ASP NET MVC Custom Validation by DataAnnotation

Question

I have a Model with 4 properties which are of type string  I know you can validate the length of a single property by using the StringLength annotation  However I want to validate the length of the 4 properties combined    What is the MVC way to do this with data annotation   I m asking this because I m new to MVC and want to do it the correct way before making my own solution

User · Answer

A bit late to answer  but for who is searching   You can easily do this by using an extra property with the data annotation   public string foo   get  set    public string bar   get  set      MinLength 20  ErrorMessage    too short    public string foobar         get               return foo   bar            That s all that is too it really  If you really want to display in a specific place the validation error as well  you can add this in your view     Html ValidationMessage  foobar    your combined text is too short     doing this in the view can come in handy if you want to do localization   Hope this helps

User · Answer

Background   Model validations are required for ensuring that the received data we receive is valid and correct so that we can do the further processing with this data  We can validate a model in an action method  The built-in validation attributes are Compare  Range  RegularExpression  Required  StringLength  However we may have scenarios wherein we required validation attributes other than the built-in ones   Custom Validation Attributes  public class EmployeeModel         Required       UniqueEmailAddress      public string EmailAddress  get set       public string FirstName  get set       public string LastName  get set       public int OrganizationId  get set       To create a custom validation attribute  you will have to derive this class from ValidationAttribute   public class UniqueEmailAddress   ValidationAttribute       private IEmployeeRepository  employeeRepository       Inject      public IEmployeeRepository EmployeeRepository               get   return  employeeRepository            set                        employeeRepository   value                      protected override ValidationResult IsValid object value                          ValidationContext validationContext                var model    EmployeeModel validationContext ObjectInstance          if model Field1    null               return new ValidationResult  Field1 is null                      if model Field2    null               return new ValidationResult  Field2 is null                      if model Field3    null               return new ValidationResult  Field3 is null                      return ValidationResult Success            Hope this helps  Cheers    References   Code Project - Custom Validation Attribute in ASP NET MVC3 Haacked - ASP NET MVC 2 Custom Validation

User · Answer

ExpressiveAnnotations gives you such a possibility    Required   AssertThat  Length FieldA    Length FieldB    Length FieldC    Length FieldD   gt  50    public string FieldA   get  set

User · Answer

You could write a custom validation attribute   public class CombinedMinLengthAttribute  ValidationAttribute       public CombinedMinLengthAttribute int minLength  params string   propertyNames                this PropertyNames   propertyNames          this MinLength   minLength             public string   PropertyNames   get  private set        public int MinLength   get  private set         protected override ValidationResult IsValid object value  ValidationContext validationContext                var properties   this PropertyNames Select validationContext ObjectType GetProperty           var values   properties Select p   gt  p GetValue validationContext ObjectInstance  null   OfType lt string gt             var totalLength   values Sum x   gt  x Length    Convert ToString value  Length          if  totalLength  lt  this MinLength                        return new ValidationResult this FormatErrorMessage validationContext DisplayName                      return null            and then you might have a view model and decorate one of its properties with it   public class MyViewModel        CombinedMinLength 20   Bar    Baz   ErrorMessage    The combined minimum length of the Foo  Bar and Baz properties should be longer than 20        public string Foo   get  set        public string Bar   get  set        public string Baz   get  set

User · Answer

Self validated model  Your model should implement an interface IValidatableObject  Put your validation code in Validate method   public class MyModel   IValidatableObject       public string Title   get  set        public string Description   get  set         public IEnumerable lt ValidationResult gt  Validate ValidationContext validationContext                if  Title    null              yield return new ValidationResult      new      nameof Title               if  Description    null              yield return new ValidationResult      new      nameof Description                Please notice  this is a server-side validation  It doesn t work on client-side  You validation will be performed only after form submission

User · Answer

To improve Darin s answer  it can be bit shorter   public class UniqueFileName   ValidationAttribute       private readonly NewsService  newsService   new NewsService         public override bool IsValid object value                if  value    null    return false             var file    HttpPostedFile  value           return  newsService IsFileNameUnique file FileName             Model    UniqueFileName ErrorMessage    This file name is not unique       Do note that an error message is required  otherwise the error will be empty

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