A popular .NET validation for building strongly-typed validation rules.
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README.md

FluentValidation

Full Documentation

A small validation library for .NET that uses a fluent interface and lambda expressions for building validation rules.

Get Started

FluentValidation can be installed using the Nuget package manager or the dotnet CLI.

Install-Package FluentValidation

For ASP.NET Core integration:

Install-Package FluentValidation.AspNetCore

For legacy ASP.NET MVC/WebApi integration:

Install-Package FluentValidation.MVC5
Install-Package FluentValidation.WebApi
Build status Tests
FluentValidation NuGet Nuget
FluentValidation.AspNetCore NuGet Nuget
FluentValidation.Mvc5 NuGet Nuget
FluentValidation.WebApi NuGet Nuget

Example

using FluentValidation;

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
  }
}

var customer = new Customer();
var validator = new CustomerValidator();
ValidationResult results = validator.Validate(customer);

bool success = results.IsValid;
IList<ValidationFailure> failures = results.Errors;

Documentation

Documentation can be found on the project site.