Table of Contents

FluentValidation Integration

Trellis.FluentValidation plugs FluentValidation validators into the Trellis Mediator validation stage and converts standalone ValidationResult runs into Result<T> failures backed by Error.UnprocessableContent.

Patterns Index

Goal Use See
Wire validators into the Mediator pipeline (AOT-safe) services.AddTrellisFluentValidation() + explicit AddScoped<IValidator<T>, ...>() Mediator integration
Wire validators with assembly scanning (non-AOT) services.AddTrellisFluentValidation(typeof(...).Assembly) Mediator integration
Run a validator outside Mediator and stay in Result<T> validator.ValidateToResult(value) Standalone validation
Same as above for async rules (DB / I/O) validator.ValidateToResultAsync(value, cancellationToken: ct) Standalone validation
Convert an existing ValidationResult you already have validationResult.ToResult(value) Converting an existing validationresult
Reject a null request without invoking FluentValidation ValidateToResult / ValidateToResultAsync (built-in null short-circuit) Null input
Aggregate FluentValidation + IValidate failures into one 422 AddTrellisFluentValidation() alongside IValidate.Validate() on the message Composing with ivalidate

Use this guide when

  • You already use FluentValidation and want failures to surface as Error.UnprocessableContent instead of exceptions or hand-rolled translations.
  • You send messages through Trellis.Mediator and want validators to run automatically inside ValidationBehavior<TMessage,TResponse> without per-handler boilerplate.
  • You need RFC 6901 JSON Pointer paths (/Lines/0/Memo) on validation failures so the ASP boundary renders them under the right field.

Surface at a glance

Trellis.FluentValidation exposes one DI extension class, one open-generic adapter, and one set of Result<T> extension methods.

Member Receiver Returns Purpose
AddTrellisFluentValidation() IServiceCollection IServiceCollection Registers the open-generic adapter as IMessageValidator<>. AOT/trim-safe, idempotent.
AddTrellisFluentValidation(params Assembly[]) IServiceCollection IServiceCollection Same as above, then scans assemblies for concrete IValidator<T> types and registers each scoped. Not AOT/trim-safe ([RequiresUnreferencedCode] / [RequiresDynamicCode]).
FluentValidationMessageValidatorAdapter<TMessage> — (DI-resolved) IMessageValidator<TMessage> Runs every injected IValidator<TMessage> sequentially, aggregates failures into one Error.UnprocessableContent. Forwards the ambient CancellationToken.
ValidateToResult<T>(value, paramName?, message?) IValidator<T> Result<T> Synchronous validate-and-convert. Short-circuits null input without invoking FluentValidation.
ValidateToResultAsync<T>(value, paramName?, message?, ct) IValidator<T> Task<Result<T>> Asynchronous validate-and-convert. Forwards CancellationToken to ValidateAsync.
ToResult<T>(value, paramName?) ValidationResult Result<T> Converts a pre-computed ValidationResult to Result<T>; preserves the validated value on success.

Full signatures: trellis-api-fluentvalidation.md.

Installation

dotnet add package FluentValidation
dotnet add package Trellis.FluentValidation

Quick start

Run a FluentValidation validator and stay inside Result<T>. No DI, no Mediator — just the standalone helper.

using FluentValidation;
using Trellis;
using Trellis.FluentValidation;

public sealed record CreateUserRequest(string Email, string FirstName, string LastName);

public sealed class CreateUserRequestValidator : AbstractValidator<CreateUserRequest>
{
    public CreateUserRequestValidator()
    {
        RuleFor(x => x.Email).NotEmpty().EmailAddress();
        RuleFor(x => x.FirstName).NotEmpty().MaximumLength(50);
        RuleFor(x => x.LastName).NotEmpty().MaximumLength(50);
    }
}

var validator = new CreateUserRequestValidator();
var request = new CreateUserRequest("sam@example.com", "Sam", "Taylor");

Result<CreateUserRequest> result = validator.ValidateToResult(request);

On success: Result.Ok(request). On failure: Result.Fail<CreateUserRequest>(new Error.UnprocessableContent(EquatableArray.Create(violations))) with one FieldViolation per FluentValidation failure.

Standalone validation

Use the IValidator<T> extension methods when validators are not driven by the Mediator pipeline — domain factories, application services, or any code path that already holds an IValidator<T> instance.

