MicroElements.Swashbuckle.FluentValidation

Use FluentValidation rules instead of ComponentModel attributes to define swagger schema.

Note: For WebApi see: https://github.com/micro-elements/MicroElements.Swashbuckle.FluentValidation.WebApi

Statuses

Supporting the project

MicroElements.Swashbuckle.FluentValidation is developed and supported by @petriashev for free in his spare time. If you find MicroElements.Swashbuckle.FluentValidation useful, please consider financially supporting the project via OpenCollective which will help keep the project going 🙏.

Usage

1. Minimal API

MinimalApi.csproj

<Project Sdk="Microsoft.NET.Sdk.Web">

    <PropertyGroup>
        <TargetFramework>net8.0</TargetFramework>
        <Nullable>enable</Nullable>
        <ImplicitUsings>enable</ImplicitUsings>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="FluentValidation.AspNetCore" Version="11.3.1" />
        <PackageReference Include="MicroElements.Swashbuckle.FluentValidation" Version="7.1.6" />
        <PackageReference Include="Swashbuckle.AspNetCore" Version="8.1.1" />
    </ItemGroup>
    
</Project>

Program.cs

using FluentValidation;
using FluentValidation.AspNetCore;
using MicroElements.Swashbuckle.FluentValidation.AspNetCore;

var builder = WebApplication.CreateBuilder(args);
var services = builder.Services;

// Asp.Net stuff
services.AddControllers();
services.AddEndpointsApiExplorer();

// Add Swagger
services.AddSwaggerGen();

// Add FV
services.AddFluentValidationAutoValidation();
services.AddFluentValidationClientsideAdapters();

// Add FV validators
services.AddValidatorsFromAssemblyContaining<Program>();

// Add FV Rules to swagger
services.AddFluentValidationRulesToSwagger();

var app = builder.Build();

// Use Swagger
app.UseSwagger();
app.UseSwaggerUI();

app.MapControllers();

app.Run();

2. AspNetCore WebApi

Reference packages in your web project

<PackageReference Include="FluentValidation.AspNetCore" Version="11.3.1" />
<PackageReference Include="MicroElements.Swashbuckle.FluentValidation" Version="7.1.6" />
<PackageReference Include="Swashbuckle.AspNetCore" Version="8.1.1" />

Change Startup.cs

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    // Asp.net stuff
    services.AddControllers();
    
    // HttpContextValidatorRegistry requires access to HttpContext
    services.AddHttpContextAccessor();

    // Register FV validators
    services.AddValidatorsFromAssemblyContaining<Startup>(lifetime: ServiceLifetime.Scoped);

    // Add FV to Asp.net
    services.AddFluentValidationAutoValidation();

    // Add swagger
    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "My API", Version = "v1" });
    });

    // [Optional] Add INameResolver (SystemTextJsonNameResolver will be registered by default)
    // services.AddSingleton<INameResolver, CustomNameResolver>();

    // Adds FluentValidationRules staff to Swagger. (Minimal configuration)
    services.AddFluentValidationRulesToSwagger();

    // [Optional] Configure generation options for your needs. Also can be done with services.Configure<SchemaGenerationOptions>
    // services.AddFluentValidationRulesToSwagger(options =>
    // {
    //     options.SetNotNullableIfMinLengthGreaterThenZero = true;
    //     options.UseAllOffForMultipleRules = true;
    // });

    // Adds logging
    services.AddLogging(builder => builder.AddConsole());
}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllers();
    });

    // Adds swagger
    app.UseSwagger();

    // Adds swagger UI
    app.UseSwaggerUI(c =>
    {
        c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1");
    });
}

Version compatibility

MicroElements.Swashbuckle.FluentValidation	Swashbuckle.AspNetCore	FluentValidation
[1.1.0, 2.0.0)	[3.0.0, 4.0.0)	>=7.2.0
[2.0.0, 3.0.0)	[4.0.0, 5.0.0)	>=8.1.3
[3.0.0, 3.1.0)	[5.0.0, 5.2.0)	>=8.3.0
[3.1.0, 4.2.1)	[5.2.0, 6.0.0)	>=8.3.0
[4.2.0, 5.0.0)	[5.5.1, 7.0.0)	[9.0.0, 10)
[5.0.0, 6.0.0)	[6.3.0, 7.0.0)	[10.0.0, 12)
[7.0.0, 8.0.0)	[8.0.0, 11.0.0)	[11.0.0, 13)

.NET 8/9 use Swashbuckle 8.x (Microsoft.OpenApi 1.x); .NET 10 uses Swashbuckle 10.x (Microsoft.OpenApi 2.x).

