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C# Tip: How to create Unit Tests for Model Validation

2023-10-24 3 min read CSharp Tips
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- Davide

Model validation is fundamental to any project: it brings security and robustness acting as a first shield against an invalid state.

You should then add Unit Tests focused on model validation. In fact, when defining the input model, you should always consider both the valid and, even more, the invalid models, making sure that all the invalid models are rejected.

BDD is a good approach for this scenario, and you can use TDD to implement it gradually.

Okay, but how can you validate that the models and model attributes you defined are correct?

Let’s define a simple model:

public class User
    public string FirstName { get; set; }

    public string LastName { get; set; }

    [Range(18, 100)]
    public int Age { get; set; }

Have we defined our model correctly? Are we covering all the edge cases? A well-written Unit Test suite is our best friend here!

We have two choices: we can write Integration Tests to send requests to our system, which is running an in-memory server, and check the response we receive. Or we can use the internal Validator class, the one used by ASP.NET to validate input models, to create slim and fast Unit Tests. Let’s use the second approach.

Here’s a utility method we can use in our tests:

public static IList<ValidationResult> ValidateModel(object model)
    var results = new List<ValidationResult>();

    var validationContext = new ValidationContext(model, null, null);

    Validator.TryValidateObject(model, validationContext, results, true);

    if (model is IValidatableObject validatableModel)

    return results;

In short, we create a validation context without any external dependency, focused only on the input model: new ValidationContext(model, null, null).

Next, we validate each field by calling TryValidateObject and store the validation results in a list, result.

Finally, if the Model implements the IValidatableObject interface, which exposes the Validate method, we call that Validate() method and store the returned validation errors in the final result list created before.

As you can see, we can handle both validation coming from attributes on the fields, such as [Required], and custom validation defined in the model class’s Validate() method.

Now, we can use this method to verify whether the validation passes and, in case it fails, which errors are returned:

public void User_ShouldPassValidation_WhenModelIsValid()
    var model = new User { FirstName = "Davide", LastName = "Bellone", Age = 32 };
    var validationResult = ModelValidationHelper.ValidateModel(mode);
    Assert.That(validationResult, Is.Empty);

public void User_ShouldNotPassValidation_WhenLastNameIsEmpty()
    var model = new User { FirstName = "Davide", LastName = null, Age = 32 };
    var validationResult = ModelValidationHelper.ValidateModel(mode);
    Assert.That(validationResult, Is.Not.Empty);

public void User_ShouldNotPassValidation_WhenAgeIsLessThan18()
    var model = new User { FirstName = "Davide", LastName = "Bellone", Age = 10 };
    var validationResult = ModelValidationHelper.ValidateModel(mode);
    Assert.That(validationResult, Is.Not.Empty);

Further readings

Model Validation allows you to create more robust APIs. To improve robustness, you can follow Postel’s law:

🔗 Postel’s law for API Robustness | Code4IT

This article first appeared on Code4IT 🐧

Model validation, in my opinion, is one of the cases where Unit Tests are way better than Integration Tests. This is a perfect example of Testing Diamond, the best (in most cases) way to structure a test suite:

🔗 Testing Pyramid vs Testing Diamond (and how they affect Code Coverage) | Code4IT

If you still prefer writing Integration Tests for this kind of operation, you can rely on the WebApplicationFactory class and use it in your NUnit tests:

🔗 Advanced Integration Tests for .NET 7 API with WebApplicationFactory and NUnit | Code4IT

Wrapping up

Model validation is crucial. Testing the correctness of model validation can make or break your application. Please don’t skip it!

I hope you enjoyed this article! Let’s keep in touch on Twitter or LinkedIn! 🤜🤛

Happy coding!