When the built-in Dependency Injection (DI) container in.NET Core is unable to instantiate a necessary service at runtime, the error message "Unable to resolve service for type" appears. One of the most frequent problems developers have while utilizing ASP.NET Core, Web APIs, Minimal APIs, background services, or microservices architecture is this exception.
Usually, this mistake manifests as:
InvalidOperationException: While trying to activate 'Y', service for type 'X' could not be resolved.
To put it simply, the framework is attempting to build an object (Y), but the Dependency Injection container does not have one of its dependents (X) registered.
Understanding how Dependency Injection functions internally in.NET Core is crucial to correctly resolving this issue.
Dependency Injection (DI) is a design pattern that enables loose coupling between components. In ASP.NET Core, services are registered in the IServiceCollection inside Program.cs (or Startup.cs in older versions). The framework automatically resolves dependencies via constructor injection.
Example:
public class ProductController : ControllerBase
{
private readonly IProductService _productService;
}
If IProductService is not registered in the service container, the application throws the “Unable to resolve service” error at runtime.
Root Causes of the Error
There are several common causes behind this exception in .NET Core applications.
This is the most frequent reason.
Problem:
IProductService is injected but not registered.
Solution:
Register it in Program.cs:
builder.Services.AddScoped<IProductService, ProductService>();
Choose the appropriate lifetime:
AddTransient → New instance per request
AddScoped → One instance per HTTP request
AddSingleton → Single instance for entire application lifecycle
Sometimes services are registered but with incompatible lifetimes.
Example Problem:
A Singleton service depends on a Scoped service.
This causes runtime failures because a longer-lived service cannot depend on a shorter-lived service.
Solution:
Align service lifetimes correctly or refactor dependencies.
If only an interface is injected but no implementation exists, DI cannot resolve it.
Incorrect:
builder.Services.AddScoped();
Correct:
builder.Services.AddScoped<IOrderService, OrderService>();
In large enterprise solutions with multiple projects, it is common to accidentally register the wrong interface or implementation from a different namespace.
Always verify:
Correct interface type
Correct implementation class
Correct project reference
If a constructor has parameters that are not registered services, the DI container fails.
Example:
public ProductService(IRepository repository, string connectionString)
Here, string connectionString is not registered as a service.
Solution:
Use IConfiguration or Options pattern instead of injecting primitive types directly.
Example fix:
public ProductService(IRepository repository, IConfiguration configuration)
{
var connectionString = configuration.GetConnectionString("Default");
}
If Service A depends on Service B and Service B depends on Service A, the DI container cannot resolve the dependency graph.
Example:
ServiceA → ServiceB
ServiceB → ServiceA
Solution:
Refactor architecture, introduce interfaces properly, or apply mediator pattern to break circular references.
7. Forgetting to Register External Services
When using external libraries such as AutoMapper, MediatR, FluentValidation, or custom middleware, their services must be registered.
Example:
builder.Services.AddAutoMapper(typeof(Program));
If not registered, injection will fail.
Consider an enterprise e-commerce Web API project structured into:
API Layer
Application Layer
Infrastructure Layer
If the Infrastructure layer contains the repository implementation but is not registered in the API layer’s DI container, controllers depending on repositories will fail during activation.
Correct approach:
builder.Services.AddInfrastructureServices();
And inside Infrastructure project:
public static IServiceCollection AddInfrastructureServices(this IServiceCollection services)
{
services.AddScoped<IProductRepository, ProductRepository>();
return services;
}
This ensures proper separation of concerns while resolving dependencies correctly.
Step-by-Step Troubleshooting Checklist
Verify the service is registered in Program.cs.
Confirm correct lifetime configuration.
Ensure correct interface-to-implementation mapping.
Check for circular dependencies.
Validate constructor parameters.
Confirm project references are added.
Rebuild the solution to refresh dependency graph.
Common Causes and Fixes Table
| Cause | Why It Happens | How to Fix |
|---|---|---|
| Service not registered | Missing AddScoped/AddTransient/AddSingleton | Register service in DI container |
| Wrong lifetime | Singleton depends on Scoped | Align lifetimes properly |
| Missing implementation | Interface registered without concrete class | Provide implementation mapping |
| Circular dependency | Services depend on each other | Refactor architecture |
| Primitive type injection | DI cannot resolve raw types | Use IConfiguration or Options pattern |
| Namespace mismatch | Wrong interface reference | Verify correct project and namespace |
| External library not registered | Required services missing | Add required service registration |
Advanced Debugging Techniques
Enable detailed logging to inspect DI behavior:
builder.Logging.SetMinimumLevel(LogLevel.Debug);
Use dependency validation during startup:
builder.Services.BuildServiceProvider(new ServiceProviderOptions
{
ValidateScopes = true,
ValidateOnBuild = true
});
This helps detect scope issues early in development.
Best Practices to Avoid This Error
Follow clean architecture principles
Group service registrations using extension methods
Keep constructors minimal
Avoid injecting primitive types
Use the Options pattern for configuration
Maintain consistent service lifetimes
Write integration tests to validate DI setup
Summary
When the dependence Injection container is unable to create a necessary dependence because of missing registrations, wrong lifetimes, circular dependencies, or incorrectly set constructors, the "Unable to resolve service for type" error in.NET Core appears. Verifying service registration in the IServiceCollection, guaranteeing accurate interface-to-implementation mapping, coordinating service lifetimes, avoiding basic type injection, and upholding clear architectural boundaries are all necessary to resolve this problem. Developers may swiftly identify and resolve this frequent runtime problem in ASP.NET Core and distributed.NET applications by comprehending how the built-in DI container functions and using structured troubleshooting techniques.
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