Sample application

See sample project: https://github.com/micro-elements/MicroElements.Swashbuckle.FluentValidation/tree/master/samples/SampleWebApi

Supported validators

• INotNullValidator (NotNull)
• INotEmptyValidator (NotEmpty)
• ILengthValidator (for strings: Length, MinimumLength, MaximumLength, ExactLength) (for arrays: MinItems, MaxItems)
• IRegularExpressionValidator (Email, Matches)
• IComparisonValidator (GreaterThan, GreaterThanOrEqual, LessThan, LessThanOrEqual)
• IBetweenValidator (InclusiveBetween, ExclusiveBetween)

File uploads (media types & size) — Issue #216

Validation rules written on nested IFormFile members (e.g. RuleFor(x => x.File.ContentType) / RuleFor(x => x.File.Length)) are not reflected in the OpenAPI document: FluentValidation names them File.ContentType / File.Length, which never match the flat File schema property, and Must(...) carries no introspectable metadata. Use the dedicated File-level rules instead:

using MicroElements.OpenApi.FluentValidation.FileUpload;

public class UploadProductImageRequestValidator : AbstractValidator<UploadProductImageRequest>
{
    public UploadProductImageRequestValidator()
    {
        RuleFor(x => x.File)
            .NotNull()                                    // required
            .FileContentType("image/jpeg", "image/png")   // allowed media types
            .MaxFileSize(2 * 1024 * 1024);                // 2 MB
    }
}

These rules enforce the constraints at runtime and drive the OpenAPI output:

multipart/form-data:
  schema:
    properties:
      File:
        type: string
        format: binary
        description: "Allowed content types: image/jpeg, image/png. Maximum file size: 2097152 bytes."
  encoding:
    File:
      contentType: "image/jpeg, image/png"

Available rules: .FileContentType(params string[]), .MaxFileSize(long), .MinFileSize(long), .FileSizeBetween(long, long).

Backend support:

Backend	size & content types in description	machine-readable encoding.contentType
Swashbuckle	✅	✅ (net8/9 = OpenAPI 3.0; net10 = OpenAPI 3.1)
NSwag	✅	✅ via FluentValidationOperationProcessor (serialized as encodingType — a known NSwag limitation)
Microsoft.AspNetCore.OpenApi	✅	✅ (net9 + net10)

The issue scenario — making the generated OpenAPI document reflect the allowed content types and size limit — works on all three backends, both via the file part description and as the machine-readable encoding.contentType.

Notes:

• File size has no standard OpenAPI/JSON-Schema byte keyword, so it is documented in description only (annotation, not enforced by consumers; enforcement stays server-side via FluentValidation).
• NSwag requires registering the operation processor: settings.OperationProcessors.Add(serviceProvider.GetService<FluentValidationOperationProcessor>()) (see the NSwag sample).
• Microsoft.AspNetCore.OpenApi: on net10 the file part is emitted as a $ref to a shared IFormFile component, so the size/content-type description is shared across all IFormFile endpoints (differing per-endpoint rules would accumulate there) — but encoding.contentType is per-operation and unaffected.

Extensibility

You can register FluentValidationRule in ServiceCollection.

User defined rule name replaces default rule with the same. Full list of default rules can be get by FluentValidationRules.CreateDefaultRules()

List or default rules:

• Required
• NotEmpty
• Length
• Pattern
• Comparison
• Between

Example of rule:

new FluentValidationRule("Pattern")
{
    Matches = propertyValidator => propertyValidator is IRegularExpressionValidator,
    Apply = context =>
    {
        var regularExpressionValidator = (IRegularExpressionValidator)context.PropertyValidator;
        context.Schema.Properties[context.PropertyKey].Pattern = regularExpressionValidator.Expression;
    }
},

Samples

Swagger Sample model and validator

public class Sample
{
    public string PropertyWithNoRules { get; set; }

    public string NotNull { get; set; }
    public string NotEmpty { get; set; }
    public string EmailAddress { get; set; }
    public string RegexField { get; set; }

    public int ValueInRange { get; set; }
    public int ValueInRangeExclusive { get; set; }

    public float ValueInRangeFloat { get; set; }
    public double ValueInRangeDouble { get; set; }
}

public class SampleValidator : AbstractValidator<Sample>
{
    public SampleValidator()
    {
        RuleFor(sample => sample.NotNull).NotNull();
        RuleFor(sample => sample.NotEmpty).NotEmpty();
        RuleFor(sample => sample.EmailAddress).EmailAddress();
        RuleFor(sample => sample.RegexField).Matches(@"(\d{4})-(\d{2})-(\d{2})");

