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Validation and Error Handling

As validation and error handling is an essential part of developing services, ServiceStack provides a rich array of error handling options that work intuitively out-of-the-box.

Optimized for developer happiness ServiceStack allows you to idiomatically throw C# exceptions directly in your services and trivially consume them on the client with minimal effort, intuitively and conventionally - allowing the opportunity to inject generic error handling routines to handle errors for all your web services.

As a bonus the most appropriate HTTP Status code is returned based upon the C# Exception type.

Typed, Structured Exceptions end-to-end

The error handling support works end-to-end where all errors get auto-serialized into your Response DTO and re-hydrated into a C# Exception on ServiceStack’s generic Service Clients. This allows you to idiomatically treat errors like normal C# Exceptions - providing easy access to rich, structured error messages in your clients.

JavaScript support included

To make it trivial to consume errors in JavaScript, you can use the lightweight ss-utils.js JavaScript library to trivially bind your response errors to your HTML form fields with a single line of code.

All Error handling and validation options described below are treated in the same way - serialized into the ResponseStatus property of your Response DTO making it possible for your clients applications to generically treat all Web Service Errors in the same way.

Note: The response DTO must follow the {Request DTO}Response naming convention and has to be in the same namespace as the Request DTO

public class Hello : IReturn<HelloResponse> {}

//Follows naming convention and is in the same namespace as 'Hello'
public class HelloResponse 
    public ResponseStatus ResponseStatus { get; set; } //Exception gets serialized here

If now an exception occurs in the service implementation, the exception is serialized.

Example JSON output:

    "ResponseStatus": {
         "ErrorCode": "NotSupportedException",
         "Message": "..."

It’s up to the client how to handle this service error.

Throw a C# Exception

The easiest way to generate an Error in ServiceStack is to simply throw a C# Exception:

public object Post(User request) 
	if (string.IsNullOrEmpty(request.Name))
		throw new ArgumentNullException("Name");

By Default C# Exceptions:

All Exceptions gets injected into the ResponseStatus property of your Response DTO that is serialized into your ServiceClient’s preferred Content-Type making error handling transparent regardless of your preferred format - i.e. the same C# Error handling code can be used for all ServiceClients.

    var client = new JsonServiceClient(BaseUri);
    var response = client.Send<UserResponse>(new User());
catch (WebServiceException webEx) 
      webEx.StatusCode  = 400
      webEx.ErrorCode   = ArgumentNullException
      webEx.Message     = Value cannot be null. Parameter name: Name
      webEx.StackTrace  = (your Server Exception StackTrace - if DebugMode is enabled)
      webEx.ResponseDto = (your populated Response DTO)
      webEx.ResponseStatus   = (your populated Response Status DTO)
      webEx.GetFieldErrors() = (individual errors for each field if any)

Enabling StackTraces

By default display StackTraces in your Response DTOs are disabled, but they’re a good to have for development, which you can enable with:

SetConfig(new HostConfig { DebugMode = true });

Customized Error Messages

If you want even finer grained control of your HTTP errors you can either throw or return a HttpError letting you customize the Http Headers and Status Code and HTTP Response body to get exactly what you want on the wire:

public object Get(User request) {
       throw HttpError.NotFound($"User {request.Name} does not exist");

the above is a short-hand for new HttpError(HttpStatusCode.NotFound, $"User {request.Name} does not exist") which returns a 404 NotFound StatusCode on the wire.

Validation Feature

For more complex validation and to be able to return multiple validation errors ServiceStack includes the excellent Fluent Validation library by @JeremySkinner - a very clean and DSL-like way to validate request DTOs. Even contextual validation for each HTTP method (GET, POST, …) is supported.

ServiceStack’s Fluent Validation feature is encapsulated in the ValidationFeature plugin which can be registered in your AppHost with:

Plugins.Add(new ValidationFeature());

FluentValidation for request dtos

The example below uses this request dto for validation:

public class User
    public string Name { get; set; }
    public string Company { get; set; }
    public int Age { get; set; }
    public int Count { get; set; }
    public string Address { get; set; }

The validation rules for this request dto are made with FluentValidation. ServiceStack makes heavy use of rule sets to provide different validation rules for each HTTP method (GET, POST, PUT…).

Tip: First read the documentation about FluentValidation before you continue reading

public interface IAddressValidator
    bool ValidAddress(string address);

public class AddressValidator : IAddressValidator
    public bool ValidAddress(string address)
	return address != null
	    && address.Length >= 20
	    && address.Length <= 250;

public class UserValidator : AbstractValidator<User>
    public IAddressValidator AddressValidator { get; set; }
    public UserValidator()
        //Validation rules for all requests
        RuleFor(r => r.Name).NotEmpty();
        RuleFor(r => r.Age).GreaterThan(0);
        RuleFor(x => x.Address).Must(x => AddressValidator.ValidAddress(x));

        //Validation rules for GET request
        RuleSet(ApplyTo.Get, () => {
            RuleFor(r => r.Count).GreaterThan(10);

        //Validation rules for POST and PUT request
        RuleSet(ApplyTo.Post | ApplyTo.Put, () => {
            RuleFor(r => r.Company).NotEmpty();

Info: ServiceStack adds another extension method named RuleSet which can handle ApplyTo enum flags. This method doesn’t exist in the core FluentValidation framework.

