Fluent Validation in .NET Core
                     Fluent Validation in .NET Core 8 - Examples & Best Practices
Fluent Validation is a popular library in the .NET ecosystem that provides a fluent interface for 
defining validation rules for your objects. It simplifies the process of validating complex models, 
ensuring your application data meets specified criteria before processing. In this article, we'll explore
Fluent Validation in .NET Core 8, offering practical examples and best practices to help you leverage 
this powerful tool in your projects.
 What is Fluent Validation  ? 
Fluent Validation is a .NET library for building strongly-typed validation rules. It offers a simple, 
fluent interface that allows developers to define validation logic for models in a clear and readable 
manner. This approach ensures your code is maintainable and less prone to errors compared to 
traditional validation techniques.
Setting Up Fluent Validation in .NET Core 8
To get started with Fluent Validation in .NET Core 8, you need to install the FluentValidation 
package. You can do this using the NuGet Package Manager or the .NET CLI.
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dotnet add package FluentValidation
After installing the package, you'll need to configure it in your project. Typically, this is done in the 
Startup.cs or Program.cs file:
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public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
        services.AddFluentValidation(fv => fv.RegisterValidatorsFromAssemblyContaining());
    }
    // Other configurations...
}
Creating a Validator
To create a validator, you need to define a class that inherits from AbstractValidator, where T is 
the type of the model you want to validate. Here’s an example of a simple validator for a User 
model:
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using FluentValidation;
public class User
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
    public int Age { get; set; }
}
public class UserValidator : AbstractValidator
{
    public UserValidator()
    {
        RuleFor(user => user.FirstName).NotEmpty().WithMessage("First name is required.");
        RuleFor(user => user.LastName).NotEmpty().WithMessage("Last name is required.");
        RuleFor(user => user.Email).NotEmpty().EmailAddress().WithMessage("Valid email is 
required.");
        RuleFor(user => user.Age).InclusiveBetween(18, 60).WithMessage("Age must be between 18 
and 60.");
    }
}
Applying Validation in Controllers
To apply validation, you need to ensure that the validators are invoked during model binding in your 
controllers. In .NET Core 8, this is usually done automatically if you’ve registered Fluent Validation in 
the service container. Here’s an example controller action:
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[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    [HttpPost]
    public IActionResult Create(User user)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }
        // Proceed with creating the user...
        return Ok(user);
    }
}
Best Practices for Using Fluent Validation
1. Keep Validation Rules Simple:
Write validation rules that are easy to read and understand. Avoid complex logic within the rules to 
maintain clarity.
2. Reuse Validators:
If you have common validation rules across multiple models, consider creating a base validator that 
can be reused. This helps reduce redundancy and ensures consistency.
3. Use Custom Validators:
For more complex validation scenarios, create custom validators. Fluent Validation allows you to 
define custom logic that can be reused across multiple rules.
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public class CustomEmailValidator : PropertyValidator
{
    public CustomEmailValidator() : base("Invalid email format.")
    {
    }
    protected override bool IsValid(PropertyValidatorContext context)
    {
        var email = context.PropertyValue as string;
        return email != null && email.Contains("@");
    }
}
4. Validate Nested Models:
Fluent Validation supports validating nested models. Use the SetValidator method to apply 
validators to properties that are complex objects.
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public class AddressValidator : AbstractValidator
{
    public AddressValidator()
    {
        RuleFor(address => address.Street).NotEmpty();
        RuleFor(address => address.City).NotEmpty();
    }
}
public class UserValidator : AbstractValidator
{
    public UserValidator()
    {
        RuleFor(user => user.FirstName).NotEmpty();
        RuleFor(user => user.Address).SetValidator(new AddressValidator());
    }
}
Conclusion
Fluent Validation is an essential tool for .NET Core 8 developers, offering a clean and fluent way to 
define validation rules. By integrating Fluent Validation into your projects, you can ensure your 
application data is accurate and reliable, leading to fewer errors and better overall performance. For 
more detailed guides and examples, visit CsharpMaster.
Visit for more  Information: https://csharpmaster.com/using-fluent-validation-in-dotnet-
core-8-with-example/