FluentValidation is a powerful library for input validation in .NET APIs, offering a fluent interface and lambda expressions to define strongly-typed validation rules. It supports dependency injection (DI), enabling dynamic rule configurations based on injected services or settings.
You Should Know:
1. Installing FluentValidation
dotnet add package FluentValidation
2. Creating a Validator Class
using FluentValidation;
public class UserValidator : AbstractValidator<User>
{
public UserValidator()
{
RuleFor(user => user.Name).NotEmpty().WithMessage("Name is required.");
RuleFor(user => user.Email).EmailAddress().WithMessage("Invalid email format.");
}
}
3. Integrating with ASP.NET Core
Register the validator in `Program.cs`:
builder.Services.AddScoped<IValidator<User>, UserValidator>();
4. Using Dependency Injection in Validation
public class ProductValidator : AbstractValidator<Product>
{
public ProductValidator(IConfiguration config)
{
int minStock = config.GetValue<int>("MinStock");
RuleFor(p => p.Stock).GreaterThanOrEqualTo(minStock);
}
}
5. Custom Validators
RuleFor(user => user.Password)
.Must(p => p.Length >= 8)
.WithMessage("Password must be at least 8 characters.");
6. Testing Validation in Postman
- Send a malformed request to verify validation responses.
- Check HTTP `400 Bad Request` responses for validation errors.
7. Advanced: RuleSets for Conditional Validation
RuleSet("Admin", () =>
{
RuleFor(user => user.Role).Equal("Admin");
});
8. Automating Validation in Minimal APIs
app.MapPost("/users", (User user, IValidator<User> validator) =>
{
var result = validator.Validate(user);
if (!result.IsValid) return Results.BadRequest(result.Errors);
return Results.Ok();
});
What Undercode Say:
FluentValidation simplifies API input validation while maintaining flexibility. Key takeaways:
– Use DI for dynamic rule configurations.
– Leverage `RuleFor` and `Must` for custom logic.
– Test validation rules rigorously.
– Combine with ASP.NET Core’s middleware for seamless integration.
For more details, check the official FluentValidation docs.
Expected Output:
A robust API with structured validation, reducing runtime errors and improving security.
Prediction:
Future .NET releases may further integrate FluentValidation with source generators for compile-time validation optimizations.
References:
Reported By: Milan Jovanovic – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
