Async validation

Some validation can't happen in-memory. Checking whether an email is already registered, verifying an invite code against a database, or calling an external API for address validation all require I/O. The standard IValidator interface is synchronous, so MADE.Data.Validation provides IAsyncValidator and AsyncValidatorCollection for these scenarios.

Implementing IAsyncValidator

IAsyncValidator has the same properties as IValidator but replaces Validate(object) with ValidateAsync(object, CancellationToken):

using MADE.Data.Validation;

public class UniqueEmailValidator : IAsyncValidator
{
    private readonly IUserRepository repository;

    public UniqueEmailValidator(IUserRepository repository)
    {
        this.repository = repository;
    }

    public string Key { get; set; } = nameof(UniqueEmailValidator);
    public bool IsInvalid { get; set; }
    public bool IsDirty { get; set; }
    public string FeedbackMessage { get; set; } = "This email address is already registered.";

    public async Task ValidateAsync(object value, CancellationToken cancellationToken = default)
    {
        var email = value?.ToString();
        IsInvalid = !string.IsNullOrWhiteSpace(email)
            && await repository.EmailExistsAsync(email, cancellationToken);
        IsDirty = true;
    }
}

Using AsyncValidatorCollection

AsyncValidatorCollection works the same way as ValidatorCollection but executes each validator asynchronously:

var validators = new AsyncValidatorCollection
{
    new UniqueEmailValidator(userRepository),
    new ActiveDomainValidator(domainService),
};

await validators.ValidateAsync(emailAddress);

if (validators.IsInvalid)
{
    foreach (var message in validators.FeedbackMessages)
    {
        Console.WriteLine(message);
    }
}

Combining sync and async validation

A common pattern is to run cheap synchronous validators first (format checks, length constraints) and only run expensive async validators if the basic checks pass:

// Step 1: synchronous validation
var formatValidators = new ValidatorCollection
{
    new RequiredValidator(),
    new EmailValidator(),
};

formatValidators.Validate(email);
if (formatValidators.IsInvalid)
{
    return formatValidators.FeedbackMessages;
}

// Step 2: async validation (only if format is valid)
var asyncValidators = new AsyncValidatorCollection
{
    new UniqueEmailValidator(userRepository),
};

await asyncValidators.ValidateAsync(email);
if (asyncValidators.IsInvalid)
{
    return asyncValidators.FeedbackMessages;
}

This avoids unnecessary database calls for obviously invalid input.

Table of Contents

Async validation

Implementing IAsyncValidator

Using AsyncValidatorCollection

Combining sync and async validation