ASP.NET Core 2.0.0通知服务选型：singleton注入scopedService及生命周期疑问

Great question! Let's break down whether using a scoped service makes sense for your notification service in ASP.NET Core 2.0, and address your concerns about state management and dependencies.

首先，Scoped服务的合理性（针对依赖注入需求） #

Absolutely, using a scoped service is a valid choice here, especially since you need access to DbContext and Identity/Entity Framework-related services:

• DbContext is registered as scoped by default in ASP.NET Core, so a scoped notification service can directly depend on it without any issues.
• Identity services like UserManager<TUser> are typically registered as scoped or transient, which also play nicely with a scoped notification service.
• Scoped services are created once per request, which aligns perfectly with the lifecycle of most web request operations (like sending a notification or marking messages as read, which are tied to a single user request).

但要注意：badgeCount成员变量的状态问题 #

Here's a critical caveat: if badgeCount is a member variable stored directly in your scoped service, it will not persist across requests. Each incoming request gets its own instance of the scoped service, so the badgeCount value will reset with every new request.

If your badgeCount is meant to represent a user-specific unread notification count (which is usually the case), you shouldn't rely on the service's in-memory state. Instead:

• Fetch the count directly from your DbContext when needed (querying unread notifications for the current user).
• Or cache the value in a distributed cache (like Redis) or in-memory cache (with user-specific keys) for better performance, updating it whenever notifications are sent or marked as read.

如果你确实需要全局状态（比如系统级通知计数） #

If you need a global badgeCount that's shared across all requests (unlikely for user-specific badges, but possible for system-level metrics), a singleton service is still an option—you just need to work around the scoped dependency limitation:

• Register your notification service as a singleton.
• Inject IServiceScopeFactory into the singleton service.
• When you need to access DbContext or Identity services, create a temporary scope using IServiceScopeFactory.CreateScope(), resolve the scoped services from that scope, and dispose the scope when done.

Example code snippet for this approach:

public class NotificationService : INotificationService
{
    private readonly IServiceScopeFactory _scopeFactory;
    private int _globalBadgeCount; // Only for system-wide counts

    public NotificationService(IServiceScopeFactory scopeFactory)
    {
        _scopeFactory = scopeFactory;
    }

    public async Task SendNotificationAsync(string userId, string message)
    {
        using var scope = _scopeFactory.CreateScope();
        var dbContext = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        
        // Your notification logic here
        dbContext.Notifications.Add(new Notification { UserId = userId, Message = message });
        await dbContext.SaveChangesAsync();

        // Update global count if needed
        _globalBadgeCount++;
    }
}

总结 #

• Use a scoped service if your operations are request-bound and badgeCount doesn't need to persist across requests (or you're fetching/storing it in a database/cache). This will simplify dependency injection and align with standard ASP.NET Core practices.
• Use a singleton service with scoped resolution only if you truly need a shared in-memory state across all requests, and be sure to handle scoped dependencies correctly with IServiceScopeFactory.

内容的提问来源于stack exchange，提问作者user6728767