Helper Sync/async Null input
ValidateToResult<T> sync Returns Fail without calling validator.Validate
ValidateToResultAsync<T> async Returns Fail without calling validator.ValidateAsync

Both helpers forward paramName from [CallerArgumentExpression], so root-level failures and null-input failures carry the caller's variable name as the field path.

using System.Threading;
using System.Threading.Tasks;
using FluentValidation;
using Trellis;
using Trellis.FluentValidation;

public interface IUserRepository
{
    Task<bool> EmailExistsAsync(string email, CancellationToken cancellationToken);
}

public sealed record RegisterUserRequest(string Email);

public sealed class RegisterUserRequestValidator : AbstractValidator<RegisterUserRequest>
{
    public RegisterUserRequestValidator(IUserRepository repository)
    {
        RuleFor(x => x.Email)
            .NotEmpty()
            .EmailAddress()
            .MustAsync(async (email, ct) => !await repository.EmailExistsAsync(email, ct))
            .WithMessage("Email is already registered.");
    }
}

public sealed class UserService(RegisterUserRequestValidator validator)
{
    public Task<Result<RegisterUserRequest>> RegisterAsync(
        RegisterUserRequest request,
        CancellationToken cancellationToken) =>
        validator.ValidateToResultAsync(request, cancellationToken: cancellationToken);
}

Validating inside domain factories

InlineValidator<T> keeps invariant rules close to the type that owns them. The factory returns Result<T> directly.

using System;
using FluentValidation;
using Trellis;
using Trellis.FluentValidation;

public sealed class Product : Entity<Guid>
{
    private static readonly InlineValidator<Product> s_validator = CreateValidator();

    public string Name { get; }
    public decimal Price { get; }

    private Product(Guid id, string name, decimal price)
        : base(id)
    {
        Name = name;
        Price = price;
    }

    public static Result<Product> Create(string name, decimal price)
    {
        var product = new Product(Guid.NewGuid(), name, price);
        return s_validator.ValidateToResult(product);
    }

    private static InlineValidator<Product> CreateValidator()
    {
        var validator = new InlineValidator<Product>();
        validator.RuleFor(x => x.Name).NotEmpty().MaximumLength(100);
        validator.RuleFor(x => x.Price).GreaterThan(0);
        return validator;
    }
}

Null input

ValidateToResult / ValidateToResultAsync reject null before invoking FluentValidation. The captured paramName becomes the field path; the optional message parameter overrides the default '{paramName}' must not be empty..

using FluentValidation;
using Trellis;
using Trellis.FluentValidation;

string? alias = null;

var validator = new InlineValidator<string?>();
validator.RuleFor(x => x).NotEmpty();

Result<string?> result = validator.ValidateToResult(alias, message: "Alias is required.");

validator.Validate(null!) is not called; the helper synthesizes a single FieldViolation for paramName with reason code "validation.error".

Converting an existing ValidationResult

When validation already happened (legacy code, custom orchestration, or a manual validator.Validate(value) call), use ToResult(value) to fold the ValidationResult into the railway.

using FluentValidation;
using FluentValidation.Results;
using Trellis;
using Trellis.FluentValidation;

public sealed record CreateUserRequest(string Email);

var validator = new InlineValidator<CreateUserRequest>();
validator.RuleFor(x => x.Email).NotEmpty().EmailAddress();

var request = new CreateUserRequest("invalid-email");
ValidationResult validation = validator.Validate(request);

Result<CreateUserRequest> result = validation.ToResult(request);

ToResult only null-checks validationResult itself — it does not reject a null value. Use ValidateToResult when null-input rejection matters.

Mediator integration

AddTrellisFluentValidation() registers FluentValidationMessageValidatorAdapter<TMessage> as the open-generic IMessageValidator<TMessage>. The existing ValidationBehavior<TMessage,TResponse> discovers it automatically — no second pipeline behavior is added.

Overload AOT/trim Behavior
AddTrellisFluentValidation() Safe Registers the open-generic adapter once. Idempotent. You register each IValidator<T> explicitly.
AddTrellisFluentValidation(params Assembly[]) Not safe ([RequiresUnreferencedCode], [RequiresDynamicCode]) Calls the parameterless overload, then scans the supplied assemblies for concrete IValidator<T> types and registers them as scoped. Deduplicates (serviceType, implementationType) pairs against existing registrations. Tolerates ReflectionTypeLoadException. 
using FluentValidation;
using Microsoft.Extensions.DependencyInjection;
using Trellis.FluentValidation;
using Trellis.Mediator;

builder.Services.AddMediator(opts => opts.ServiceLifetime = ServiceLifetime.Scoped);
builder.Services.AddTrellisBehaviors();
builder.Services.AddTrellisFluentValidation();

builder.Services.AddScoped<IValidator<SubmitBatchTransfersCommand>, SubmitBatchTransfersValidator>();

For non-AOT apps, scan instead:

builder.Services.AddTrellisFluentValidation(typeof(SubmitBatchTransfersValidator).Assembly);

Adapter behavior

Situation Result
No IValidator<TMessage> registered Result.Ok() — no allocations
All registered validators pass Result.Ok()
One or more validators report failures Result.Fail(new Error.UnprocessableContent(EquatableArray.Create(violations))) aggregating every failure
FluentValidation PropertyName is null/whitespace Pointer derived from typeof(TMessage).Name
FluentValidation ErrorCode is null/whitespace FieldViolation.ReasonCode defaults to "validation.error"
CancellationToken cancelled mid-run Forwarded to validator.ValidateAsync; cancellation propagates

Validators run sequentially and every failure is collected — the adapter does not short-circuit on the first failing validator.

