NetArchTest

A fluent API for .Net Standard that can enforce architectural rules in unit tests.

Inspired by the ArchUnit library for Java.

Rationale

This project allows you create tests that enforce conventions for class design, naming and dependency in .Net code bases. These can be used with any unit test framework and incorporated into a build pipeline. It uses a fluid API that allows you to string together readable rules that can be used in test assertions.

There are plenty of static analysis tools that can evaluate application structure, but they are aimed more at enforcing generic best practice rather than application-specific conventions. The better tools in this space can be press-ganged into creating custom rules for a specific architecture, but the intention here is to incorporate rules into a test suite and create a self-testing architecture.

The project is inspired by ArchUnit, a java-based library that attempts to address the difficulties of preserving architectural design patterns in code bases over the long term. Many patterns can only be enforced by convention, which tends to rely on a rigorous and consistent process of code review. This discipline often breaks down as projects grow, use cases become more complex and developers come and go.

Examples

// Classes in the presentation should not directly reference repositories
var result = Types.InCurrentDomain()
    .That()
    .ResideInNamespace("NetArchTest.SampleLibrary.Presentation")
    .ShouldNot()
    .HaveDependencyOn("NetArchTest.SampleLibrary.Data")
    .GetResult()
    .IsSuccessful;

// Classes in the "data" namespace should implement IRepository
result = Types.InCurrentDomain()
    .That().HaveDependencyOn("System.Data")
    .And().ResideInNamespace(("ArchTest"))
    .Should().ResideInNamespace(("NetArchTest.SampleLibrary.Data"))
    .GetResult()
    .IsSuccessful;

// All the service classes should be sealed
result = Types.InCurrentDomain()
    .That().ImplementInterface(typeof(IWidgetService))
    .Should().BeSealed()
    .GetResult()
    .IsSuccessful;

Getting started

The main rules library is available as a package on NuGet: NetArchTest.Rules.

It is a .Net Standard 2.0 library that is compatible with .Net Framework 4.6.1 or better and .Net Core 2.0 or better.

Writing rules

The fluent API should direct you in building up a rule based on a combination of predicates, conditions and conjunctions.

The starting point for any rule is the static Types class, where you load a set of types from a path, assembly or namespace.

var types = Types.InAssembly(typeof(MyClass).Assembly);

Once you have selected the types you can filter them using one or more predicates. These can be chained together using And() or Or() conjunctions:

types.That().ResideInNamespace(“MyProject.Data”);

Once the set of classes have been filtered you can apply a set of conditions using the Should() or ShouldNot() methods, e.g.

types.That().ResideInNamespace(“MyProject.Data”).Should().BeSealed();

Finally, you obtain a result from the rule by using an executor, i.e. use GetTypes() to return the types that match the rule or GetResult() to determine whether the rule has been met. Note that the result will also return a list of types that failed to meet the conditions.

var isValid = types.That().ResideInNamespace(“MyProject.Data”).Should().BeSealed().GetResult().IsSuccessful;

Custom rules

You can extend the library by writing custom rules that implement the ICustomRule interface. These can be applied as both predicates and conditions using a MeetsCustomRule() method, e.g.

var myRule = new CustomRule();

// Write your own custom rules that can be used as both predicates and conditions
result = Types.InCurrentDomain()
    .That().AreClasses()
    .Should()
    .MeetCustomRule(myRule)
    .GetResult().IsSuccessful;

Grouping rules into Policies

Rules can be grouped into policies using the fluent interface exposed by the Policy class, e.g.

var architecturePolicy = Policy.Define("Example Policy", "This is an example policy")
                .For(Types.InCurrentDomain)
                .Add(t =>
                   t.That()
                   .ResideInNamespace("NetArchTest.SampleLibrary.Presentation")
                   .ShouldNot()
                   .HaveDependencyOn("NetArchTest.SampleLibrary.Data"),
                   "Enforcing layered architecture", "Controllers should not directly reference repositories"
                )
                ...
                .Add(t =>
                    t.That()
                    .AreInterfaces()
                    .Should()
                    .HaveNameStartingWith("I"),
                    "Generic implementation rules", "Interface names should start with an 'I'"
                );

The rules are loaded lazily and executed when the Evaluate() method is called. This method returns a PolicyResults instance that can be passed to a reporting mechanism.

The ExamplePolicies class in the samples demonstrates how to do this.

Source code

I welcome contributions. Please refer to the contributing guidelines.

The solution contains projects in three directories:

• src: The main Rules library that is available as a package on NuGet. The main dependency is Mono.Cecil.
• test: A set of unit tests for the rules based on XUnit.
• samples: A couple of sample projects that demonstrate some of the possible usage scenarios.

Further reading

A more extensive blog post describing the implementation detail is available in my blog.