        RuleFor(sample => sample.ValueInRange).GreaterThanOrEqualTo(5).LessThanOrEqualTo(10);
        RuleFor(sample => sample.ValueInRangeExclusive).GreaterThan(5).LessThan(10);

        // WARNING: Swashbuckle implements minimum and maximim as int so you will loss fraction part of float and double numbers
        RuleFor(sample => sample.ValueInRangeFloat).InclusiveBetween(1.1f, 5.3f);
        RuleFor(sample => sample.ValueInRangeDouble).ExclusiveBetween(2.2, 7.5f);
    }
}

Swagger Sample model screenshot

Validator with Include

public class CustomerValidator : AbstractValidator<Customer>
{
    public CustomerValidator()
    {
        RuleFor(customer => customer.Surname).NotEmpty();
        RuleFor(customer => customer.Forename).NotEmpty().WithMessage("Please specify a first name");

        Include(new CustomerAddressValidator());
    }
}

internal class CustomerAddressValidator : AbstractValidator<Customer>
{
    public CustomerAddressValidator()
    {
        RuleFor(customer => customer.Address).Length(20, 250);
    }
}

Nested objects (SetValidator / ChildRules)

Rules for a nested object are applied to the child component schema, and the parent property keeps a $ref to it. Both a standalone child validator (SetValidator) and inline ChildRules are supported (inline ChildRules $ref preservation was fixed in 7.1.6, see #198):

public class CreateUserRequest
{
    public CreateUserParams User { get; set; }
}

public class CreateUserParams
{
    public string Email { get; set; }
    public string Name { get; set; }
}

public class CreateUserRequestValidator : AbstractValidator<CreateUserRequest>
{
    public CreateUserRequestValidator()
    {
        // Inline child rules — no separate validator class required
        RuleFor(x => x.User)
            .NotEmpty()
            .ChildRules(user =>
            {
                user.RuleFor(u => u.Email).NotEmpty().EmailAddress();
                user.RuleFor(u => u.Name).NotEmpty().MaximumLength(510);
            });

        // Equivalent with a standalone validator:
        // RuleFor(x => x.User).NotEmpty().SetValidator(new CreateUserParamsValidator());
    }
}

The Email/Name constraints (and required) end up on the CreateUserParams component, while the parent stays a reference:

"CreateUserRequest": {
  "required": [ "user" ],
  "type": "object",
  "properties": {
    "user": { "$ref": "#/components/schemas/CreateUserParams" }
  }
}

Get params bounded to validatable models

MicroElements.Swashbuckle.FluentValidation updates swagger schema for operation parameters bounded to validatable models.

Nested [FromQuery] parameters

When a [FromQuery] model has nested objects, ASP.NET Core flattens them into dot-path parameters (e.g. RequiredSubType.SubProperty). The validation rules for such a nested parameter are reflected in the OpenAPI document only when they are actually enforced at runtime:

• The nested validator must be wired from the root validator via SetValidator/ChildRules (since 7.1.7). FluentValidation never auto-validates a child object just because a validator for it is registered in DI — so an unwired nested validator no longer leaks required/length/pattern constraints onto the parameter.
• A nested parameter is marked required only when every ancestor segment of the dot-path is required (see #209).

Note: if there is no validator registered for the root [FromQuery] type (only a leaf/child validator), the flattened nested parameter is left unconstrained — matching the default runtime, where no validation runs without a root validator. If you instead validate the child manually in the controller (e.g. new SubValidator().Validate(filter.Child)), those constraints cannot be detected statically and so are not reflected in the schema — register/wire a validator for the root type if you want them documented.

Document filter pipeline (UseDocumentFilter)

Since 7.2.0 the library offers a supported alternative pipeline: a single document filter instead of the default schema + operation filters.

services.AddFluentValidationRulesToSwagger(
    configureRegistration: options => options.UseDocumentFilter = true);

The document filter processes the whole document at once — component schemas, operation parameters (including required-marking, #209), request bodies and encoding.contentType (#216) — and performs the unused-query-schema cleanup once at the end.

