如何在FluentValidation中实现WithMessageAsync异步消息翻译

要实现WithMessageAsync并复用内置验证规则，核心思路是通过包装内置验证器，重写其异步消息获取逻辑，同时保留链式调用的体验。以下是具体实现步骤：

1. 创建异步消息包装验证器 #

这个类负责包裹FluentValidation的内置验证器，替换其错误消息的获取逻辑为异步方式：

public class AsyncMessagePropertyValidator<T, TProperty> : PropertyValidator<T, TProperty>
{
    private readonly PropertyValidator<T, TProperty> _innerValidator;
    private readonly Func<ValidationContext<T>, Task<string>> _messageProvider;

    public AsyncMessagePropertyValidator(PropertyValidator<T, TProperty> innerValidator, Func<ValidationContext<T>, Task<string>> messageProvider)
    {
        _innerValidator = innerValidator;
        _messageProvider = messageProvider;
        RuleSets = innerValidator.RuleSets;
    }

    public override async Task<bool> IsValidAsync(ValidationContext<T> context, TProperty value, CancellationToken cancellationToken)
    {
        return await _innerValidator.IsValidAsync(context, value, cancellationToken);
    }

    protected override async Task<string> GetErrorMessageAsync(ValidationContext<T> context, TProperty value)
    {
        return await _messageProvider(context);
    }

    public override bool IsValid(ValidationContext<T> context, TProperty value)
    {
        return _innerValidator.IsValid(context, value);
    }

    protected override string GetErrorMessage(ValidationContext<T> context, TProperty value)
    {
        throw new InvalidOperationException("请使用异步验证方法调用此规则");
    }
}

2. 编写扩展方法实现链式调用 #

通过扩展IRuleBuilderOptions，添加WithMessageAsync方法，将内置验证器替换为我们的包装器：

public static class FluentValidationAsyncExtensions
{
    public static IRuleBuilderOptions<T, TProperty> WithMessageAsync<T, TProperty>(
        this IRuleBuilderOptions<T, TProperty> rule,
        Func<ValidationContext<T>, Task<string>> messageProvider)
    {
        var innerValidator = rule.CurrentValidator as PropertyValidator<T, TProperty>;
        if (innerValidator == null)
        {
            throw new InvalidOperationException("WithMessageAsync仅适用于属性验证器");
        }

        rule.ClearValidators();
        rule.SetValidator(new AsyncMessagePropertyValidator<T, TProperty>(innerValidator, messageProvider));

        return rule;
    }

    // 重载：结合翻译服务的快捷方式
    public static IRuleBuilderOptions<T, TProperty> WithMessageAsync<T, TProperty>(
        this IRuleBuilderOptions<T, TProperty> rule,
        ITranslationService translationService,
        string translationKey,
        params object[] formatArgs)
    {
        return rule.WithMessageAsync(async context =>
        {
            var translatedText = await translationService.GetTranslationAsync(translationKey, context.CultureInfo);
            return string.Format(translatedText, formatArgs);
        });
    }
}

3. 实际使用示例 #

假设你有一个从数据库获取译文的ITranslationService，注入到验证器后即可像原生WithMessage一样链式调用：

public class UserValidator : AbstractValidator<User>
{
    private readonly ITranslationService _translationService;

    public UserValidator(ITranslationService translationService)
    {
        _translationService = translationService;

        // 基础用法：复用NotEmpty规则，异步获取翻译消息
        RuleFor(x => x.FirstName)
            .NotEmpty()
            .WithMessageAsync(_translationService, "FirstName.Empty");

        // 自定义异步逻辑：针对邮箱格式验证设置异步消息
        RuleFor(x => x.Email)
            .NotEmpty()
            .WithMessageAsync(_translationService, "Email.Empty")
            .EmailAddress()
            .WithMessageAsync(async ctx => 
                await _translationService.GetTranslationAsync("Email.InvalidFormat", ctx.CultureInfo));
    }
}

关键注意点 #

• 必须使用异步验证方法：在Controller或服务中调用await _validator.ValidateAsync(userModel)，同步验证会抛出异常（可根据需求修改包装器的同步方法逻辑）。
• 多规则独立设置：如果一个字段有多个验证规则（如邮箱的NotEmpty和EmailAddress），需为每个规则单独调用WithMessageAsync。
• 上下文支持：ValidationContext包含当前模型、文化信息等，可用于多语言翻译场景。

内容的提问来源于stack exchange，提问作者AlwaysNeedingHelp