Asp.net-mvc – Validate object based on external factors (ie. data store uniqueness)

asp.net-mvcdata-annotationsvalidationxval

Description

My solution has these projects:

DAL = Modified Entity Framework
DTO = Data Transfer objects that are able to validate themselves
BL = Business Layer Services
WEB = presentation Asp.net MVC application

DAL, BL and WEB all reference DTO which is great.
The process usually executes this way:

A web request is made to the WEB
WEB gets DTOs posted
- DTOs get automagically validated via custom ActionFilter
- validation errors are auto-collected
(Validation is OK) WEB calls into BL providing DTOs
BL calls into DAL by using DTOs (can either pass them through or just use them)

DTO Validation problem then…

My DTOs are able to validate themselves based on their own state (properties' values). But right now I'm presented with a problem when this is not the case. I need them to validate using BL (and consequently DAL).

My real-life example: User registers and WEB gets a User DTO that gets validated. The problematic part is username validation. Its uniqueness should be checked against data store.
How am I supposed to do this?

There's additional info that all DTOs implement an interface (ie. User DTO implements IUser) for IoC purposes and TDD. Both are part of the DTO project.

Impossible tries

I can't reference BL in DTO because I'll get circular reference.
Compilation error
I can't create an additional DTO.Val project that would reference partial DTO classes and implement their validation there (they'd reference BL + DTO).
Partial classes can't span assemblies.

Possible tries

Create a special ActionFilter that would validate object against external conditions. This one would be created within WEB project thus seeing DTO and BL that would be used here.
Put DTOs in BL and keep DTO interfaces as actual DTOs referenced by other projects and refactor all code to use interfaces instead of concrete classes.
Don't handle external dependant validation and let external dependencies throw an exception – probably the worst solution to this issue

What would you suggest?

Best Solution

I would suggest an experiment that i have only been trialling for the last week or so.

Based on this inspiration i am creating DTOs that validate a little differently to that of the DataAnnotations approach. Sample DTO:

public class Contact : DomainBase, IModelObject
{
    public int ID { get; set; }
    public string Name { get; set; }
    public LazyList<ContactDetail> Details { get; set; }
    public DateTime Updated { get; set; }


    protected override void ConfigureRules()
    {
        base.AddRule(new ValidationRule()
        {
            Properties = new string[] { "name" },
            Description = "A Name is required but must not exceed 300 characters in length and some special characters are not allowed",
            validator = () => this.Name.IsRequired300LenNoSpecial()
        });

        base.AddRule(new ValidationRule()
        {
            Properties = new string[] { "updated" },
            Description = "required",
            validator = () => this.Updated.IsRequired()
        });
    }
}

This might look more work than DataAnnotations and well, that's coz it is, but it's not huge. I think it's more presentable in the class (i have some really ugly DTO classes now with DataAnnotations attributes - you can't even see the properties any more). And the power of anonymous delegates in this application is almost book-worthy (so i'm discovering).

Base class:

public partial class DomainBase : IDataErrorInfo
{
    private IList<ValidationRule> _rules = new List<ValidationRule>();

    public DomainBase()
    {
        // populate the _rules collection
        this.ConfigureRules();
    }

    protected virtual void ConfigureRules()
    {
        // no rules if not overridden
    }

    protected void AddRule(ValidationRule rule)
    {
        this._rules.Add(rule);
    }





    #region IDataErrorInfo Members

    public string Error
    {
        get { return String.Empty; }    // Validation should call the indexer so return "" here
    }                                   // ..we dont need to support this property.

    public string this[string columnName]
    {
        get
        {
            // get all the rules that apply to the property being validated
            var rulesThatApply = this._rules
                .Where(r => r.Properties.Contains(columnName));

            // get a list of error messages from the rules
            StringBuilder errorMessages = new StringBuilder();
            foreach (ValidationRule rule in rulesThatApply)
                if (!rule.validator.Invoke())   // if validator returns false then the rule is broken
                    if (errorMessages.ToString() == String.Empty)
                        errorMessages.Append(rule.Description);
                    else
                        errorMessages.AppendFormat("\r\n{0}", rule.Description);

            return errorMessages.ToString();
        }
    }

    #endregion
}

ValidationRule and my validation functions:

public class ValidationRule
{
    public string[] Properties { get; set; }
    public string Description { get; set; }
    public Func<bool> validator { get; set; }
}


