Skip to main content

Validation is not a Cross Cutting Concern

Attributes in C# are also known as decorators in Python or annotations in Java. Other languages may have similar constructs. This post will use attribute throughout but refers to the same concept.


While attributes prove useful for cross cutting concerns such as authorization or logging, they can be misused. Attributes should act as metadata, providing no direct behaviour. Failing to do so will make DI, testability and composition very difficult.

These flaws are especially true for validation. Despite all input requiring validation, the manner in which validation is performed is dependent on the entry point to the code. Context matters.

Consider order information that requires a billing address and by definition, its children to be populated. An attribute works a treat here in this simple case.

A problem arises if you only want the billing address validation to activate if the billing address and delivery address differ.

Complexity quickly starts to take over. With a more fully featured example attributes can start to overwhelm the class. This example becomes worse if the validation is required to be performed by a third party library or service. Finding a hook to integrate becomes troublesome.

Solution

Avoid attributes for validation in all but the simplest scenarios. Even simple scenarios lead to some churn if you do decide to switch. My personal preference is to now avoid attributes all together, instead opting to use a validation service.

The obvious downside to this is approach is the appearance of more code. While this is true, composed object graphs can benefit from the ability of reuse. Additionally in the case of attributes some degree of testing is required. These usually fall into the category of asserting the presence of attributes on properties which is far from ideal. The use of validation services do not suffer this problem. Internally the implementation can be switched, altered or refactored without fear of breaking any tests.

Example

The RootValidator is a composite of zero or more actual validators. Each validator can be specific to a particular task. The only requirement being the interface must be the parent object. This is to ensure the context is not lost when making decisions. The actual interface in this case could be made to use generic types if required. The ValidationResults are a simple value type representing an aggregation of validation failures. This could be extended or modified for further enhancements.

Benefits

  • Composition makes it possible to provide multiple validators that all do one thing well.
  • Testing is much easy as you can test each validator in isolation.
  • Null validators provides easier higher level testing as you can provide a no-op validator. Removing the need to build up complex object graphs for other test cases.
  • Developers can follow, debug and understand simple conditional logic more so than framework specific metadata.
  • Open to extension and additions such as third party code.
  • Services never lose context which allows easy runtime decisions to be made.

Comments

Popular posts from this blog

Constant Object Anti Pattern

Most constants are used to remove magic numbers or variables that lack context. A classic example would be code littered with the number 7. What does this refer to exactly? If this was replaced with DaysInWeek or similar, much clarity is provided. You can determine that code performing offsets would be adding days, rather than a mysterious number seven.Sadly a common pattern which uses constants is the use of a single constant file or object. The beauty of constants is clarity, and the obvious fact such variables are fixed. When a constant container is used, constants are simply lumped together. These can grow in size and often become a dumping ground for all values within the application.A disadvantage of this pattern is the actual value is hidden. While a friendly variable name is great, there will come a time where you will want to know the actual value. This forces you to navigate, if only to peek at the value within the constant object. A solution is to simple perform a refactor …

Three Steps to Code Quality via TDD

Common complaints and problems that I've both encountered and hear other developers raise when it comes to the practice of Test Driven Development are: Impossible to refactor without all the tests breakingMinor changes require hours of changes to test codeTest setup is huge, slow to write and difficult to understandThe use of test doubles (mocks, stubs and fakes is confusing)Over the next three posts I will demonstrate three easy steps that can resolve the problems above. In turn this will allow developers to gain one of the benefits that TDD promises - the ability to refactor your code mercifully in order to improve code quality.StepsStop Making Everything PublicLimit the Amount of Dependencies you Use A Unit is Not Always a Method or ClassCode quality is a tricky subject and highly subjective, however if you follow the three guidelines above you should have the ability to radically change implementation details and therefore improve code quality when needed.

DRY vs DAMP in Tests

In the previous post I mentioned that duplication in tests is not always bad. Sometimes duplication becomes a problem. Tests can become large or virtually identically excluding a few lines. Changes to these tests can take a while and increase the maintenance overhead. At this point, DRY violations need to be resolved.SolutionsTest HelpersA common solution is to extract common functionality into setup methods or other helper utilities. While this will remove and reduce duplication this can make tests a bit harder to read as the test is now split amongst unrelated components. There is a limit to how useful such extractions can help as each test may need to do something slightly differently.DAMP - Descriptive and Meaningful PhrasesDescriptive and Meaningful Phrases is the alter ego of DRY. DAMP tests often use the builder pattern to construct the System Under Test. This allows calls to be chained in a fluent API style, similar to the Page Object Pattern. Internally the implementation wil…