# Scrutor — Assembly Scanning and DI Decoration for .NET

> Scrutor extends Microsoft.Extensions.DependencyInjection with assembly scanning and decorator pattern support, enabling convention-based service registration without manually wiring every interface-to-implementation pair.

## Install

Save in your project root:

# Scrutor — Assembly Scanning and DI Decoration for .NET

## Quick Use
```bash
dotnet add package Scrutor
```
```csharp
builder.Services.Scan(scan => scan
    .FromAssemblyOf<Startup>()
    .AddClasses(c => c.AssignableTo<IRepository>())
    .AsImplementedInterfaces()
    .WithScopedLifetime());
// Decorator pattern:
builder.Services.Decorate<INotificationService, CachingNotificationService>();
```

## Introduction
ASP.NET Core's built-in DI container handles basic registrations well, but manually registering dozens of services is tedious and error-prone. Scrutor adds assembly scanning to auto-register services by convention and a `Decorate` method to wrap existing registrations with cross-cutting behavior like caching, logging, or retry logic.

## What Scrutor Does
- Scans assemblies to auto-discover and register services matching conventions
- Registers classes by their implemented interfaces, base types, or custom selectors
- Applies the decorator pattern to existing DI registrations without changing consuming code
- Supports filtering by namespace, attribute, or custom predicate
- Works with transient, scoped, and singleton lifetimes

## Architecture Overview
Scrutor is a thin extension library over `IServiceCollection`. The scanning API builds a fluent pipeline: select assemblies, filter classes, choose registration strategy (as interface, as self, as matching interface), and set lifetime. Under the hood, it reflects over types once and calls the standard `IServiceCollection.Add()` methods. The `Decorate<TInterface, TDecorator>()` method replaces an existing registration by wrapping the original factory, so the container resolves the decorator with the original service injected.

## Self-Hosting & Configuration
- Install via NuGet: `dotnet add package Scrutor`
- Call `services.Scan()` in your DI setup (Program.cs or Startup.cs)
- No configuration files or external dependencies
- Works with the built-in Microsoft DI container (no Autofac or StructureMap needed)
- Chain multiple scan operations for different assemblies or conventions

## Key Features
- Convention-based registration: match interfaces by name, namespace, or attribute
- Decorator support without replacing the DI container
- Duplicate handling strategies: Skip, Replace, or Append
- Assembly source selection from entry assembly, calling assembly, or explicit types
- Composable filter pipeline with `AddClasses(predicate)` for precise control

## Comparison with Similar Tools
- **Autofac** — full-featured DI container with scanning; Scrutor adds scanning to the built-in container
- **Manual registration** — explicit control but does not scale; Scrutor automates the pattern
- **Lamar (StructureMap)** — alternative container with conventions; Scrutor avoids replacing the container
- **Castle Windsor** — mature DI with interceptors; heavier than Scrutor's focused feature set
- **Simple Injector** — convention registration available but requires replacing the default container

## FAQ
**Q: Does Scrutor replace the built-in DI container?**
A: No. Scrutor extends `IServiceCollection` with scanning and decoration. It works with the default Microsoft container.

**Q: Can I decorate a service multiple times?**
A: Yes. Each `Decorate<TInterface, TDecorator>()` call wraps the previous registration, creating a chain of decorators.

**Q: How do I exclude certain classes from scanning?**
A: Use the `AddClasses(c => c.Where(t => !t.Name.EndsWith("Helper")))` predicate to filter unwanted types.

**Q: Does Scrutor support keyed or named services?**
A: Scrutor works with the standard DI abstractions. For keyed services in .NET 8+, combine Scrutor scanning with the built-in keyed service registration.

## Sources
- https://github.com/khellang/Scrutor

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Author: AI Open Source