.NET Core：如何在IHostedService间引用且避免重复实例？

Let's break down exactly what's happening here, and how to fix it cleanly:

The Root Cause #

When you use serviceCollection.AddHostedService<Domain>(), you're not registering Domain as a resolvable service type in the DI container—you're only registering it as an implementation of IHostedService.

Under the hood, AddHostedService<T> does something like this:

services.AddSingleton<IHostedService, T>();

This means the container knows how to provide an IHostedService instance (which happens to be a Domain), but it has no idea how to resolve a direct request for the Domain type itself. That's why when ComplianceService tries to inject a Domain in its constructor, the container throws that InvalidOperationException—it can't find a registered service for Domain.

Why Your Temporary Fix Creates Two Instances #

When you added serviceCollection.AddSingleton<Domain>(), you told the container how to resolve Domain directly. But then AddHostedService<Domain>() still creates a second Domain instance, because it's registering a new IHostedService implementation separate from your singleton Domain registration.

Your workaround using AddHostedService(sp => sp.GetRequiredService<Domain>()) fixes this by reusing the singleton instance, but it feels clunky because you're manually wiring things up.

The Clean, Intuitive Solution #

The fix is straightforward: register Domain as a singleton first, then use it as your hosted service. This way, the DI container knows how to resolve Domain for ComplianceService, and you only get one instance running as a hosted service.

Here's the corrected code:

// First, register Domain as a singleton so the container can resolve it for dependencies
serviceCollection.AddSingleton<Domain>()
    // Keep your other dependency registrations
    .AddSingleton<RulesActivator>()
    .AddSingleton<Accruals>()
    .AddSingleton<IRuleRepository, ComplianceRuleRepository>()
    .AddSingleton<IViolation, Violation>();

// Now add Domain as a hosted service by reusing the singleton instance
serviceCollection.AddHostedService(sp => sp.GetRequiredService<Domain>());

// Conditionally add ComplianceService as before
if (this.Configuration.GetValue<bool>("ComplianceEnabled")) { 
    serviceCollection.AddHostedService<ComplianceService>(); 
}

This approach:

• Lets ComplianceService successfully inject Domain (since it's registered in the container)
• Ensures only one Domain instance exists (both the hosted service and ComplianceService use the same singleton)
• Follows DI best practices by making your service registrations explicit and intuitive

Key Takeaway #

AddHostedService<T> is for registering something as a background service, not for making that type available as a dependency to other services. If you need a hosted service to be injectable elsewhere, you must first register the type itself in the DI container, then reference that registered instance when adding it as a hosted service.

内容的提问来源于stack exchange，提问作者Don Airey