Automatic Module Discovery With Autofac – Simplified Registration

In the realm of software development, modularity stands as a cornerstone principle. It not only ensures that software components remain organized but also facilitates easier maintenance and scalability. One tool that has gained traction in the .NET ecosystem for its prowess in module management is Autofac. Serving as a dependency injection container, Autofac offers developers a structured way to instantiate, configure, and retrieve application components. But what about automatic module discovery?

In this article, we’ll look through why we might want automatic module discovery and how we can achieve this with Autofac! I’ll do my best to cover both the pros and the cons because it’s important that we remain pragmatic. While I love using Autofac and automatic module discovery like this for my plugin-based solutions, there are always tradeoffs!


Basics of Autofac and Module Management

As mentioned, Autofac is a popular Inversion of Control (IoC) container for .NET. At its core, it manages the creation and disposal of services or components. One of its standout features is its ability to manage components across various lifetimes, ensuring that resources are efficiently utilized and disposed of.

A significant aspect of Autofac is its module system. In Autofac, a module is a small, organized unit that encapsulates a set of related component registrations. This system allows developers to group related services together, making it easier to manage and configure them. By leveraging modules, developers can ensure that their applications remain organized, especially as they scale and evolve.


The Need for Automatic Module Discovery

As applications grow and evolve, the number of modules and services they encompass can increase significantly. Manually registering each module can become a tedious and error-prone task. This manual approach can lead to oversights, where certain modules might be left unregistered, leading to runtime errors. Moreover, as the number of modules increases, the complexity of managing dependencies between them can become overwhelming.

Automating the module discovery process offers a solution to these challenges. By automatically discovering and registering modules, developers can ensure that all necessary components are correctly initialized without manual intervention. This not only reduces the potential for errors but also streamlines the development process, allowing developers to focus on building functionality rather than managing module registrations.


Scanning Directories for Assemblies

Setting Up the Environment

Before diving into the code, it’s essential to ensure the environment is correctly set up. For this demonstration, you’ll need:

  1. A .NET Core application.
  2. Autofac NuGet package installed in your project.
  3. A directory containing the assemblies you wish to scan.

This article assumes you have some prior basic knowledge of Autofac, but if not, you can check this article out to get started with Autofac!

Code Example: Directory Scanning

To scan a directory for assemblies, we can leverage the .NET Core’s built-in System.Reflection namespace. Let’s walk through this, step by step, so that we can see how to facilitate automatic module discovery with Autofac!

First, we’ll want to decide which directory or directories we want to look at, and then from there, consider which files we are interested in. This can look very different from project to project depending on what your needs are. Don’t marry yourself to this exact solution but listing the files could look like the following:

string pathToAssemblies = @"C:\path\to\your\assemblies";
var allFiles = Directory.GetFiles(pathToAssemblies, "*.dll");

You may want to consider other types of filters (i.e. not just *.dll), go the multi-folder route, or even switch away from using LINQ entirely if you prefer writing it out with loops. Once we have the files in question though, we’ll want to load the assemblies:

var assemblies = allFiles.Select(file => Assembly.LoadFrom(file)).ToArray();

Again, the code above can be changed from LINQ to using loops depending on your style. You could also consider safety mechanisms for loading assemblies and handling exceptions, but this will be situational for your application.

Finally, we’ll want to register these assemblies with our Autofac container builder:

var builder = new ContainerBuilder();
foreach (var assembly in assemblies)
{
    builder.RegisterAssemblyModules(assembly);
}

Or without LINQ:

string pathToAssemblies = @"C:\path\to\your\assemblies";
var builder = new ContainerBuilder();

foreach (var file in Directory.GetFiles(pathToAssemblies, "*.dll"))
{
    var assembly = Assembly.LoadFrom(file);
    builder.RegisterAssemblyModules(assembly);
}

With these steps, you’ve successfully scanned a directory for assemblies and registered the contained Autofac modules. This approach ensures that as you add new modules to the directory, they’ll be automatically discovered and registered without any additional manual steps as long as they meet the criteria that you’ve defined.


Advanced Scenario: Reflection-Based Type Discovery

The Power of Reflection in C#

Reflection is a powerful feature in C# that allows for runtime type discovery and introspection. With reflection, developers can inspect the metadata of types, create instances of types, and even invoke methods. This capability is particularly useful when you want to discover specific types in an assembly without knowing them at compile time.

Code Example: Discovering Types with Reflection

Let’s say you want to discover all types in an assembly that implement a specific interface:

var targetType = typeof(IMyTargetInterface);
var types = assembly
    .GetTypes()
    .Where(p => targetType.IsAssignableFrom(p) && !p.IsInterface && !p.IsAbstract)
    .ToList();

This code will give you a list of types from the specified assembly that implement IMyTargetInterface. Notice how we’re asking an assembly reference for the list of types? If you think back to the earlier example about scanning directories for assemblies to register modules, this might get some gears turning!

Integrating with Autofac

Once you’ve discovered the desired types using reflection, you can register them in Autofac:

foreach (var type in types)
{
    builder
        .RegisterType(type)
        .AsImplementedInterfaces();
}

var container = builder.Build();

This code registers each discovered type with Autofac, mapping it to the interfaces it implements. It is important to note the registration strategies being used above (and following this pattern) get applied across all types being discovered this way. That is, if you want some to be SingleInstance() but others to not be, this approach will not work out of the box this way but may be feasible with attributes. Additionally, if you don’t want to use AsImplementedInterfaces(), you will need to consider how to map these to be resolved properly.


Potential Pitfalls and Solutions

Handling Duplicate Registrations

One of the challenges with automatic Autofac module discovery is the potential for duplicate registrations. If the same Autofac module is discovered and registered multiple times, it can lead to unexpected behavior or errors.

Some strategies to address this include:

  1. Unique Naming Convention: Ensure that module names are unique. This can be enforced by adopting a consistent naming convention across the project.
  2. Check Before Registration: Before registering a module, check if it’s already been registered. Autofac provides functionalities like IsRegistered to help with this.
  3. Idempotent Registrations: Design your Autofac modules so that registering them multiple times doesn’t have adverse effects. This might not always be feasible but can be a useful strategy in some scenarios.

Ensuring Compatibility

When Autofac modules are loaded dynamically, there’s always a risk that they might not be fully compatible with the host application, leading to runtime errors.

Some tips to help ensure compatibility include:

  1. Versioning: Implement a versioning system for your modules. Before loading an Autofac module, check its version against a list of supported versions.
  2. Interface Contracts: Ensure that modules adhere to a strict interface contract. This can help in catching incompatible modules at the time of loading.
  3. Testing: Regularly test the application with different combinations of modules to catch compatibility issues early on.

Generally speaking, you will want to consider error handling when loading plugins automatically. It may be such that you want to handle failures gracefully, or perhaps it should halt the entire application.


Conclusion

Automatic Autofac module discovery in Autofac, combined with the power of C# reflection, introduces a level of efficiency that can significantly streamline the development process. By dynamically loading and registering modules, developers can build more modular and scalable applications.

However, with this flexibility comes the responsibility of ensuring compatibility and avoiding potential pitfalls. As with any advanced technique, a thorough understanding and careful implementation are key.

I encourage you to integrate automatic Autofac module discovery thoughtfully! Ensuring that your C# projects remain robust and maintainable, as that’s likely one of the main reasons you were entertaining a modular approach this way!

Nick Cosentino

Principal Software Engineering Manager at Microsoft. Views are my own.

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