MVC2: Validation (Data Annotations) with two field dependency

Question

Example: We have a conditional field. This conditional field is a radio button with the following two values “yes” and “no”. Lets say the name of this radiobutton is “AAA”.

This conditional field “AAA” should only be displayed when another radio button field “BBB” is set to “yes”. (Values of radio button “BBB” are also “yes” and no”) .

But the conditional field “AAA” should be displayed with NO pre-set value, means “yes” nor “no” should be set when the field is first displayed.

The problem occurs based on the requirement that the conditional field “AAA” should ONLY be required when (the non-conditional) field “BBB” is set to “yes” – and not required when the field “BBB” is set to “no”.

(Sounds, that I didn’t heard anything about an if statement, or? But hold on and continue reading ...)

Believe me, it would not be a problem for me to solve this topic when we would use the “Modelstate” – but we are talking here about Validation (Data Annotations) that looks like this here:

public class Input1FormModel 
{
     [Required(ErrorMessageResourceName="Error_Field_AAA_Empty",
               ErrorMessageResourceType=typeof(Resources.MyDialog))]
     public int AAA { get; set; }
}

I fully understand ALSO these lines of code - I believe ;-)

...

//property limits
public int UpperBound { get { return DateTime.Now.Year; } }
public int LowerBound { get { return 1900; } }

...

[NotNullValidator]
[PropertyComparisonValidator("LowerBound", ComparisonOperator.GreaterThan)]
[PropertyComparisonValidator("UpperBound", ComparisonOperator.LessThanEqual)]
public int? XYZ { get; set; }

But, how to solve the above described dependency (AAA <-> BBB)?

Changing “return DateTime.Now.Year;” to a function call which checks first the other field and returns then true or false? But how to fetch there the value of the other field?

Answer 1

You can do this using data annotations but the annotation needs to be operating on the class level and not on the property level as validationattributes are for single properties.

Here is an example I created because post code is optional and state not required if people have said they're in New Zealand, but it is compulsory in Australia. This composite validation with take the whole model as the input value and use reflection to compare the values of the property names passed in from the data annotation.

    [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public class NZPostcodeAttribute : ValidationAttribute
{
    public const string _defaultErrorMessage = "Postcode and State are required for Australian residents";
    private readonly object _typeId = new object();

    public NZPostcodeAttribute(string countryProperty, string postcodeProperty, string stateProperty)
    {
        CountryProperty = countryProperty;
        PostcodeProperty = postcodeProperty;
        StateProperty = stateProperty;
    }

    public string CountryProperty { get; private set; }
    public string StateProperty { get; private set; }
    public string PostcodeProperty { get; private set; }

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

    public override string FormatErrorMessage(string name)
    {
        return _defaultErrorMessage;
    }

    public override bool IsValid(object value)
    {
        PropertyDescriptorCollection props = TypeDescriptor.GetProperties(value);
        object countryValue = props.Find(CountryProperty, true).GetValue(value);
        object postcodeValue = props.Find(PostcodeProperty, true).GetValue(value);
        object stateValue = props.Find(StateProperty, true).GetValue(value);

        string countryString = countryValue == null ? "" : countryValue.ToString();
        string postcodeString = postcodeValue == null ? "" : postcodeValue.ToString();
        string stateString = stateValue == null ? "" : stateValue.ToString();


        bool isValid = true;

        if (countryString.ToString().ToLower() == "australia")
        {
            if (String.IsNullOrEmpty(postcodeString) || String.IsNullOrEmpty(stateString))
            {
                isValid = false;
            }
        }
        if (!String.IsNullOrEmpty(postcodeString))
        {
            string isNumeric = "^[0-9]+";
            if (!Regex.IsMatch(postcodeString, isNumeric))
                isValid = false;
        }
        return isValid;
    }
}

When you want to apply this to your model, it needs to be done on a class level on the model (see the flag AttributeTargets.Class at the top).

Do it as follows:

[NZPostcode("Country", "Postcode", "State")]
public class UserRegistrationModel 
{....

You need to point the validation attribute to the property names. It is also possible to add client side validation to this as well, but that would be a whole article on its own.

You can easily adapt the above to your scenario.

Answer 2

You may need to use IDataErrorInfo.

See this question, where I answered this:

Check out IDataErrorInfo and this question I asked about IDataErrorInfo vs. DataAnnotations.