Warning: If a validator for a request dto is created, all rules which aren’t in any rule set are executed + the rules in the matching rule set. Normally FluentValidation only executes the matching rule set and ignores all other rules (whether they’re in a rule set or not) and the rules which don’t belong to any rule set are normally only executed, if no rule set-name was given to the validate method of the validator.

Like services registered in the IoC container, validators are also auto-wired, so if there’s a public property which can be resolved by the IoC container, the IoC container will inject it. In this case, the IoC container will resolve the property AddressValidator, if an object of the type IAddressValidator was registered.

Tip: You can access the current IRequest in your Custom Validator from base.Request

Async Validators

Async validators can be registered using the MustAsync validator where you could simulate the following built-in Not Empty validation:

public class MyRequestValidator : AbstractValidator<MyRequest>
    public MyRequestValidator()
        RuleFor(x => x.Name).NotEmpty();

And replace it with an Async version that uses the Service Gateway to call a custom Async GetStringLength Service that returns the same ErrorCode and Error Message as the Not Empty validator:

public class MyRequestValidator : AbstractValidator<MyRequest>
    public MyRequestValidator()
        RuleFor(x => x.Name).MustAsync(async (s, token) => 
            (await Gateway.SendAsync(new GetStringLength { Value = s })).Result > 0)
        .WithMessage("'Name' should not be empty.")

Register Validators

The ValidationFeature plugin automatically scans and auto-wires all validators in the AppHost.ServiceAssemblies that’s injected in the AppHost constructor.

This default behavior can be disabled with:

Plugins.Add(new ValidationFeature {
    ScanAppHostAssemblies = false

Otherwise if the validators are in other assembles they can be registered using RegisterValidators(), e.g:

//This method scans the assembly for validators

Optionally each validator can be registered individually:

//Add the IAdressValidator which will be injected into the UserValidator
container.Register<IAddressValidator>(new AddressValidator());

Now the service etc can be created and the validation rules are checked every time a request comes in.

If you try now for example to send this request:

POST localhost:50386/validated
    "Name": "Max"

You’ll get this JSON response:

    "ErrorCode": "GreaterThan",
    "Message": "'Age' must be greater than '0'.",
    "Errors": [
            "ErrorCode": "GreaterThan",
            "FieldName": "Age",
            "Message": "'Age' must be greater than '0'."
            "ErrorCode": "NotEmpty",
            "FieldName": "Company",
            "Message": "'Company' should not be empty."

As you can see, the ErrorCode and the FieldName provide an easy way to handle the validation error at the client side. If you want, you can also configure a custom ErrorCode for a validation rule:

RuleFor(x => x.Name).NotEmpty().WithErrorCode("ShouldNotBeEmpty"); 

If the rule fails, the JSON response will look like that:

    "ErrorCode": "ShouldNotBeEmpty",
    "FieldName": "Name",
    "Message": "'Name' should not be empty."

Custom Validation

ServiceStack’s internal implementation of FluentValidation uses the latest 7.2 version which lets you take advantage of new features like implementing Custom Validators, e.g:

public class CustomValidationValidator : AbstractValidator<CustomValidation>
    public CustomValidationValidator()
        RuleFor(request => request.Code).NotEmpty();
        RuleFor(request => request)
            .Custom((request, context) => {
                if (request.Code?.StartsWith("X-") != true)
                    var propName = context.ParentContext.PropertyChain.BuildPropertyName("Code");
                    context.AddFailure(new ValidationFailure(propName, error:"Incorrect prefix") {
                        ErrorCode = "NotFound"

Use FluentValidation everywhere!

Of course FluentValidation can be used for any other classes (not only request DTOs), too:

public class TestClass
    public string Text { get; set; }
    public int Length { get; set; }

Now the validator:

public class TestClassValidator : AbstractValidator<TestClass>
    public TestClassValidator()
        RuleFor(x => x.Text).NotEmpty();
        RuleFor(x => x.Length).GreaterThan(0);

Info: If FluentValidation isn’t used for request DTOs, it behaves the same as documented in the Fluent Validation documentation.

Inside some service code you can validate an instance of this class:

public class SomeService : Service
    //You should have registered your validator in the IoC container to inject the validator into this property
    public IValidator<TestClass> Validator { get; set; }

    public object Get(Validated request)
        TestClass instance = new TestClass();

        ValidationResult result = this.Validator.Validate(instance);

        if (!result.IsValid)
            //The result will be serialized into a ValidationErrorException and throw this one
            //The errors will be serialized in a clean, human-readable way (as the above JSON example)
            throw result.ToException();


Populating the Response DTO Manually

All the error handling methods illustrated above are just sugar coating for serializing exceptions into your Response DTOs ResponseStatus property. You don’t have use any of this as you can simply populate the ResponseStatus property yourself, coupled with the HttpResult class you have complete control of your HTTP Response - this gives you the necessary freedom so you are able to define your own Error handling API and helpers if you have an existing validation library that you would like to use instead.


World Validation

See the annotated World Validation Docs for a detailed example of Fluent Validation that walks through and showcases the implementation of how the most popular Server HTML rendered approaches and Client UI rendered technologies which are able all to use the same single suite of Fluent Validators and ServiceStack Services.