Why Avoid Service Locators in Dependency Injection?

Why Avoid Service Locators?

Service Locator is an anti-pattern where you inject IServiceProvider and manually resolve dependencies:

// ❌ Bad - Service Locator pattern
public class OrderService
{
    private readonly IServiceProvider _serviceProvider;

    public OrderService(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public void ProcessOrder()
    {
        var repo = _serviceProvider.GetRequiredService<IOrderRepository>();
        var emailer = _serviceProvider.GetRequiredService<IEmailService>();
        // ...
    }
}

Problems with this approach:

Hidden dependencies - You can't tell what the class needs just by looking at its constructor. Dependencies are obscured inside the implementation.
Runtime failures - Missing dependencies only fail at runtime when that code path executes, not at application startup.
Hard to test - You have to mock IServiceProvider and set up complex mock behaviors instead of just passing in the dependencies.
Breaks IoC principle - The class is now coupled to the DI container itself, defeating the purpose of dependency injection.

Better approach - Constructor Injection:

// ✅ Good - Dependencies are explicit
public class OrderService
{
    private readonly IOrderRepository _repository;
    private readonly IEmailService _emailer;

    public OrderService(IOrderRepository repository, IEmailService emailer)
    {
        _repository = repository;
        _emailer = emailer;
    }

    public void ProcessOrder()
    {
        // Use _repository and _emailer directly
    }
}

Why Isolate Registrations Per Module?

Isolation means organizing your DI registrations by feature/module rather than dumping everything in Program.cs:

// ❌ Bad - Everything in one place
public class Program
{
    public static void Main(string[] args)
    {
        var builder = WebApplication.CreateBuilder(args);

        // 200+ lines of service registrations for all modules...
        builder.Services.AddScoped<IOrderRepository, OrderRepository>();
        builder.Services.AddScoped<IOrderService, OrderService>();
        builder.Services.AddScoped<IProductRepository, ProductRepository>();
        builder.Services.AddScoped<IUserRepository, UserRepository>();
        builder.Services.AddScoped<IAuthService, AuthService>();
        // ... many more
    }
}

Better approach - Extension methods per module:

// ✅ Good - Orders module owns its registrations
public static class OrdersServiceExtensions
{
    public static IServiceCollection AddOrdersModule(this IServiceCollection services)
    {
        services.AddScoped<IOrderRepository, OrderRepository>();
        services.AddScoped<IOrderService, OrderService>();
        services.AddScoped<IOrderValidator, OrderValidator>();
        return services;
    }
}

// Auth module
public static class AuthServiceExtensions
{
    public static IServiceCollection AddAuthModule(this IServiceCollection services)
    {
        services.AddScoped<IUserRepository, UserRepository>();
        services.AddScoped<IAuthService, AuthService>();
        return services;
    }
}

// Program.cs stays clean
builder.Services.AddOrdersModule();
builder.Services.AddAuthModule();
builder.Services.AddProductsModule();

Benefits:

Maintainability - Each module's dependencies are colocated with that module's code.
Discoverability - Easy to find what services a module provides.
Modularity - Modules can be enabled/disabled, or even moved to separate assemblies.
Testability - You can register just the modules needed for integration tests.
Clean startup - Program.cs remains readable and high-level.

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Why Avoid Service Locators?

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