Introduction
If you work with ASP.NET Core, you will eventually face this error:
InvalidOperationException: Unable to resolve service for type ‘X’ while attempting to activate ‘Y’.
This error confuses beginners and frustrates experienced developers because it often appears at runtime, not compile time.
The good news?
👉 This error has limited root causes, and once you understand Dependency Injection (DI) properly, you will never fear it again.
In this guide, you will learn:
- Why this error occurs
- How ASP.NET Core Dependency Injection works internally
- All possible causes with real project examples
- Step-by-step debugging techniques
- Best practices used in production-grade applications
- Interview-ready explanations
By the end, you will fix this error in minutes, not hours.
What Does “Unable to Resolve Service for Type” Mean?
This error means ASP.NET Core cannot create an object because it does not know how to construct one of its dependencies.
In simple words:
You asked ASP.NET Core to inject something, but you never told it how to create that thing.
Understanding Dependency Injection in ASP.NET Core
ASP.NET Core uses built-in Dependency Injection.
What Is Dependency Injection?
Dependency Injection is a design pattern where:
- Objects do not create their own dependencies
- Dependencies are provided from outside
ASP.NET Core uses a Service Container to manage dependencies.
How ASP.NET Core Creates Objects
When ASP.NET Core creates a controller or service:
- It looks at the constructor
- It checks required parameters
- It searches the DI container
- If any dependency is missing, it throws:
Unable to resolve service for type
Basic Example That Causes the Error
Controller Code
public class ProductController : Controller
{
private readonly IProductService _productService;
public ProductController(IProductService productService)
{
_productService = productService;
}
}
Missing Registration ❌
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();
Result ❌
InvalidOperationException:
Unable to resolve service for type 'IProductService'
Root Cause #1: Service Not Registered in DI Container (MOST COMMON)
Why It Happens
You created:
- Interface
- Implementation
- Constructor injection
But forgot to register the service.
❌ Wrong Setup
public interface IProductService
{
List<string> GetProducts();
}
public class ProductService : IProductService
{
public List<string> GetProducts() => new();
}
No registration in Program.cs.
✅ Correct Fix
builder.Services.AddScoped<IProductService, ProductService>();
Root Cause #2: Registering Concrete Class but Injecting Interface
❌ Wrong
builder.Services.AddScoped<ProductService>();
Controller:
public ProductController(IProductService productService)
✅ Correct
builder.Services.AddScoped<IProductService, ProductService>();
Root Cause #3: Injecting Concrete Class Without Registration
ASP.NET Core cannot auto-create services unless registered.
❌ Wrong
public ProductController(ProductService productService)
No registration.
✅ Fix
builder.Services.AddScoped<ProductService>();
Root Cause #4: Lifetime Mismatch (Scoped into Singleton)
This is a classic interview and production bug.
❌ Wrong
builder.Services.AddScoped<IUserService, UserService>();
builder.Services.AddSingleton<IEmailService, EmailService>();
public class EmailService
{
public EmailService(IUserService userService) { }
}
❌ Error
Cannot consume scoped service from singleton
✅ Fix Options
Option 1: Match Lifetimes
builder.Services.AddScoped<IEmailService, EmailService>();
Option 2: Use Factory Pattern
public EmailService(IServiceProvider provider)
{
using var scope = provider.CreateScope();
var userService = scope.ServiceProvider.GetRequiredService<IUserService>();
}
Root Cause #5: Missing Registration of Nested Dependency
Even if top-level service is registered, inner dependencies matter.
Example
ProductController → ProductService → ProductRepository
❌ Only Registered
builder.Services.AddScoped<IProductService, ProductService>();
❌ Missing
IProductRepository
✅ Fix
builder.Services.AddScoped<IProductRepository, ProductRepository>();
Root Cause #6: Constructor Has Unregistered Parameter
ASP.NET Core only supports constructor injection.
