Understanding ValidationContext in DataAnnotations

Question

I want to utilize Validator.TryValidateValue() but don't understand the mechanics. Say, i have the following:

public class User {
    [Required(AllowEmptyStrings = false)]
    [StringLength(6)]
    public string Name { get; set; }
}

and the method:

public void CreateUser(string name) {...}

My validation code is:

ValidationAttribute[] attrs = bit of reflection here to populate from User class
var ctx = new ValidationContext(name, null, null);
var errors = new List<ValidationResult>();
bool valid = Validator.TryValidateValue(name, ctx, errors, attrs);

It works fine until value of name is null. I'm getting ArgumentNullException when instantiating ValidationContext and don't understand why. TryValidateValue() also demands non-null context. I have a value and a list of attributes to validate against. What is that ValidationContext for?

Solution

The only thing that's wrong about your code is the instance object for your validation context. The instance does not need to be the value that's being validated. For Validator.ValidateProperty, yes, it does need to be the object that owns the property, but for Validator.ValidateValue, "this" is sufficient.

I wrote a validation helper class to do the setup; this lets me pass in arbitrary values from anywhere.

public class ValidationHelper
{
    private List<ValidationResult> m_validationResults = new List<ValidationResult>();
    private List<ValidationAttribute> m_validationAttributes = new List<ValidationAttribute>();

    public Tuple<bool, List<string>> ValidateDOB(DateTime? dob)
    {
        m_validationAttributes.Add(new CustomValidationAttribute(typeof(DateOfBirthValidator), "ValidateDateOfBirth"));
        bool result = Validator.TryValidateValue(dob, 
                             new ValidationContext(this, null, null), 
                             m_validationResults, 
                             m_validationAttributes);
        if (result)
        {
            return Tuple.Create(true, new List<string>());
        }
        List<string> errors = m_validationResults.Select(vr => vr.ErrorMessage).ToList();
        return Tuple.Create(false, errors);
    }
}

If you are validating properties that have the validation attributes on the property, it's a lot easier:

internal void Validate(T value)
{
    if (!m_Initializing && TrackChanges && !Entity.IsImmutable)
    {
        Validator.ValidateProperty(value, new ValidationContext(Entity, null, null) { MemberName = PropertyName ?? ModelName });
    }
}

"Entity" is a property of the current class that references the object that I want to validate. This lets me validate properties on other objects. If you're already inside the objct, "this" will again be sufficient.