FluentValidation exception after NotNull check

If you are validating a reference type which can be null, along with its properties, you may encounter a situation where the parent object being null causes FluentValidation to throw a NullReferenceException when validating the properties on that object.

Take the following rule in a custom Validator class:

1RuleFor(request => request.ZipFile)
2    .MustBeValidZipFile();
Copy

which uses the following extension method:

1public static IRuleBuilderOptions<T, IFormFile> MustBeValidZipFile<T>(
2    this IRuleBuilder<T, IFormFile> ruleBuilder)
3{
4    return ruleBuilder
5        .NotNull()
6            .WithMessage(ValidationMessages.FileIsNull)
7        .Must(file => file.FileName.Length < MaxFilenameSize)
8            .WithMessage(ValidationMessages.FilenameTooLong)
9        .Must(file => file.Length > 0)
10            .WithMessage(ValidationMessages.FileSizeMustNotBeZero)
11        .Must(file => file.FileName.ToLower().EndsWith(".zip"))
12            .WithMessage(ValidationMessages.ZipFilenameMustEndDotZip);
13}
Copy

Even though the IFormFile is null, subsequent checks on its properties will continue to be performed, therefore, attempting to check the FileName property on that object will result in a NullReferenceException.

This can be avoided by using the Cascade method to stop execution on the first error.

1RuleFor(request => request.ZipFile)
2    .Cascade(CascadeMode.Stop)
3    .MustBeValidZipFile();
Copy

This is covered in the FluentValidation documentation, but is tucked away under "other features" and not immediately obvious.