C# FluentValidation

last modified July 5, 2023

Thi article is a short introduction to data validation in C# using the FluentValidation library.

FluentValidation is a validation library of .NET. It uses fluent interface and lambda expressions for building strongly-typed validation rules.

$ dotnet add package FluentValidation

We add the package to the project.

C# FluentValidation simple example

The following is a simple example that uses FluentValidation.

Program.cs

using FluentValidation;

var validator = new UserValidator(); var u1 = new User(“John Doe”, “gardener”); var u2 = new User(“Roger Roe”, “”);

var res = validator.Validate(u1);

if (res.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); }

var res2 = validator.Validate(u2);

if (res2.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); }

class User { public string Name; public string Occupation;

public User(string Name, string Occupation) =>
    (this.Name, this.Occupation) = (Name, Occupation);

public override string ToString() =>
    $"User {{ {this.Name} {this.Occupation} }}";

}

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty(); RuleFor(user => user.Occupation).NotEmpty(); } }

In the program, we have a User class. With our validation code we ensure that the user properties are not empty.

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty(); RuleFor(user => user.Occupation).NotEmpty(); } }

A validator class inherits from AbstractValidator. It defines rules for its properties with RuleFor. In our case, we use the built-in NotEmpty rule.

var validator = new UserValidator();

We create an instance of the UserValidator.

var u1 = new User(“John Doe”, “gardener”); var u2 = new User(“Roger Roe”, “”);

We create two user objects.

if (res.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); }

We check the validity of an object with IsValid.

$ dotnet run instance is valid instance not valid

C# FluentValidation errors

The validation result contains the Errors property which contains the validation error messages.

Program.cs

using FluentValidation;

var validator = new UserValidator(); var u1 = new User(“John Doe”, “gardener”); var u2 = new User(“Roger Roe”, “”);

var res = validator.Validate(u1);

if (res.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); ShowErrors(res); }

var res2 = validator.Validate(u2);

if (res2.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); ShowErrors(res2); }

void ShowErrors(FluentValidation.Results.ValidationResult res) { foreach (var e in res.Errors) { Console.WriteLine(e); } }

class User { public string Name; public string Occupation;

public User(string Name, string Occupation) =>
    (this.Name, this.Occupation) = (Name, Occupation);

public override string ToString() =>
    $"User {{ {this.Name} {this.Occupation} }}";

}

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty(); RuleFor(user => user.Occupation).NotEmpty(); } }

In this program, we also display the error messages.

void ShowErrors(FluentValidation.Results.ValidationResult res) { foreach (var e in res.Errors) { Console.WriteLine(e); } }

We define the ShowErrors function which goes over the Errors property and displays them to the console.

$ dotnet run instance is valid instance not valid ‘Occupation’ must not be empty.

C# FluentValidation custom error messages

Custom error messages can be provided with WithMessage.

Program.cs

using FluentValidation;

var validator = new UserValidator(); var u = new User(“Roger Roe”, “”);

var res = validator.Validate(u);

if (res.IsValid) { Console.WriteLine(“instance is valid”); } else { Console.WriteLine(“instance not valid”); ShowErrors(res); }

void ShowErrors(FluentValidation.Results.ValidationResult res) { foreach (var e in res.Errors) { Console.WriteLine(e); } }

class User { public string Name; public string Occupation;

public User(string Name, string Occupation) =>
    (this.Name, this.Occupation) = (Name, Occupation);

public override string ToString() =>
    $"User {{ {this.Name} {this.Occupation} }}";

}

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty() .WithMessage(“Validation failed: Name field must not be empty”); RuleFor(user => user.Occupation).NotEmpty() .WithMessage(“Validation failed: Occupation field must not be empty”); } }

In the program, we add custom error messages.

RuleFor(user => user.Name).NotEmpty() .WithMessage(“Validation failed: Name field must not be empty”); RuleFor(user => user.Occupation).NotEmpty() .WithMessage(“Validation failed: Occupation field must not be empty”);

Right after the NotEmpty rule, we all the WithMessage with the custom error message.

$ dotnet run instance not valid Validation failed: Occupation field must not be empty

C# FluentValidation chaining rules

The validation rules can be chained.

Program.cs

using FluentValidation;

var validator = new UserValidator();

var u1 = new User(); ValidateUser(u1);

Console.WriteLine("——————–");

var u2 = new User(); u2.Name = “John Doe”; ValidateUser(u2);

Console.WriteLine("——————–");

var u3 = new User(); u3.Name = “John Doe”; u3.Occupation = “gar”; ValidateUser(u3);

Console.WriteLine("——————–");

var u4 = new User(); u4.Name = “John Doe”; u4.Occupation = “gardener”; ValidateUser(u4);

void ValidateUser(User u) { var res = validator.Validate(u);

if (res.IsValid)
{
    Console.WriteLine("instance is valid");
}
else
{
    Console.WriteLine("instance not valid");
    ShowErrors(res);
}

}

void ShowErrors(FluentValidation.Results.ValidationResult res) { foreach (var e in res.Errors) { Console.WriteLine(e); } }

class User { public string? Name { get; set; } public string? Occupation { get; set; }

public override string ToString() =>
    $"User {{ {this.Name} {this.Occupation} }}";

}

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty().MinimumLength(2); RuleFor(user => user.Occupation).NotEmpty().MinimumLength(5); } }

In the program, we use two rules for our properties: NotEmpty and MinimumLength.

class UserValidator : AbstractValidator<User> { public UserValidator() { RuleFor(user => user.Name).NotEmpty().MinimumLength(2); RuleFor(user => user.Occupation).NotEmpty().MinimumLength(5); } }

In our validation class we assume that a name has at least two letters and an occupation five.

$ dotnet run instance not valid ‘Name’ must not be empty. ‘Occupation’ must not be empty.

instance not valid ‘Occupation’ must not be empty.

instance not valid The length of ‘Occupation’ must be at least 5 characters. You entered 3 characters.

instance is valid

FluentValidation documentation

This article was a quick introduction to the C# FluentValidation library.

Source

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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