Software architecture is a blueprint for how you should structure your system. You can follow this blueprint strictly, or you can give yourself varying levels of freedom.

But when deadlines are tight, and you start cutting corners, that beautiful software architecture you built crumbles like a house of cards.

How can you enforce your software architecture?

By writing architecture tests.

Architecture tests are automated tests that verify the structure and design of your code.

You can use them to enforce your software architecture and the direction of dependencies of your projects.

In this week's issue I'll explain how to:

• Write architecture tests
• Enforce architecture
• Enforce design rules

Let's dive in.

Writing Architecture Tests

You write architecture tests the same as any unit test in your application. There's an excellent library for writing architecture tests that already implements the boilerplate code we need to start writing tests.

We're going to use the NetArchTest.Rules library for writing architecture tests.

First, you have to install the NuGet package:

Install-Package NetArchTest.Rules

And now you can use it to write rules in your test project.

The starting point for writing architecture tests is the static Types class, which you can use to load a set of types.

Once you have loaded your types you can further filter them to find a more specific set of types.

Some of the available filtering methods:

• ResideInNamespace
• AreClasses
• AreInterfaces
• HaveNameStartingWith
• HaveNameEndingWith

Finally, when you are satisfied with your selection, you can write the rule you want to enforce by calling Should or ShouldNotand applying the condition you want to check.

Here's an example checking that all classes in the domain assembly are sealed:

var result = Types
  .InAssembly(DomainAssembly)
  .That()
  .AreClasses()
  .Should()
  .BeSealed()
  .GetResult();

Assert.True(result.IsSuccessful);

Enforcing Architecture Rules

Architecture tests are particularly useful to enforce software architecture rules in a layered architecture or Modular Monolith.

Let's take the example of the Clean architecture:

• Domain should not have any dependencies
• Application should not depend on Infrastructure
• Infrastructure should depend on Application and Domain

Here's how you can write tests for enforcing architecture rules.

Domain should not have any dependencies

var result = Types
  .InAssembly(DomainAssembly)
  .ShouldNot()
  .HaveDependencyOnAny("Application", "Infrastructure")
  .GetResult();

Assert.True(result.IsSuccessful);

Application should not depend on Infrastructure

var result = Types
  .InAssembly(AplicationAssembly)
  .Should()
  .NotHaveDependencyOn("Infrastructure")
  .GetResult();

Assert.True(result.IsSuccessful);

Infrastructure should depend on Application and Domain

How the NetArchTest.Rules library works is by scanning the imported namespaces of your types.

Because of this, writing negative conditions like in the previous two examples is straightforward.

But writing positive conditions has to be scoped to a more specific set of types.

For example, we can validate this dependency by checking that all repositories must have a dependency on the Domain namespace.

var result = Types
  .InAssembly(InfrastructureAssembly)
  .HaveNameEndingWith("Repository")
  .Should()
  .HaveDependencyOn("Domain")
  .GetResult();

Assert.True(result.IsSuccessful);

Enforcing Design Rules

Another valuable use case for architecture tests is enforcing design rules in your application.

Design rules are more specific than project references, and focus on the implementation details of your classes.

Here are some design rules that you can enforce:

• Services must be internal
• Entities and Value objects must be sealed
• Controllers can't depend on repositories directly
• Command (or query) handlers must follow a naming convention

The possibilities are endless, and it's up to you how many design rules you want to enforce.

Here's how you can write tests for enforcing design rules.

Command handlers must end with CommandHandler

var result = Types
    .InAssembly(ApplicationAssembly)
    .That()
    .ImplementInterface(typeof(ICommandHandler))
    .Should()
    .HaveNameEndingWith("CommandHandler")
    .GetResult();

Assert.True(result.IsSuccessful);

Controllers can't directly reference repositories

var result = Types
    .InAssembly(ApiAssembly)
    .That()
    .HaveNameEndingWith("Controller")
    .ShouldNot()
    .HaveDependencyOn("Infrastructure.Repositories")
    .GetResult();

Assert.True(result.IsSuccessful);

Takeaway

Architecture tests are an easy way to enforce software architecture and design rules with automated tests.

One of the best investments you can make as a software engineer is writing automated tests. You write the tests once, and use them to verify your system forever. Granted, you also have to maintain the tests over time as your system grows.

Manually enforcing software architecture with pair programming and constant PR reviews is:

• Error prone
• Time consuming
• Not cost effective

Architecture tests really shine here, since you can write them quickly and reduce the cost of enforcing your software architecture rules to zero.

Thanks for reading.

Hope that was helpful!

P.S. Whenever you're ready, there are 2 ways I can help you:

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