Data validation is a crucial aspect of any application, ensuring that the data processed is correct, complete, and formatted properly. In .NET applications, validation is often handled using built-in attributes or custom logic. However, FluentValidation simplifies and enhances this process by providing a robust, expressive, and maintainable way to validate objects.
In this guide, we’ll explore FluentValidation in .NET, covering its installation, usage, and best practices to streamline data validation in your applications.
What is FluentValidation?
FluentValidation is a powerful .NET library for building strongly typed validation rules in a fluent manner. It eliminates the need for cluttered and scattered validation logic by centralizing all validation concerns within dedicated classes.
Why Use FluentValidation?
- Expressive Syntax: Uses a fluent API for defining validation rules.
- Reusability: Promotes modular validation logic.
- Customization: Supports custom rules, error messages, and localization.
- Performance: Lightweight and efficient for high-performance applications.
Installing FluentValidation in .NET
To get started with FluentValidation, install the NuGet package:
Install-Package FluentValidation
Or using .NET CLI:
dotnet add package FluentValidation
Creating a Simple Validator
Let’s define a basic validation class for a User
model:
using FluentValidation;
public class User {
public string Name { get; set; }
public string Email { get; set; }
public int Age { get; set; }
}
public class UserValidator : AbstractValidator<User> {
public UserValidator() {
RuleFor(user => user.Name)
.NotEmpty().WithMessage("Name is required.")
.MaximumLength(50).WithMessage("Name cannot exceed 50 characters.");
RuleFor(user => user.Email)
.NotEmpty().WithMessage("Email is required.")
.EmailAddress().WithMessage("Invalid email format.");
RuleFor(user => user.Age)
.InclusiveBetween(18, 60).WithMessage("Age must be between 18 and 60.");
}
}
Validating Data with FluentValidation
Once the validator is defined, we can validate an object as follows:
var user = new User { Name = "", Email = "invalid-email", Age = 17 };
var validator = new UserValidator();
var results = validator.Validate(user);
if (!results.IsValid) {
foreach (var failure in results.Errors) {
Console.WriteLine($"Property {failure.PropertyName} failed validation. Error: {failure.ErrorMessage}");
}
}
Advanced Validation Scenarios
1. Conditional Validation
You can apply rules conditionally based on other properties:
RuleFor(user => user.Email)
.NotEmpty().When(user => !string.IsNullOrEmpty(user.Name))
.WithMessage("Email is required if Name is provided.");
2. Custom Validators
Create custom validation rules for complex logic:
public class CustomEmailValidator : AbstractValidator<User> {
public CustomEmailValidator() {
RuleFor(user => user.Email)
.Must(email => email.EndsWith("@example.com"))
.WithMessage("Email must belong to example.com domain.");
}
}
3. Collection Validation
Validate lists of objects:
RuleForEach(user => user.Addresses).SetValidator(new AddressValidator());
Integrating FluentValidation with ASP.NET Core
FluentValidation seamlessly integrates with ASP.NET Core by adding validation to the dependency injection system.
1. Install Required Packages
Install-Package FluentValidation.AspNetCore
2. Register FluentValidation in Program.cs
builder.Services.AddControllers()
.AddFluentValidation(fv => fv.RegisterValidatorsFromAssemblyContaining<UserValidator>());
3. Automatic Model Validation in Controllers
Once registered, FluentValidation automatically validates models in API requests:
[HttpPost]
public IActionResult CreateUser([FromBody] User user) {
return Ok("User is valid");
}
Best Practices for FluentValidation
- Keep validators separate from models for better maintainability.
- Use dependency injection for better testability.
- Leverage built-in rules before writing custom logic.
- Use localization for error messages in multi-language applications.
Frequently Asked Questions (FAQ)
1. Is FluentValidation better than Data Annotations?
Yes, FluentValidation provides a more flexible and maintainable approach compared to Data Annotations.
2. Can I use FluentValidation with Blazor?
Yes, FluentValidation can be integrated into Blazor applications using the FluentValidation.Blazor
package.
3. Does FluentValidation support dependency injection?
Yes, validators can be injected via constructor-based dependency injection in ASP.NET Core.
4. How can I customize error messages?
You can specify custom error messages using .WithMessage("Custom error message")
.
5. Is FluentValidation free to use?
Yes, FluentValidation is open-source and free to use under the Apache License 2.0.
Conclusion
FluentValidation simplifies and enhances data validation in .NET applications with its expressive API and seamless integration with ASP.NET Core. By adopting FluentValidation, developers can write cleaner, reusable, and maintainable validation rules.
Start using FluentValidation today and improve your data integrity with ease!
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