .ConfigureServices((hostContext, services) =>
         {
             services.AddSingleton<LoginWindow>();
             services.AddSingleton<LoginViewModel>();

             services.AddDbContext<KnitterNotebookContext>(
                 options =>
                 {
                     string appSettingsPath = Path.Combine(ProjectDirectory.ProjectDirectoryFullPath, "appsettings.json");
                     string appSettingsString = File.ReadAllText(appSettingsPath);
                     AppSettings AppSettings = JsonConvert.DeserializeObject<AppSettings>(appSettingsString)!;
                     options.UseSqlServer(AppSettings.KnitterNotebookConnectionString);
                 });
         })
         .Build();
         
    ConfigureServiceProvider(AppHost.Services);
}

private static void ConfigureServiceProvider(IServiceProvider appHost)
{
    ServiceDispatcher.Resolver = (type) =>
    {
        return appHost.GetRequiredService(type);
    };
}

Now you can use your ServiceDispatcher to dynamically provide DataContext instance for your Views using syntax below.

<Window.DataContext>
    <base:ServiceDispatcher Type="{x:Type local:LoginViewModel}"/>
</Window.DataContext>

Thank you so much. You were right, I had configured DataContext as you thought. Thank you for help, I will fix my application.

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