Question

I found a RequiredIfAttribute on the internet which I modified to RequiredNotIf. The attribute can be used like this.

[RequiredNotIf("LastName", null, ErrorMessage = "You must fill this.")]
public string FirstName { get; set; }

[RequiredNotIf("FirstName", null, ErrorMessage = "You must fill this")]
public string LastName { get; set; }

And the source code to the attribute…

[AttributeUsageAttribute(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = true)]
public class RequiredNotIfAttribute : RequiredAttribute, IClientValidatable
{
    private string OtherProperty { get; set; }
    private object Condition { get; set; }

    public RequiredNotIfAttribute(string otherProperty, object condition)
    {
        OtherProperty = otherProperty;
        Condition = condition;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(OtherProperty);
        if (property == null)
        {
            return new ValidationResult(String.Format("Property {0} not found.", OtherProperty));
        }

        var propertyValue = property.GetValue(validationContext.ObjectInstance, null);
        var conditionIsMet = !Equals(propertyValue, Condition);
        return conditionIsMet ? base.IsValid(value, validationContext) : null;
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = FormatErrorMessage(metadata.GetDisplayName()),
            ValidationType = "requiredif",
        };

        var depProp = BuildDependentPropertyId(metadata, context as ViewContext);

        var targetValue = (Condition ?? "").ToString();
        if (Condition != null && Condition is bool)
        {
            targetValue = targetValue.ToLower();
        }

        rule.ValidationParameters.Add("otherproperty", depProp);
        rule.ValidationParameters.Add("condition", targetValue);

        yield return rule;
    }

    private string BuildDependentPropertyId(ModelMetadata metadata, ViewContext viewContext)
    {
        var depProp = viewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(OtherProperty);
        var thisField = metadata.PropertyName + "_";
        if (depProp.StartsWith(thisField))
        {
            depProp = depProp.Substring(thisField.Length);
        }
        return depProp;
    }
}

The drawback with this – as I see it – is the magic string in the attribute “header”. How can I get rid of it?

Answer 1

You can’t get rid of it because attributes are metadata and values must be known at compile time. If you want to do more advanced validation without magic strings I would very strongly recommend you FluentValidation.NET. Performing validation with attributes in a declarative manner is very limiting IMHO. Just look at the quantity of source code you have to write for something as standard and easy as RequiredIf or RequiredNotIf. I don’t know what the designers of the framework were thinking when they choose Data Annotations for validation. It’s just ridiculous. Maybe in the future they will enrich it and allow for more complex scenarios but until then I stick with FV.