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Discover how to configure `FluentValidation` to avoid NullReferenceExceptions by skipping validation of subfields that aren't included in requests.
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How to Prevent FluentValidation from Checking Subfields When Not Present in Request

When working with FluentValidation, a powerful and popular .NET library for building strong validation rules, developers may encounter some challenges. One common issue arises when the object being validated includes nested properties or subfields not included in the incoming request. This can sometimes lead to frustrating scenarios such as NullReferenceException errors, which occur when the validator attempts to access properties of a null object.

In this guide, we'll dive deep into this problem and explore effective solutions that will help you configure FluentValidation correctly, enabling it to skip validation for subfields of fields that are not included in the request.

Understanding the Problem

Consider the following JSON input:

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With this data, your FluentValidation rules might look like this:

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If you send a request without geographyInfo as follows:

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You may encounter a NullReferenceException, which is not the expected behavior. This happens because FluentValidation tries to execute the rules for CountryCode, leading to an attempt to access a property of an object that doesn't exist.

Solutions to Prevent NullReferenceExceptions

The good news is that there are efficient ways to handle this validation scenario in FluentValidation. Here are two primary approaches you can implement:

1. Stop Execution on First Failure

Setting CascadeMode to StopOnFirstFailure can help. This instructs FluentValidation to stop evaluating the remaining rules as soon as it encounters a failure in the validation.

To do this, simply add the following line to the configuration:

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Once this is set, if GeographyInfo is null or empty, the validation for CountryCode won't even be executed, thus preventing the NullReferenceException.

2. Use the When Method for Conditional Validation

Another approach is to utilize the When method to specify a condition that must be true before proceeding to validate the nested properties. In your case, you want to ensure that the GeographyInfo object is not null before checking the CountryCode.

Here’s how you can do that:

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By employing this pattern, the validation for CountryCode will only execute if GeographyInfo is present, thus eliminating the chance of running into NullReferenceException errors.

Conclusion

Handling nested object validation in FluentValidation can be tricky, especially when dealing with optional fields. However, by either stopping execution on the first failure with CascadeMode or using the When method to conditionally validate subfields, you can create robust validation rules that gracefully handle missing data.

Using these strategies will not only improve the resilience of your application but also lead to a better user experience by providing clearer error messages when necessary fields are missing.

Remember, the goals are clarity and user-friendliness—both in error handling and overall code quality. Happy validating!