Table of Contents

BLAZMVVM0009: Service Injection

Summary

Services should be injected via constructor in ViewModels, not using [Inject] attribute.

What This Rule Checks

BLAZMVVM0009 reports properties decorated with Microsoft.AspNetCore.Components.InjectAttribute when the containing type is a ViewModel. Constructor-injected dependencies do not produce this diagnostic.

This rule does not inspect the application's dependency injection registrations or determine whether constructor parameters are registered. It also does not detect service locator usage or services created directly with new.

Severity

Warning - Using [Inject] in ViewModels is not the recommended pattern.

Why This Rule Exists

  • ViewModels should use constructor injection, not property injection
  • Makes dependencies explicit and testable
  • Follows dependency injection best practices
  • [Inject] is for Blazor components, not ViewModels
  • Enables proper unit testing

✅ DO: Correct Usage

// ✅ Correct: Constructor injection in ViewModel
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    private readonly IProductService _productService;
    private readonly ILogger<ProductViewModel> _logger;

    public ProductViewModel(
        IProductService productService,
        ILogger<ProductViewModel> logger)
    {
        _productService = productService;
        _logger = logger;
    }

    [RelayCommand]
    private async Task LoadProductsAsync()
    {
        Products = await _productService.GetAllAsync();
    }

    public List<Product> Products { get; set; } = [];
}

// ✅ Correct: [Inject] in Blazor components is fine
public class ProductPage : MvvmComponentBase<ProductViewModel>
{
    [Inject]
    public required NavigationManager Navigation { get; set; }
}

❌ DON'T: Incorrect Usage

// ❌ Wrong: [Inject] in ViewModel
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    [Inject]  // Wrong! Use constructor injection
    public IProductService ProductService { get; set; } = null!;

    [Inject]  // Wrong! Use constructor injection
    public ILogger<ProductViewModel> Logger { get; set; } = null!;
}

// Related anti-pattern, but not detected by BLAZMVVM0009: Service locator
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    private IProductService? _productService;

    protected override Task OnInitializedAsync()
    {
        // Anti-pattern: service locator
        _productService = ServiceProvider.GetService<IProductService>();
        return Task.CompletedTask;
    }
}

// Related anti-pattern, but not detected by BLAZMVVM0009: Creating services directly
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    private readonly ProductService _service = new ProductService();  // Wrong!
}

How to Fix

Convert to Constructor Injection

// Before
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    [Inject]
    public IProductService ProductService { get; set; } = null!;

    [RelayCommand]
    private async Task LoadAsync()
    {
        await ProductService.LoadDataAsync();
    }
}

// After
[ViewModelDefinition]
public class ProductViewModel : ViewModelBase
{
    private readonly IProductService _productService;

    public ProductViewModel(IProductService productService)
    {
        _productService = productService;
    }

    [RelayCommand]
    private async Task LoadAsync()
    {
        await _productService.LoadDataAsync();
    }
}

Benefits

Testability

// Easy to test with constructor injection
[Fact]
public void Test_LoadProducts()
{
    var mockService = new Mock<IProductService>();
    var viewModel = new ProductViewModel(mockService.Object);

    // Test the ViewModel
}

Explicit Dependencies

// Constructor shows all dependencies at a glance
public ProductViewModel(
    IProductService productService,
    ILogger<ProductViewModel> logger,
    IMapper mapper)
{
    // Dependencies are clear
}

Null Safety

// Constructor injection ensures non-null
private readonly IProductService _productService;

public ProductViewModel(IProductService productService)
{
    _productService = productService;  // Never null
}

// vs property injection with nullability issues
[Inject]
public IProductService ProductService { get; set; } = null!;  // Suppression needed

Service Registration

// In Program.cs or Startup.cs
builder.Services.AddScoped<IProductService, ProductService>();
builder.Services.AddViewModels();  // Registers ViewModels with DI

Testing Example

public class ProductViewModelTests
{
    [Fact]
    public async Task LoadProducts_Success()
    {
        // Arrange
        var mockService = new Mock<IProductService>();
        mockService.Setup(s => s.GetAllAsync())
            .ReturnsAsync(new List<Product> { new() { Id = 1 } });

        var viewModel = new ProductViewModel(mockService.Object);

        // Act
        await viewModel.LoadProductsCommand.ExecuteAsync(null);

        // Assert
        Assert.Single(viewModel.Products);
    }
}

Code Fix

No automatic code fix is currently provided for BLAZMVVM0009. Convert property injection to constructor injection manually, as shown above.

Additional Resources

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