Is there a way to reuse data annotations?

Question

Is there a way to implement the idea of a data domains (at a property level) inside of a class that is used as a model in a view in ASP.Net MVC 4?

Consider this code:

public class LoginProfileModel {

    [DisplayName("Login ID")]
    [Required(ErrorMessage = "Login ID is required.")]
    public string LogonID { get; set; }

    [DisplayName("Password")]
    [Required(ErrorMessage = "Password cannot be blank.")]
    [StringLength(20, MinimumLength = 3)]
    [DataType(DataType.Password)]
    public string Password { get; set; }
}

Here is a LoginProfileModel in for ASP.Net MVC 4. It uses a variety of metadata/data annotations so that I can create a clean view with this code:

@model myWebSite.Areas.People.Models.LoginProfileModel

@using ( Html.BeginForm( "Index" , "Login" ) ) {
    @Html.ValidationSummary()
    @Html.EditorForModel()
    <input type="submit" value="Login" />
}

I use the idea of a "Login ID" and a "Password" in more than one view, and therefore, in more than one view model. I want to be able to define the attributes that a Password uses, or possibly the Password itself with all of it's data annotations in a single location so that I can reuse all those definitions where needed, rather than respecifying them every time they are used:

    [DisplayName("Password")]
    [Required(ErrorMessage = "Password cannot be blank.")]
    [StringLength(20, MinimumLength = 3)]
    [DataType(DataType.Password)]
    public string Password { get; set; }

Is that possible some way?

Answer 1

You can do this using a buddy class, which provides metadata for your view model. Like so:

public partial class LogonMetaData
{
    [DisplayName("Login ID")]
    [Required(ErrorMessage = "Login ID is required.")]
    public string LogonID { get; set; }

    [DisplayName("Password")]
    [Required(ErrorMessage = "Password cannot be blank.")]
    [StringLength(20, MinimumLength = 3)]
    [DataType(DataType.Password)]
    public string Password { get; set; }
}

Then your view models:

using System.ComponentModel.DataAnnotations;

[MetadataType(typeof(LogonMetaData))]
public partial class FirstViewModel
{
    public string LogonID { get; set; }
    public string Password { get; set; }
}

using System.ComponentModel.DataAnnotations;

[MetadataType(typeof(LogonMetaData))]
public partial class SecondViewModel
{
    public string LogonID { get; set; }
    public string Password { get; set; }
}

Note the use of partial in the class definitions. This is what allows this approach to work. One caveat, other than the obvious problem with DRY, is that I believe the metadata class has to reside in the same namespace as your view models, else it complains. Other than that, this should do what you want.

Answer 2

The following attributes affect the validation process of your View.

[Required(ErrorMessage = "Password cannot be blank.")]
[StringLength(20, MinimumLength = 3)]

For the Validation attributes, you can create a class like this:

public class PasswordRuleAttribute : ValidationAttribute
    {    
        public override bool IsValid(object value)
        {

            if (new RequiredAttribute { ErrorMessage = "Password cannot be blank." }.IsValid(value) && new StringLengthAttribute(20) { MinimumLength=3 }.IsValid(value) )
                return true;

            return false;
        }
    }

You can use it as follows:

[PasswordRule]
public string Password{get;set;}

The other two attributes that you mentioned are Directly derived from the Attribute class, and I don't think there's a way to consolidate them into a single attribute.

I'll update you with an edit soon.

So now we're left with:

[DisplayName("Password")]
[DataType(DataType.Password)]
[PasswordRule]
public string Password{get;set;}

EDIT:

According to this post: Composite Attribute, it's not possible to merge attributes.

Answer 3

As a corollary to John H's answer, you could just use inheritance and make those view models that have the "idea of a LogonId and password" inherit from that base view model. This would solve the MetaData issues that were mentioned in the previous answer.

      public class LoginProfileModel {

        [DisplayName("Login ID")]
        [Required(ErrorMessage = "Login ID is required.")]
        public string LogonID { get; set; }

        [DisplayName("Password")]
        [Required(ErrorMessage = "Password cannot be blank.")]
        [StringLength(20, MinimumLength = 3)]
        [DataType(DataType.Password)]
        public string Password { get; set; }
    }

    public SomeOtherClassThatNeedsLoginInfo : LoginProfileModel{

         public string Property {get;set;}
}

Now in the SomeOtherClassThatNeedsLoginInfo, those properties and their related DataAnnotations will be available to you.

Another idea would be just to pass that LoginInfo as a property on your other view models.

public SomeOtherClassThatNeedsLoginInfo{

             public string Property {get;set;}

             public LoginProfileModel LoginModel {get;set;}
    }