❌ Wrong
public ProductService(string apiKey)
✅ Fix Using Options Pattern
builder.Services.Configure<ApiSettings>(
builder.Configuration.GetSection("ApiSettings"));
public ProductService(IOptions<ApiSettings> options)
Root Cause #7: Wrong Namespace or Duplicate Interface
Sometimes the interface exists twice.
❌ Issue
- Same interface name
- Different namespaces
- Registered one, injected another
✅ Fix
Always verify:
using Correct.Namespace;
Root Cause #8: Generic Service Not Registered
❌ Error Example
Unable to resolve service for type IRepository<Product>
✅ Fix Generic Registration
builder.Services.AddScoped(typeof(IRepository<>), typeof(Repository<>));
Root Cause #9: Middleware Injecting Scoped Service
❌ Wrong
public class CustomMiddleware
{
public CustomMiddleware(RequestDelegate next, IUserService userService)
}
✅ Correct Way
public async Task InvokeAsync(HttpContext context, IUserService userService)
Root Cause #10: Background Services Injection Issue
❌ Wrong
public class Worker : BackgroundService
{
public Worker(IUserService userService) { }
}
Scoped service injected into singleton.
✅ Fix
using IServiceScopeFactory
How to Debug This Error Step-by-Step
Step 1: Read Full Error Message
ASP.NET Core clearly shows:
- Missing type
- Consuming class
Step 2: Check Constructor
Verify every constructor parameter.
Step 3: Verify Program.cs Registrations
Ensure every dependency chain is registered.
Step 4: Check Lifetimes
Singleton → Scoped ❌
Scoped → Transient ✅
Best Practices to Avoid This Error Forever
✅ Always use interfaces
✅ Keep service registrations grouped
✅ Follow clean architecture
✅ Use AddScoped for business logic
✅ Avoid injecting services into middleware constructors
✅ Validate DI at startup:
builder.Host.UseDefaultServiceProvider(options =>
{
options.ValidateScopes = true;
options.ValidateOnBuild = true;
});
FAQs – Unable to Resolve Service for Type in ASP.NET Core
1. What does “Unable to resolve service for type” mean in ASP.NET Core?
This error means ASP.NET Core cannot create an instance of a controller or service because one of its constructor dependencies is not registered or cannot be resolved by the Dependency Injection container.
2. What is the most common reason for this error?
The most common reason is forgetting to register a service in Program.cs using AddScoped, AddTransient, or AddSingleton.
3. Can this error occur even if the service is registered?
Yes. This error can still occur if any nested dependency used inside the service is missing from the DI container. ASP.NET Core must resolve the entire dependency chain.
4. Does ASP.NET Core support property injection?
No. ASP.NET Core officially supports constructor injection only. Property injection is not supported by default and can lead to this error.
5. Can lifetime mismatch cause this error?
Yes. Injecting a scoped service into a singleton causes lifetime mismatch and runtime exceptions. ASP.NET Core blocks this to prevent memory and state issues.
6. Why does this error commonly occur in middleware?
Middleware is created once at application startup. Injecting scoped services into the middleware constructor causes this error. Scoped services must be injected into the Invoke or InvokeAsync method instead.
7. Can incorrect namespace usage trigger this error?
Yes. If multiple interfaces share the same name in different namespaces, ASP.NET Core may register one and inject another, resulting in this error.
8. How do I quickly debug this error?
Read the full exception message, identify the missing service type, check the constructor parameters, and verify all related service registrations in Program.cs.
9. How can I catch this error before runtime?
Enable Dependency Injection validation during startup using:
builder.Host.UseDefaultServiceProvider(options =>
{
options.ValidateOnBuild = true;
options.ValidateScopes = true;
});
This catches DI issues early.
10. Is this a common ASP.NET Core interview question?
Yes. This is a very common interview question for mid to senior ASP.NET Core roles. Interviewers expect clear understanding of DI registration, service lifetimes, and constructor injection.
Final Thoughts
The “Unable to resolve service for type” error is not scary.
It is actually ASP.NET Core protecting you from bad architecture.
Once you understand:
- DI container
- Service lifetimes
- Constructor injection
👉 This error becomes your best debugging friend.