FluentValidation

https://fluentvalidation.net/

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FluentValidation is a popular .NET library for building strongly-typed validation rules.

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### CustomerValidator with AbstractValidator and RuleFor

Source: https://fluentvalidation.net

Shows a complete validator class that defines rules for a Customer object, including NotEmpty, WithMessage, When, Length, and Must with a custom postcode check. The BeAValidPostcode method is a placeholder for custom logic.

```csharp
public class CustomerValidator : AbstractValidator<Customer>
{
  public CustomerValidator()
  {
    RuleFor(x => x.Surname).NotEmpty();
    RuleFor(x => x.Forename).NotEmpty().WithMessage("Please specify a first name");
    RuleFor(x => x.Discount).NotEqual(0).When(x => x.HasDiscount);
    RuleFor(x => x.Address).Length(20, 250);
    RuleFor(x => x.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");
  }

  private bool BeAValidPostcode(string postcode)
  {
    // custom postcode validating logic goes here
  }
}

```

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