FluentValidation自定义扩展调用MustAsync访问属性报CS1061错误如何解决

问题原因 #

你遇到的CS1061报错由泛型参数命名冲突导致：
你在CountryRules类的三个扩展方法声明时，额外加了和实体类同名的泛型参数<Country>，这个泛型占位符的优先级高于你定义的Country实体类，编译器会将方法内所有的Country类型识别为无约束的泛型参数，自然无法识别Id属性。Intellisense的类型提示属于误判，编译时会按实际的泛型参数逻辑处理，所以会触发报错。

解决方案 #

直接移除三个扩展方法名后的<Country>泛型声明即可，修改后的代码如下：

public static class CountryRules
{
    public static IRuleBuilderOptions<Country, string> Name(this IRuleBuilder<Country, string> ruleBuilder, IApplicationDbContext dbContext) =>
        ruleBuilder.NotNull().NotEmpty().MaximumLength(64)
            .MustAsync((country, name, cancellationToken) =>
            {
                return dbContext.Countries.AllAsync(x => x.Id == country.Id || x.Name != name, cancellationToken);
            }).WithMessage(FvConstants.UNIQUE);

    public static IRuleBuilderOptions<Country, string> Code2(this IRuleBuilder<Country, string> ruleBuilder, IApplicationDbContext dbContext) =>
        ruleBuilder.NotEmpty().Length(2).Matches("^[A-Z]{2}$").WithMessage("{PropertyName} must be two uppercase letters")
            .MustAsync((country, code2, cancellationToken) =>
            {
                return dbContext.Countries.AllAsync(x => x.Id == country.Id || x.Code2 != code2, cancellationToken);
            }).WithMessage(FvConstants.UNIQUE);

    public static IRuleBuilderOptions<Country, string> Code3(this IRuleBuilder<Country, string> ruleBuilder, IApplicationDbContext dbContext) =>
        ruleBuilder.NotEmpty().Length(3).Matches("^[A-Z]{3}$").WithMessage("{PropertyName} must be three uppercase letters")
            .MustAsync((country, code3, cancellationToken) =>
            {
                return dbContext.Countries.AllAsync(x => x.Id == country.Id || x.Code3 != code3, cancellationToken);
            }).WithMessage(FvConstants.UNIQUE);
}

内容的提问来源于stack exchange，提问作者Sam