FluentValidation：C#后端输入验证框架的官方文档解读

using FluentValidation;

public class CustomerValidator: AbstractValidator<Customer> {
  public CustomerValidator() {
    RuleFor(customer => customer.Surname).NotEmpty();
    RuleFor(customer => customer.Forename).NotEmpty().WithMessage("Please specify a first name");
    RuleFor(customer => customer.Discount).NotEqual(0).When(customer => customer.HasDiscount);
    RuleFor(customer => customer.Address).Length(20, 250);
    RuleFor(customer => customer.Postcode).Must(BeAValidPostcode).WithMessage("Please specify a valid postcode");
  }

  private bool BeAValidPostcode(string postcode) {
    // custom postcode validating logic goes here
  }
}

Customer customer = new Customer();
CustomerValidator validator = new CustomerValidator();
ValidationResult results = validator.Validate(customer);

bool validationSucceeded = results.IsValid;
IList<ValidationFailure> failures = results.Errors;

    public class Customer
    {
        public int Id { get; set; }
        public string Surname { get; set; }
        public string Forename { get; set; }
        public decimal Discount { get; set; }
        public Address Address { get; set; }
        public List<string> AddressLines { get; set; } = new List<string>();
        public IList<Order> Orders { get; set; }
    }

    /// <summary>
    /// 为Customer类定义一组规则，继承自AbstractValidator<Customer>
    /// </summary>
    public class CustomerValidator : AbstractValidator<Customer>
    {
        public CustomerValidator()
        {
            //多个验证规则
            RuleFor(customer => customer.Surname).NotNull().NotEqual("foo");

            //对集合中的每一项进行验证
            RuleForEach(x => x.AddressLines).NotNull();

            //复合属性验证的重新利用
            RuleFor(customer => customer.Address).SetValidator(new AddressValidator());

            //复合的集合属性验证的重新利用
            RuleFor(x => x.Orders).SetCollectionValidator(new OrderValidator());
            //用Where方法选择性验证某些项
            RuleFor(x => x.Orders).SetCollectionValidator(new OrderValidator()).Where(x => x.Cost != null);

            //定义名为“Names”的规则集合
            RuleSet("Names", () =>
            {
                RuleFor(x => x.Surname).NotNull();
                RuleFor(x => x.Forename).NotNull();
            });
        }
    }

    public class Address
    {
        public string Line1 { get; set; }
        public string Line2 { get; set; }
        public string Town { get; set; }
        public string County { get; set; }
        public string Postcode { get; set; }
    }
    public class AddressValidator : AbstractValidator<Address>
    {
        public AddressValidator()
        {
            RuleFor(address => address.Postcode).NotNull();
            //etc
        }
    }

    public class Order
    {
        public string ProductName { get; set; }
        public decimal? Cost { get; set; }
    }
    public class OrderValidator : AbstractValidator<Order>
    {
        public OrderValidator()
        {
            RuleFor(x => x.ProductName).NotNull();
            RuleFor(x => x.Cost).GreaterThan(0);
        }
    }

        public void Validate()
        {
            Customer customer = new Customer();
            customer.Orders = new List<Order> {
                new Order { ProductName = "Foo" },
                new Order { Cost = 5 }
            };

            CustomerValidator validator = new CustomerValidator();
            ValidationResult results = validator.Validate(customer);

            //将复杂的验证定义分解到较小的片段中，可独立使用
            //把多个规则聚集都一个规则集合中
            var result1 = validator.Validate(customer, ruleSet: "Names");
            //多个规则集合
            var result2 = validator.Validate(customer, ruleSet: "Names,MyRuleSet,SomeOtherRuleSet");
            //不在任何规则集合中的default
            var result3 = validator.Validate(customer, ruleSet: "default,MyRuleSet");

            if (!results.IsValid)
            {
                foreach (var failure in results.Errors)
                {
                    Console.WriteLine("Property " + failure.PropertyName + " failed validation. Error was: " + failure.ErrorMessage);
                }
            }

            //抛出异常
            validator.ValidateAndThrow(customer);

        }

//Ensures that the specified property is not null.
RuleFor(customer => customer.Surname).NotNull();

//Ensures that the specified property is not null, an empty string or whitespace (or the default value for value types, eg 0 for int)
RuleFor(customer => customer.Surname).NotEmpty();

//Not equal to a particular value
RuleFor(customer => customer.Surname).NotEqual("Foo");
//Not equal to another property
RuleFor(customer => customer.Surname).NotEqual(customer => customer.Forename);

