Table of Contents

BLAZMVVM0002: ViewModelDefinition Attribute

Summary

ViewModels must be decorated with the [ViewModelDefinition] attribute to ensure proper dependency injection registration.

Description

This analyzer ensures that ViewModel classes that inherit from ViewModelBase are decorated with the [ViewModelDefinition] attribute. This attribute is essential for automatic service registration in the dependency injection container.

Severity

Warning - This should be addressed so the ViewModel is properly registered and resolved.

Why This Rule Exists

  • Ensures ViewModels are registered in the DI container
  • Specifies the service lifetime (Transient, Scoped, Singleton)
  • Enables automatic ViewModel resolution in Views
  • Prevents runtime errors when navigating to Views

✅ DO: Correct Usage

using Blazing.Mvvm.ComponentModel;
using Microsoft.Extensions.DependencyInjection;

// ✅ Correct: Transient lifetime (default, recommended for most ViewModels)
[ViewModelDefinition(Lifetime = ServiceLifetime.Transient)]
public class ProductViewModel : ViewModelBase
{
    public string Name { get; set; }
}

// ✅ Correct: Singleton lifetime (for app-wide state)
[ViewModelDefinition(Lifetime = ServiceLifetime.Singleton)]
public class AppStateViewModel : ViewModelBase
{
    public bool IsAuthenticated { get; set; }
}

// ✅ Correct: Scoped lifetime (for request-scoped data)
[ViewModelDefinition(Lifetime = ServiceLifetime.Scoped)]
public class ShoppingCartViewModel : ViewModelBase
{
    private readonly List<Item> _items = new();
}

// ✅ Correct: Abstract base ViewModels don't need the attribute
public abstract class BaseViewModel : ViewModelBase
{
    // Shared logic
}

[ViewModelDefinition(Lifetime = ServiceLifetime.Transient)]
public class ConcreteViewModel : BaseViewModel
{
    // Specific implementation
}

❌ DON'T: Incorrect Usage

using Blazing.Mvvm.ComponentModel;

// ❌ Wrong: Missing ViewModelDefinition attribute
public class ProductViewModel : ViewModelBase
{
    public string Name { get; set; }
}

// ❌ Wrong: Wrong attribute used
[Serializable]
public class UserViewModel : ViewModelBase
{
    public string Username { get; set; }
}

// ❌ Wrong: Manual service registration conflicts
// In Program.cs
services.AddTransient<OrderViewModel>(); // Don't do this!

// In ViewModel file
[ViewModelDefinition(Lifetime = ServiceLifetime.Singleton)]
public class OrderViewModel : ViewModelBase
{
    // This creates a conflict!
}

How to Fix

Add the ViewModelDefinition Attribute

// Before
using Blazing.Mvvm.ComponentModel;

public class ProductViewModel : ViewModelBase
{
    public string Name { get; set; }
}

// After
using Blazing.Mvvm.ComponentModel;
using Microsoft.Extensions.DependencyInjection;

[ViewModelDefinition(Lifetime = ServiceLifetime.Transient)]
public class ProductViewModel : ViewModelBase
{
    public string Name { get; set; }
}

Choosing the Right Lifetime

  • New instance created each time the ViewModel is resolved
  • Use for most ViewModels
  • Prevents state leakage between navigations
[ViewModelDefinition(Lifetime = ServiceLifetime.Transient)]
public class ProductDetailsViewModel : ViewModelBase { }

Scoped

  • Single instance per scope (request/circuit in Blazor)
  • Use for shopping carts, user sessions
  • Shared within the same user session
[ViewModelDefinition(Lifetime = ServiceLifetime.Scoped)]
public class ShoppingCartViewModel : ViewModelBase { }

Singleton

  • Single instance for the entire application lifetime
  • Use for app-wide state, settings
  • Be careful with thread safety
[ViewModelDefinition(Lifetime = ServiceLifetime.Singleton)]
public class AppSettingsViewModel : ViewModelBase { }

Code Fix

This analyzer provides an automatic code fix that:

  1. Adds [ViewModelDefinition(Lifetime = ServiceLifetime.Transient)] attribute
  2. Adds the necessary using directives
  • BLAZMVVM0001: ViewModelBase Inheritance - Ensures proper base class
  • BLAZMVVM0013: MvvmOwningComponentBase Usage - For ViewModels with scoped dependencies

Additional Resources

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