/// <summary>
/// These extention methods return true if the validation condition is met.
/// </summary>
public static class ValidationFunctions
{
    #region IsRequired

    public static bool IsRequired(this String str)
    {
        return !str.IsNullOrTrimEmpty();
    }

    public static bool IsRequired(this int num)
    {
        return num != 0;
    }

    public static bool IsRequired(this long num)
    {
        return num != 0;
    }

    public static bool IsRequired(this double num)
    {
        return num != 0;
    }

    public static bool IsRequired(this Decimal num)
    {
        return num != 0;
    }

    public static bool IsRequired(this DateTime date)
    {
        return date != DateTime.MinValue;
    }

    #endregion


    #region String Lengths

    public static bool IsLengthLessThanOrEqual(this String str, int length)
    {
        return str.Length <= length;
    }

    public static bool IsRequiredWithLengthLessThanOrEqual(this String str, int length)
    {
        return !str.IsNullOrTrimEmpty() && (str.Length <= length);
    }

    public static bool IsRequired300LenNoSpecial(this String str)
    {
        return !str.IsNullOrTrimEmpty() &&
            str.RegexMatch(@"^[- \r\n\\\.!:*,@$%&""?\(\)\w']{1,300}$",
                RegexOptions.Multiline) == str;
    }

    #endregion

}

If my code looks messy well that's because i've only been working on this validation approach for the last few days. I need this idea to meet a few requirements:

I need to support the IDataErrorInfo interface so my MVC layer validates automatically
I need to be able to support complex validation scenarios (the whole point of your question i guess): I want to be able to validate against multiple properties on the same object (ie. StartDate and FinishDate); properties from different/multiple/associated objects like i would have in an object graph; and even other things i haven't thought of yet.
I need to support the idea of an error applying to more than one property
As part of my TDD and DDD journey i want my Domain Objects to describe more my 'domain' than my Service layer methods, so putting these complex conditions in the model objects (not DTOs) seems to achieve this

This approach i think will get me what i want, and maybe you as well.

I'd imagine if you jump on board with me on this that we'd be pretty 'by ourselves' but it might be worth it. I was reading about the new validation capabilities in MVC 2 but it still doesn't meet the above wish list without custom modification.

Hope this helps.

Related Solutions

C# – Data type validation with xVal

What I ended up doing was creating some DTOs that represent the forms. These DTOs would take a Request.Form in and automatically map all the form values into the internal properties (ex public string email, public string firstname) based on them being the same name as the form values.

They would have all string properties and I'd put the xVal attributes on them. I'd then use xVal and regular expressions to make sure the data coming in was valid (ex a valid date, email, number, etc). This way there would never be an exception thrown because it was always going into a string, as opposed to .Net trying to parse it into a date or something.

This would make sure that the data always made it to xVal where I could run the validation I want, and then convert it to the proper type like DateTime once I know I have valid data.

Custom model validation of dependent properties using Data Annotations

MVC2 comes with a sample "PropertiesMustMatchAttribute" that shows how to get DataAnnotations to work for you and it should work in both .NET 3.5 and .NET 4.0. That sample code looks like this:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public sealed class PropertiesMustMatchAttribute : ValidationAttribute
{
    private const string _defaultErrorMessage = "'{0}' and '{1}' do not match.";

    private readonly object _typeId = new object();

    public PropertiesMustMatchAttribute(string originalProperty, string confirmProperty)
        : base(_defaultErrorMessage)
    {
        OriginalProperty = originalProperty;
        ConfirmProperty = confirmProperty;
    }

    public string ConfirmProperty
    {
        get;
        private set;
    }

    public string OriginalProperty
    {
        get;
        private set;
    }

    public override object TypeId
    {
        get
        {
            return _typeId;
        }
    }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString,
            OriginalProperty, ConfirmProperty);
    }

    public override bool IsValid(object value)
    {
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(value);
        object originalValue = properties.Find(OriginalProperty, true /* ignoreCase */).GetValue(value);
        object confirmValue = properties.Find(ConfirmProperty, true /* ignoreCase */).GetValue(value);
        return Object.Equals(originalValue, confirmValue);
    }
}

When you use that attribute, rather than put it on a property of your model class, you put it on the class itself:

[PropertiesMustMatch("NewPassword", "ConfirmPassword", ErrorMessage = "The new password and confirmation password do not match.")]
public class ChangePasswordModel
{
    public string NewPassword { get; set; }
    public string ConfirmPassword { get; set; }
}

When "IsValid" gets called on your custom attribute, the whole model instance is passed to it so you can get the dependent property values that way. You could easily follow this pattern to create a date comparison attribute, or even a more general comparison attribute.

Brad Wilson has a good example on his blog showing how to add the client-side portion of the validation as well, though I'm not sure if that example will work in both .NET 3.5 and .NET 4.0.