How to Use FluentValidation Properly for List Validation in Unit Tests

Описание: Learn how to effectively validate lists using FluentValidation in C# . Solve your unit testing issues with tips for proper validator setup.
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This video is based on the question https://stackoverflow.com/q/67387602/ asked by the user 'mehul9595' ( https://stackoverflow.com/u/366096/ ) and on the answer https://stackoverflow.com/a/67390397/ provided by the user 'Pawel Maga' ( https://stackoverflow.com/u/4859671/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: FluentValidation and List Validation Errors

If you've been working with FluentValidation for C# and encountered issues when validating lists, you're not alone. One common problem arises when using the TestValidate method to unit test rules on a list of objects. Specifically, you might find that the validator doesn't raise validation errors as expected, even when you've set rules like NotNull. This can lead to confusion, especially when you're trying to assert validations for elements of a list that might not even exist.

In this post, we’ll explore a typical scenario where a TestValidationException is thrown because the list in question is empty. We’ll also break down how to structure your validators to ensure proper validation, and share an effective solution to your unit testing woes.

The Validator Setup

To better understand the solution, let's first take a look at how the validators are structured:

RequestValidator

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In the RequestValidator, you're trying to validate a list of EntryRequests. You utilize RuleForEach in conjunction with SetValidator to apply a child validator. However, if the EntryRequests list is empty, the validation for NotNull will effectively skip any checks because there are no items to validate.

EntryRequestValidator

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The EntryRequestValidator is straightforward, validating individual entries and their messages.

The Issue at Hand

In your unit test setup, when you try to test validation rules, the following setup is used:

[[See Video to Reveal this Text or Code Snippet]]

The attempt was to assert an error for EntryRequests, but due to the validator’s configuration, TestValidate raises a TestValidationException instead of evaluating the list for validation errors.

The Solution: Adjusting the Validator Rules

To ensure proper validation, you'll need to modify your rules in the RequestValidator. Instead of only checking each entry with NotNull, change the validation rules as follows:

[[See Video to Reveal this Text or Code Snippet]]

Why This Works

By implementing NotEmpty on the list of EntryRequests, you enforce that the list must contain at least one element. This covers multiple scenarios:

If EntryRequests is null, the validation fails.

If EntryRequests is an empty list, the validation fails.

If any element in the list is null, the inner validation fails as well (alongside the validation from the EntryRequestValidator).

Conclusion

Using FluentValidation effectively ensures your data integrity and improves the user experience by catching issues up front. With the adjustments explained here, your unit tests will now properly validate the EntryRequests list and its entries, helping you build robust applications. No more silent failures or unexpected exceptions!

Feel free to implement these practices in your project, and happy coding!