Fluent Validation
Last modified on Wed 23 Mar 2022

We use fluent validation for determining "rules" which we want to include for our class, somewhat similar to Data annotations, but with more flexibility and control.

How to use it:

Firstly, add the package...

Fluent Validation package should be referenced in your project, which you can simply do by using NuGet package manager or dotnet CLI command:

Install-Package FluentValidation

or

dotnet add package FluentValidation

Secondly, find or write a class you would like to validate...

In our scenario, we would like to validate members of this DTO (data transfer objects) class so we can use this class as a "middleman" for saving data to the database. By doing that we don't expose our internal data structure to everybody 🙊.

public class LoginDetailsDto
{
    public string Username { get; set; }
    public DateTime DateTime { get; set; }
    public string AuthType { get; set; }
    public string IpAddress { get; set; }
}

Thirdly, create the validator...

Here we have an example Validator class where we validate the LoginDetailsDto class members.

using FluentValidaton;

public class LoginDetailsDtoValidator : AbstractValidator<LoginDetailsDto>
{
    public LoginDetailsDtoValidator()
    {
         RuleFor(loginDetailsDto => loginDetailsDto.Username)
             .NotEmpty()
             .WithMessage("Username is empty")
             .Must(ValidateUsername)
             .WithMessage("Username already exists");

         RuleFor(loginDetailsDto => loginDetailsDto.AuthType)
             .NotEmpty()
             .WithMessage("Authtype is empty");

         RuleFor(loginDetailsDto => loginDetailsDto.DateTime)
             .NotEmpty()
             .WithMessage("Date is empty");

         RuleFor(loginDetailsDto => loginDetailsDto.IpAddress)
             .NotEmpty()
             .NotEqual("1234");
    }

    public bool ValidateUsername(string username)
    {
         if(doesNameAlreadyExistInDatabase)
         {
             return false;
         }
         else
         {
             return true;
         }
    }
}

Later we can use this validation where we see fit, in this case, we are validating if the input data is correct before we save it to the database.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        .AddFluentValidation(config =>
        {
            options.RunDefaultMvcValidationAfterFluentValidationExecutes = false;
            options.ValidatorOptions.PropertyNameResolver = FluentValidationResolvers.CamelCasePropertyNameResolver;
            options.RegisterValidatorsFromAssemblyContaining(typeof(LoginDetailsDtoValidator));
        });
}
public async Task CreateExternalLoginDetails(LoginDetailsDto googleUserInfo)
{
    if (ModelState.IsValid)
    {
        var loginDetails = new LoginDetails
        {
            Username = googleUserInfo.Username,
            AuthType = googleUserInfo.AuthType,
            IpAddress = googleUserInfo.IpAddress,
            DateTime = DateTime.UtcNow,
        };
        await _uow.LoginDetailsRepo.CreateAsync(loginDetails);
        await _uow.SaveChangesAsync();
    }
    else
    {
        foreach(var failure in results.Errors)
        {
            Console.WriteLine("Property" + failure.PropertyName + "failed validation.");
            Console.WriteLine("Error was: " + failure.ErrorMessage);
        }       
    }
}
...
var validator = new LoginDetailsDtoValidator();
if (validator.Validate(googleUserInfo).IsValid)`{...}
...

Errors and exceptions

Combining all error messages into a single string:

validator.Validate(googleUserInfo).ToString("~");

You can also throw exceptions, by using ValidateAndThrow, like this:

using FluentValidation;
...
validator.ValidateAndThrow(googleUserInfo)

Configuration checker

With fluent validation we can validate any class, and there is nothing stopping us from using it to validate configuration properties in the project. This is a very neat way of checking if all the configuration is set up before we fire up the solution. The project will build normally, but when started it will throw an error and remind us of which configuration we are missing. It's a really good solution for the dev-ops team when they are setting application configuration. We will also bypass annoying unspecified errors which we usually get when the configuration is missing.

How to do it?

public class SomeOption
{
    public string Option1 {get; set;}
}
public class SomeOptionValidator : AbstractValidator<SomeOption>
{
    public SomeOptionValidator()
    {
        RuleFor(c=>c.Option1).NotEmpty().WithMessage("{PropertyName} cannot be empty or null.");
    }
}
public static class ServiceCollectionExtensions
    {
        public static void BindOptions(
            this IServiceCollection services,
            IConfiguration config)
        {
            services.ConfigureOptionsSettings<SomeOption>(config);
        }

        public static void ConfigureOptionsSettings<T>(
            this IServiceCollection services,
            IConfiguration config,
            string? name = null) where T : class
        {
            var configurationSectionName = name ?? typeof(T).Name;

            var value = GetConfigurationValue<T>(config, configurationSectionName);

            var validationResult = IsValid(value, out string validationMessage);

            services.AddOptions<T>()
                .Bind(config.GetSection(configurationSectionName))
                .Validate(x => validationResult, validationMessage)
                .ValidateOnStart();
        }

        private static T GetConfigurationValue<T>(
            IConfiguration config,
            string configurationSectionName) where T : class
        {
            var value = config
                .GetSection(configurationSectionName)
                .Get<T>();

            if (value == default)
            {
                throw new Exception($"Validation error: Configuration section is missing for {configurationSectionName} settings.");
            }

            return value;
        }

        private static bool IsValid<T>(T value, out string message) where T : class
        {
            var validator = GenerateValidator<T>();
            var result = validator.Validate(value);
            message = FormatError(result);
            return result.IsValid;
        }

        private static BaseAbstractValidator<T> GenerateValidator<T>() where T : class
        {
            var type = FindValidatorImplementationType<T>();
            return Activator.CreateInstance(type) as BaseAbstractValidator<T>;
        }

        private static string FormatError(ValidationResult result)
            => $"Validation error: {string.Join(' ', result?.Errors?.Select(x => x.ErrorMessage))}";

        private static Type FindValidatorImplementationType<T>() where T : class
        {
            var type = Assembly
                .GetAssembly(typeof(BaseAbstractValidator<>))?
                .GetTypes()
                .FirstOrDefault(type => type.BaseType == typeof(BaseAbstractValidator<T>));
            if (type == default)
            {
                throw new ArgumentException($"Validator is not set for {typeof(T).Name} configuration.");
            }
            return type;
        }
    }
public class Startup
{
    ...
    public void ConfigureServices(IServiceCollection services)
    {
        ...
        services.BindOptions(Configuration);
        ...
    }
}

Disclamer: this will validate the property as well as the configuration section!

Congratulations, we covered the basics, if you want to know more visit this link: Fluent Validation Documentation