Using Dependency Injection in WPF MVVM Applications

TLDR

• Introducing Microsoft.Extensions.DependencyInjection in WPF enables modern dependency injection, replacing traditional Singleton or manual management patterns.
• Using CommunityToolkit.Mvvm simplifies MVVM architecture development by automatically generating properties and commands via Source Generators.
• You must remove the StartupUri attribute from App.xaml and manually instantiate MainWindow in OnStartup; otherwise, the application will throw an error due to failed constructor parameter injection.
• ViewModels must inherit from ObservableObject and be declared as a partial class to utilize [ObservableProperty] and [RelayCommand] attributes.
• Pay attention to naming conventions during binding: properties generated by [ObservableProperty] use Pascal Case, and commands generated by [RelayCommand] require a Command suffix.

Using Dependency Injection in WPF

When introducing a DI container (such as Microsoft.Extensions.DependencyInjection) into a WPF project, you must adjust the application startup process.

App.xaml Configuration and Startup Logic

When you encounter this issue: When you modify MainWindow to inject dependencies via a DI container (e.g., injecting a ViewModel into the constructor).

Since the default WPF StartupUri mechanism attempts to call a parameterless constructor, if you switch to using DI to create MainWindow, you must remove the StartupUri attribute from App.xaml and manually initialize it in the OnStartup method of App.xaml.cs:

public partial class App : Application {
    protected override void OnStartup(StartupEventArgs e) {
        IConfigurationBuilder builder = new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);

        IConfiguration configuration = builder.Build();

        ServiceCollection serviceCollection = new ServiceCollection();
        ConfigureServices(serviceCollection, configuration);

        ServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();

        MainWindow mainWindow = serviceProvider.GetRequiredService<MainWindow>()!;
        mainWindow.Show();
    }

    private static void ConfigureServices(IServiceCollection services, IConfiguration configuration) {
        services.Configure<AppOptions>(configuration!.GetSection("App"));
        services.AddTransient<MainWindow>();
        services.AddTransient<ViewModel>();
    }
}

Building a WPF MVVM Application

Using CommunityToolkit.Mvvm can significantly reduce boilerplate code.

ViewModel Implementation Standards

When you encounter this issue: When you need to implement data binding and commands but do not want to manually write tedious INotifyPropertyChanged implementations.

ViewModels must adhere to the following standards:

• Inherit from ObservableObject.
• Be declared as a partial class.
• Use the [ObservableProperty] attribute on fields (the Source Generator will automatically generate the corresponding Pascal Case property).
• Use the [RelayCommand] attribute on methods (the Source Generator will automatically generate the corresponding Command property).

public partial class ViewModel : ObservableObject {
    [ObservableProperty]
    private string? input;

    [RelayCommand]
    private void Submit() {
        MessageBox.Show("Input value: " + Input);
        Input += "_Modified";
        MessageBox.Show("Changed value: " + Input);
    }
}

MainWindow Binding Configuration

When you encounter this issue: When performing data binding in XAML, failing to follow the automatically generated naming rules will result in binding failures.

Inject the ViewModel into the MainWindow constructor and set the DataContext:

public partial class MainWindow : Window {
    public MainWindow(ViewModel viewModel) {
        InitializeComponent();
        DataContext = viewModel;
    }
}

In XAML, binding names must correspond to the results generated by the Source Generator:

• TextBox binds to Input (corresponding to the input field).
• Button binds to SubmitCommand (corresponding to the Submit method).

<Window x:Class="WpfApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <TextBox Text="{Binding Input}"/>
        <Button Content="Submit" Command="{Binding SubmitCommand}"/>
    </Grid>
</Window>

Execution Results

• After clicking "Submit", the Submit() method executes successfully and retrieves the Input value.
• When Submit() modifies the Input property, the TextBox on the UI updates automatically.

Change Log

• 2023-02-15 Initial documentation created.