//Equal to a particular value
RuleFor(customer => customer.Surname).Equal("Foo");
//Equal to another property
RuleFor(customer => customer.Password).Equal(customer => customer.PasswordConfirmation);

//must be between 1 and 250 chars (inclusive)
RuleFor(customer => customer.Surname).Length(1, 250);

//Less than a particular value
RuleFor(customer => customer.CreditLimit).LessThan(100);
//Less than another property
RuleFor(customer => customer.CreditLimit).LessThan(customer => customer.MaxCreditLimit);

//Less than a particular value
RuleFor(customer => customer.CreditLimit).LessThanOrEqualTo(100);
//Less than another property
RuleFor(customer => customer.CreditLimit).LessThanOrEqualTo(customer => customer.MaxCreditLimit);

//Greater than a particular value
RuleFor(customer => customer.CreditLimit).GreaterThan(0);
//Greater than another property
RuleFor(customer => customer.CreditLimit).GreaterThan(customer => customer.MinimumCreditLimit);

//Greater than a particular value
RuleFor(customer => customer.CreditLimit).GreaterThanOrEqualTo(1);
//Greater than another property
RuleFor(customer => customer.CreditLimit).GreaterThanOrEqualTo(customer => customer.MinimumCreditLimit);

//Passes the value of the specified property into a delegate that can perform custom validation logic on the value
//Also known as "Must"
RuleFor(customer => customer.Surname).Must(surname => surname == "Foo");
//Note that there is an additional overload for Must that also accepts an instance of the parent object being validated. 
//This can be useful if you want to compare the current property with another property from inside the predicate:
//Note that in this particular example, it would be better to use the cross-property version of NotEqual
RuleFor(customer => customer.Surname).Must((customer, surname) => surname != customer.Forename);

//Ensures that the value of the specified property matches the given regular expression. Example:
RuleFor(customer => customer.Surname).Matches("some regex here");

//Ensures that the value of the specified property is a valid email address format. Example:
RuleFor(customer => customer.Email).EmailAddress();

RuleFor(customer => customer.Surname).NotNull().WithMessage("Please ensure that you have entered your Surname");
//‘{PropertyName}‘ will be replaced with the name of the property being validated
//and the value ‘Surname‘ will be inserted.
RuleFor(customer => customer.Surname).NotNull().WithMessage("Please ensure you have entered your {PropertyName}");

//Using static values in a custom message:
RuleFor(customer => customer.Surname).NotNull()
    .WithMessage(customer => string.Format("This message references some constant values: {0} {1}", "hello", 5));
//Result would be "This message references some constant values: hello 5"

//Referencing other property values:
RuleFor(customer => customer.Surname).NotNull()
    .WithMessage(customer => $"This message references some other properties: Forename: {customer.Forename} Discount: {customer.Discount}");
//Result would be: "This message references some other properties: Forename: Jeremy Discount: 100"

//The default error message would be ‘Surname‘ must not be empty. 
RuleFor(customer => customer.Surname).NotNull();

//Replace just the property name by calling WithName
//Now the error message would be ‘Last name‘ must not be empty.
RuleFor(customer => customer.Surname).NotNull().WithName("Last name");

//Completely rename the property, including the Errors collection on the ValidationResult
RuleFor(customer => customer.Surname).NotNull().OverridePropertyName("Last name");

//Property name resolution is also pluggable.By default, the name of the property extracted from the MemberExpression passed to RuleFor. If you want change this logic, you can set the DisplayNameResolver property on the ValidatorOptions class.
//这段不是太懂
ValidatorOptions.DisplayNameResolver = (type, member) => {
    if (member != null)
    {
        return member.Name + "Foo";
    }
    return null;
};

//另外，FluentValidation 可采用 DisplayName and Display attributes，来生成错误信息中的属性名，同 WithName
public class Person
{
    [Display(Name = "Last name")]
    public string Surname { get; set; }
}

//当满足条件时才执行验证规则（Unless则相反）
RuleFor(customer => customer.CustomerDiscount).GreaterThan(0).When(customer => customer.IsPreferredCustomer);

//同时应用到多条规则
When(customer => customer.IsPreferred, () =>
{
    RuleFor(customer => customer.CustomerDiscount).GreaterThan(0);
    RuleFor(customer => customer.CreditCardNumber).NotNull();
});

//默认是第一个验证失败后，继续执行第二个验证
RuleFor(x => x.Surname).NotNull().NotEqual("foo");

