FluentValidation in Blazor: A Complete Guide

Ever feel like you’re fighting Blazor’s default validation just to keep your forms clean and your logic sane? You’re not alone. Most Blazor devs stumble into the world of DataAnnotations and soon realize the limitations: they’re verbose, not very expressive, and get messy as your models grow. That’s where FluentValidation enters the scene like a breath of fresh C# air.

Why Choose FluentValidation for Blazor?

FluentValidation is more than just another library — it’s a developer-friendly toolkit that brings expressive, reusable, and clean validation rules to your form models. With FluentValidation, you can:

• Define rules in a clean and readable syntax
• Keep validation logic out of your data models
• Reuse rules across multiple components
• Support complex conditional logic and localization

In a nutshell, FluentValidation plays nicely with Blazor while freeing you from the clunky confines of DataAnnotations.

The Problem with Default Validation in Blazor

Blazor’s built-in validation relies heavily on DataAnnotations, which work fine for simple cases but fall apart when things get complex. Here are some pain points:

• Tight coupling between model and validation logic
• Verbose attribute-based rules
• No support for advanced scenarios like conditionals, rule sets, or custom messages
• Difficult to reuse validation logic across models or components

If you’ve ever tried validating nested objects or needed different rule sets per UI state, you’ve probably already outgrown DataAnnotations.

Introducing FluentValidation

FluentValidation is a .NET library that lets you write validation rules with a fluent API. It’s expressive, testable, and supports advanced scenarios out-of-the-box.

Here’s a taste:

public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        RuleFor(x => x.Name).NotEmpty().WithMessage("Name is required");
        RuleFor(x => x.Age).InclusiveBetween(18, 60);
    }
}

Readable? Definitely. Reusable? Absolutely. Cross-platform? Yep — works with .NET Core, ASP.NET, Blazor, and more.

Setting Up FluentValidation in a Blazor Project

Installing the Required Packages

First, grab the necessary NuGet packages:

Install-Package FluentValidation
Install-Package FluentValidation.DependencyInjectionExtensions
Install-Package BlazorFluentValidation

Configuring FluentValidation in Dependency Injection

In your Program.cs, register the validators:

builder.Services.AddValidatorsFromAssemblyContaining<PersonValidator>();
builder.Services.AddFluentValidation();

This ensures Blazor knows how to wire up the validators when rendering forms.

Creating and Using Validators

Writing a Basic Validator Class

Let’s say you have a RegisterModel:

public class RegisterModel
{
    public string Email { get; set; }
    public string Password { get; set; }
}

Here’s the FluentValidation validator:

public class RegisterModelValidator : AbstractValidator<RegisterModel>
{
    public RegisterModelValidator()
    {
        RuleFor(x => x.Email).NotEmpty().EmailAddress();
        RuleFor(x => x.Password).MinimumLength(6);
    }
}

Integrating Validators with Blazor Forms

Use the FluentValidationValidator component inside your EditForm:

<EditForm Model="model" OnValidSubmit="HandleValidSubmit">
    <FluentValidationValidator />
    <InputText @bind-Value="model.Email" />
    <ValidationMessage For="@(() => model.Email)" />
    <!-- More fields -->
    <button type="submit">Submit</button>
</EditForm>

This automatically hooks the validator to the form.

Advanced Validation Scenarios

FluentValidation shines in complex rules:

RuleFor(x => x.Password)
    .NotEmpty()
    .MinimumLength(8)
    .When(x => x.RequiresStrongPassword);

You can also define rule sets and nested validators for hierarchical data models.

Enhancing User Experience with Real-Time Feedback

Displaying Validation Errors Dynamically

Blazor re-renders components reactively, making it easy to display error messages as users type. Just include <ValidationMessage> per field.

Customizing UI Based on Validation State

You can bind CSS classes conditionally:

<InputText @bind-Value="model.Email" class="@(IsValid(nameof(model.Email)) ? "valid" : "invalid")" />

Use helper methods or validation state checks to drive visual cues like icons or border colors.

Best Practices and Performance Tips

Keeping Validators Maintainable

• Use descriptive rule chains
• Group rules logically with comments or regions
• Use extension methods for shared rules

Validating Nested Objects and Collections

FluentValidation supports .SetValidator() for nested models:

RuleFor(x => x.Address).SetValidator(new AddressValidator());

Performance Considerations

• Avoid validating on every keystroke for large forms
• Validate on blur or submit for better performance
• Use debouncing if real-time validation is essential

FAQ: Common Questions About FluentValidation in Blazor

Conclusion: Stop Fighting Forms and Start Writing Code

Blazor’s forms don’t have to be a pain. With FluentValidation, you gain cleaner code, more control, and happier users. The setup is simple, the rules are powerful, and the result is a better developer experience.

Give FluentValidation a try in your next Blazor form. You might just wonder why you didn’t do it sooner.