How can I use the Data Validation Attributes in C# in a non-ASP.net context?

Question

I'd like to use the data validation attributes in a library assembly, so that any consumer of the data can validate it without using a ModelBinder (in a console application, for instance). How can I do it?

Answer 1

Actually this is pretty cool. I used it in a WFP validation implementation recently. Most people end up writing lots of code using reflection to iterate the attributes, but there's a built in function for this.

var vc = new ValidationContext(myObject, null, null);
return Validator.TryValidateObject(myObject, vc, null, true);

You can also validate attributes on a single named property. You can also optionally pass in a list in order to access the error messages :

var results = new List<ValidationResult>();
var vc = new ValidationContext(myObject, null, null) { MemberName = "UserName"};
var isValid = Validator.TryValidateProperty(value, vc, results);

// get all the errors
var errors = Array.ConvertAll(results.ToArray(), o => o.ErrorMessage);

Answer 2

The System.ComponentModel.DataAnnotations.ValidationAttribute classes have IsValid methods that perform the validation logic. They take an Object (the value of the field they decorate) and return true or false.

You can use these attributes, and a little reflection, to roll your own aspect-oriented validator. Pass your validator an object, and the validator will get a list of PropertyInfos and FieldInfos. For each of these, it can call GetCustomAttributes to look for those that inherit from ValidationAttribute, and for each of these, call IsValid, passing the value of the property or field. This can be done totally dynamically without knowing the structure of the class to be validated at design-time.

Answer 3

TryValidateProperty is just badly written - you have to jump through hoops to get it to work outside a Controller, and even then, if you use it twice, it will end up quietly setting ModelState to valid/invalid and stop alterring that state, and stop returning accurate results from then on.

I gave up on it and just wrote my own validator. This'll loop over any set of objects in any context and tell you if they're valid:

bool isValid = true;
var invalidFields = new List<string>();

foreach (var o in viewModels)
{
    var properties = o.GetType()
        .GetProperties(BindingFlags.Public | BindingFlags.Instance);
    foreach(var prop in properties)
    {
        var attrs = prop.GetCustomAttributes(true);
        if (attrs != null)
        {
            var val = prop.GetValue(o);
            ValidationAttribute[] validatorAttrs = attrs
                .Where(a => a is ValidationAttribute)
                .Select(a => (ValidationAttribute)a).ToArray();

            foreach(var attr in validatorAttrs)
            {                       
                bool thisFieldIsValid = attr.IsValid(val);
                if (!thisFieldIsValid)
                    invalidFields.Add(prop.Name);

                isValid = isValid && thisFieldIsValid;
            }
        }
    }
}