Composing with IValidate

A message can implement Trellis.Mediator.IValidate for cross-cutting business invariants and also have one or more IValidator<TMessage> implementations registered for property-shaped rules. ValidationBehavior<TMessage,TResponse> runs every source and merges all Error.UnprocessableContent failures into a single response.

using System.Collections.Generic;
using FluentValidation;
using Mediator;
using Trellis;
using Trellis.Mediator;

public sealed record SubmitBatchTransfersCommand(
    AccountId FromId,
    BatchMetadata Metadata,
    IReadOnlyList<BatchTransferLine> Lines)
    : ICommand<Result<BatchTransferReceipt>>, IValidate
{
    public IResult Validate()
    {
        var violations = new List<FieldViolation>();
        if (Lines.Count == 0)
            violations.Add(new FieldViolation(InputPointer.ForProperty(nameof(Lines)), "batch.empty")
            { Detail = "At least one line is required." });
        for (var i = 0; i < Lines.Count; i++)
            if (Lines[i].ToAccountId == FromId)
                violations.Add(new FieldViolation(new InputPointer($"/Lines/{i}/ToAccountId"), "batch.self-transfer")
                { Detail = "A line may not target the source account." });

        return violations.Count == 0
            ? Result.Ok()
            : Result.Fail(new Error.UnprocessableContent(EquatableArray.Create(violations.ToArray())));
    }
}

public sealed class SubmitBatchTransfersValidator : AbstractValidator<SubmitBatchTransfersCommand>
{
    public SubmitBatchTransfersValidator()
    {
        RuleFor(c => c.Metadata.Reference)
            .NotEmpty().Matches(@"^BATCH-\d{4}-\d{3}$");

        RuleForEach(c => c.Lines).ChildRules(line =>
            line.RuleFor(l => l.Memo).NotEmpty().MaximumLength(200));
    }
}

A request that violates both sources at once produces one 422 with every violation aggregated under its proper JSON Pointer.

Note

Any non-UnprocessableContent failure (Error.Conflict, Error.Forbidden, …) returned by IValidate or any validator short-circuits the stage immediately and propagates as-is. Aggregation only applies to UnprocessableContent.

JSON Pointer normalization

The adapter and ToResult translate FluentValidation property names to RFC 6901 JSON Pointers before constructing InputPointer. The same JsonPointerNormalizer is used in both code paths.

FluentValidation PropertyName FieldViolation.Field
Email /Email
Address.PostCode /Address/PostCode
Items[0].Sku /Items/0/Sku
Items[0].Tags[2] /Items/0/Tags/2

Special characters in segments are escaped per RFC 6901 (~ → ~0, / → ~1). Names that already begin with / are passed through unchanged.

Composition

Once a validation step yields Result<T>, it composes with the rest of Trellis (Bind, Map, Ensure, async variants). For commands that produce no payload, return Result<Unit>.

using System.Threading;
using System.Threading.Tasks;
using FluentValidation;
using Trellis;
using Trellis.FluentValidation;

public sealed record RegisterUserRequest(string Email, string FirstName, string LastName);
public sealed record User(string Email, string FirstName, string LastName);

public interface IUserRepository
{
    Task<Result<Unit>> AddAsync(User user, CancellationToken cancellationToken);
}

public sealed class UserService(
    IValidator<RegisterUserRequest> validator,
    IUserRepository repository)
{
    public Task<Result<Unit>> RegisterAsync(RegisterUserRequest request, CancellationToken ct) =>
        validator.ValidateToResultAsync(request, cancellationToken: ct)
            .MapAsync(valid => new User(valid.Email, valid.FirstName, valid.LastName), ct)
            .BindAsync((user, token) => repository.AddAsync(user, token), ct);
}

Practical guidance

  • Prefer the parameterless AddTrellisFluentValidation() overload. It is AOT/trim-safe and idempotent. Register each IValidator<T> explicitly with AddScoped.
  • Reach for ValidateToResultAsync whenever rules touch I/O. It forwards CancellationToken straight to validator.ValidateAsync.
  • Use ToResult only when you already hold a ValidationResult. For all other paths use ValidateToResult / ValidateToResultAsync so null input is rejected before FluentValidation runs.
  • Combine IValidate and FluentValidation for layered rules. Cross-field invariants live on IValidate.Validate(); per-property rules live in the validator. Both contribute to a single aggregated 422.
  • Let pointer normalization carry structure. Do not pre-format property names — the adapter handles dotted chains and indexers automatically.
  • Validators do not parse primitives. Keep primitive-to-value-object parsing at the transport seam; validate already-shaped commands and value objects.

Cross-references

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