//第一个验证失败后停止，不再验证之后的规则
RuleFor(x => x.Surname).Cascade(CascadeMode.StopOnFirstFailure).NotNull().NotEqual("foo");

//在应用程序启动入口设置全局级联模式
//这会被具体的验证器类和具体的验证规则重写
ValidatorOptions.CascadeMode = CascadeMode.StopOnFirstFailure;

//在单个验证器类中设置级联模式
public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        // First set the cascade mode
        CascadeMode = CascadeMode.StopOnFirstFailure;

        //Rule definitions follow
        //RuleFor(...)
        //RuleFor(...)
    }
}

public class Person
{
    public IList<Pet> Pets { get; set; } = new List<Pet>();
}

public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        RuleFor(x => x.Pets).Must(list => list.Count <= 10).WithMessage("The list must contain fewer than 10 items");
    }
}

//封装成扩展方法，使其可重用
public static class MyCustomValidators
{
    public static IRuleBuilderOptions<T, IList<TElement>> ListMustContainFewerThan<T, TElement>(this IRuleBuilder<T, IList<TElement>> ruleBuilder, int num)
    {
        return ruleBuilder.Must(list => list.Count < num).WithMessage("The list contains too many items");
    }
}

RuleFor(x => x.Pets).ListMustContainFewerThan(10);

//The resulting message will now be ‘Pets‘ must contain fewer than 10 items.
public static IRuleBuilderOptions<T, IList<TElement>> ListMustContainFewerThan<T, TElement>(this IRuleBuilder<T, IList<TElement>> ruleBuilder, int num)
{
    return ruleBuilder.Must((rootObject, list, context) =>
    {
        context.MessageFormatter.AppendArgument("MaxElements", num);
        return list.Count < num;
    })
    .WithMessage("{PropertyName} must contain fewer than {MaxElements} items.");
}

public static IRuleBuilderOptions<T, IList<TElement>> ListMustContainFewerThan<T, TElement>(this IRuleBuilder<T, IList<TElement>> ruleBuilder, int num)
{
    return ruleBuilder.Must((rootObject, list, context) =>
    {
        context.MessageFormatter
          .AppendArgument("MaxElements", num)
          .AppendArgument("TotalElements", list.Count);

        return list.Count < num;
    })
    .WithMessage("{PropertyName} must contain fewer than {MaxElements} items. The list contains {TotalElements} element");
}

//This method allows you to manually create the ValidationFailure instance associated with the validation error. 
RuleFor(x => x.Pets).Custom((list, context) =>
{
    if (list.Count > 10)
    {
        context.AddFailure("The list must contain 10 items or fewer");
        // It allows you to return multiple errors for the same rule 
        context.AddFailure("SomeOtherProperty", "The list must contain 10 items or fewer");
        // Or you can instantiate the ValidationFailure directly:
        context.AddFailure(new ValidationFailure("SomeOtherProperty", "The list must contain 10 items or fewer");
    }
});

//若定制的逻辑非常复杂，则可将定制逻辑放入单独的验证类中
public class ListCountValidator<T> : PropertyValidator
{
    private int _max;

    public ListCountValidator(int max)
        : base("{PropertyName} must contain fewer than {MaxElements} items.")
    {
        _max = max;
    }

    protected override bool IsValid(PropertyValidatorContext context)
    {
        var list = context.PropertyValue as IList<T>;

        if (list != null && list.Count >= _max)
        {
            context.MessageFormatter.AppendArgument("MaxElements", _max);
            return false;
        }

        return true;
    }
}

//调用方法一
RuleFor(person => person.Pets).SetValidator(new ListCountValidator<Pet>(10));


//调用方法二
public static IRuleBuilderOptions<T, IList<TElement>> ListMustContainFewerThan3<T, TElement>(this IRuleBuilder<T, IList<TElement>> ruleBuilder, int num)
{
    return ruleBuilder.SetValidator(new ListCountValidator<Pet>(10));
}

RuleFor(x => x.Pets).ListMustContainFewerThan(10);

//从7.0版本开始弃用，推荐上文中使用的 RuleFor(x => x).Custom((x, context) => ... )
public class PersonValidator : AbstractValidator<Person>
{
    public PersonValidator()
    {
        Custom(person => {
            return person.Pets.Count >= 10
               ? new ValidationFailure("Pets", "More than 9 pets is not allowed.")
               : null;
        });
    }
}