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.
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.
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.
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.
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.
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