Why opt in: the default pipeline runs per operation, so a DTO shared between several endpoints (or between [FromQuery] and [FromBody] bindings) used to hit per-operation state bugs (#223, #226); those were fixed, but the net8.0/net9.0 targets keep a narrower guarantee (the healing API only exists in Swashbuckle 10+). Under the document filter this class of issue cannot occur, on any target framework.

Caveats:

• The default pipeline is unchanged — UseDocumentFilter is opt-in. A future major version may make it the default.
• On net10.0 the cleanup also clears Swashbuckle's internal reserved-ids for the container types it processed; transitively registered child schemas are not covered. On net8.0/net9.0, a custom IDocumentFilter registered after this one that regenerates a removed [FromQuery] container type may still observe a reserved-but-removed schema state.
• ExperimentalUseDocumentFilter still works as an obsolete alias and forwards to UseDocumentFilter.

The samples/MinimalApi sample runs on this pipeline.

Defining rules dynamically from database

See BlogValidator in sample.

Common problems and workarounds

Error: System.InvalidOperationException: 'Cannot resolve 'IValidator<T>' from root provider because it requires scoped service 'TDependency'

Workarounds in order or preference:

Workaround 1 (Use HttpContextServiceProviderValidatorFactory) by @WarpSpideR

public void ConfigureServices(IServiceCollection services)
{
    // HttpContextServiceProviderValidatorFactory requires access to HttpContext
    services.AddHttpContextAccessor();

    services
        .AddMvc()
        // Adds fluent validators to Asp.net
        .AddFluentValidation(c =>
        {
            c.RegisterValidatorsFromAssemblyContaining<Startup>();
            // Optionally set validator factory if you have problems with scope resolve inside validators.
            c.ValidatorFactoryType = typeof(HttpContextServiceProviderValidatorFactory);
        });

Workaround 2 (Use ScopedSwaggerMiddleware)

Replace UseSwagger for UseScopedSwagger:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app
        .UseMvc()
        // Use scoped swagger if you have problems with scoped services in validators
        .UseScopedSwagger();

Workaround 3 (Set ValidateScopes to false)

public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        // Needed for using scoped services (for example DbContext) in validators
        .UseDefaultServiceProvider(options => options.ValidateScopes = false)
        .UseStartup<Startup>()
        .Build();

Problem: I cant use several validators of one type

Example: You split validator into several small validators but AspNetCore uses only one of them.

Workaround: Hide dependent validators with internal and use Include to include other validation rules to one "Main" validator.

Problem: I'm using FluentValidation or FluentValidation.DependencyInjectionExtensions instead of FluentValidation.AspNetCore

If you are using the more basic FluentValidation or FluentValidation.DependencyInjectionExtensions libraries, then they will not automatically register IValidatorFactory and you will get an error at runtime: "ValidatorFactory is not provided. Please register FluentValidation." In that case you must register it manually (see issue 97 for more details):

services.TryAddTransient<IValidatorFactory, ServiceProviderValidatorFactory>();
services.AddFluentValidationRulesToSwagger();

Problem: Newtonsoft.Json DefaultNamingStrategy, SnakeCaseNamingStrategy does not work

Startup.cs:

    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.PropertyNamingPolicy = new NewtonsoftJsonNamingPolicy(new SnakeCaseNamingStrategy());
        //options.JsonSerializerOptions.DictionaryKeyPolicy = new NewtonsoftJsonNamingPolicy(new SnakeCaseNamingStrategy());
    })


    /// <summary>
    /// Allows use Newtonsoft <see cref="NamingStrategy"/> as System.Text <see cref="JsonNamingPolicy"/>.
    /// </summary>
    public class NewtonsoftJsonNamingPolicy : JsonNamingPolicy
    {
        private readonly NamingStrategy _namingStrategy;

        /// <summary>
        /// Creates new instance of <see cref="NewtonsoftJsonNamingPolicy"/>.
        /// </summary>
        /// <param name="namingStrategy">Newtonsoft naming strategy.</param>
        public NewtonsoftJsonNamingPolicy(NamingStrategy namingStrategy)
        {
            _namingStrategy = namingStrategy;
        }

        /// <inheritdoc />
        public override string ConvertName(string name)
        {
            return _namingStrategy.GetPropertyName(name, false);
        }
    }

Credits

Initial version of this project was based on Mujahid Daud Khan answer on StackOverflow: https://stackoverflow.com/questions/44638195/fluent-validation-with-swagger-in-asp-net-core/49477995#49477995