Question

Is it possible to have unobtrusive validation to make a field required but only if other properties change?

For Example

[Required]
public Decimal Income {get; set;}
[Required]
public Decimal Tax {get; set;}
//Required if tax or income changes
public string ChangeReason {get; set;}

I thought about having multiple backing store fields and writing a Custom Validator to compare these, but wondered if anyone had a better suggestion?

Answer 1

Custom Validator is the way to go. I had to build something similar a while back.

I’d set up a hidden value – “Changed” – set it to true whenever the user mods the fields of interest.

Set a RequiredIf validator on the 2 properties of interest:

 [RequiredIf("Changed", true, ErrorMessage = "Required")]

The code for a RequiredIf validator is shown below:

public class RequiredIfAttribute : ValidationAttribute, IClientValidatable
{
    private RequiredAttribute _innerAttribute = new RequiredAttribute();

    public string DependentProperty { get; set; }
    public object TargetValue { get; set; }

    public RequiredIfAttribute(string dependentProperty, object targetValue)
    {
        this.DependentProperty = dependentProperty;
        this.TargetValue = targetValue;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        // get a reference to the property this validation depends upon
        var containerType = validationContext.ObjectInstance.GetType();
        var field = containerType.GetProperty(this.DependentProperty);

        if (field != null)
        {
            // get the value of the dependent property
            var dependentvalue = field.GetValue(validationContext.ObjectInstance, null);

            // compare the value against the target value
            if ((dependentvalue == null && this.TargetValue == null) ||
                (dependentvalue != null && dependentvalue.Equals(this.TargetValue)))
            {
                // match => means we should try validating this field
                if (!_innerAttribute.IsValid(value))
                    // validation failed - return an error
                    return new ValidationResult(this.ErrorMessage, new[] { validationContext.MemberName });
            }
        }

        return null;
    }

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

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

        // find the value on the control we depend on;
        // if it's a bool, format it javascript style 
        // (the default is True or False!)
        string targetValue = (this.TargetValue ?? "").ToString();
        if (this.TargetValue.GetType() == typeof(bool))
            targetValue = targetValue.ToLower();

        rule.ValidationParameters.Add("dependentproperty", depProp);
        rule.ValidationParameters.Add("targetvalue", targetValue);

        yield return rule;
    }

    private string BuildDependentPropertyId(ModelMetadata metadata, ViewContext viewContext)
    {
        // build the ID of the property
        string depProp = viewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(this.DependentProperty);
        // unfortunately this will have the name of the current field appended to the beginning,
        // because the TemplateInfo's context has had this fieldname appended to it. Instead, we
        // want to get the context as though it was one level higher (i.e. outside the current property,
        // which is the containing object (our Person), and hence the same level as the dependent property.
        var thisField = metadata.PropertyName + "_";
        if (depProp.StartsWith(thisField))
            // strip it off again
            depProp = depProp.Substring(thisField.Length);
        return depProp;
    }
}

Javascript:

/// <reference path="jquery-1.4.4-vsdoc.js" />
/// <reference path="jquery.validate.unobtrusive.js" />

$.validator.addMethod('requiredif',
function (value, element, parameters) {
    var id = '#' + parameters['dependentproperty'];

    // get the target value (as a string, 
    // as that's what actual value will be)
    var targetvalue = parameters['targetvalue'];
    targetvalue =
      (targetvalue == null ? '' : targetvalue).toString();

    // get the actual value of the target control
    // note - this probably needs to cater for more 
    // control types, e.g. radios
    var control = $(id);
    var controltype = control.attr('type');
    var actualvalue =
        controltype === 'checkbox' ?
        control.attr('checked').toString() :
        control.val();

    // if the condition is true, reuse the existing 
    // required field validator functionality
    if (targetvalue === actualvalue)
        return $.validator.methods.required.call(
          this, value, element, parameters);

    return true;
 }
 );

 $.validator.unobtrusive.adapters.add(
'requiredif',
['dependentproperty', 'targetvalue'],
function (options) {
    options.rules['requiredif'] = {
        dependentproperty: options.params['dependentproperty'],
        targetvalue: options.params['targetvalue']
    };
    options.messages['requiredif'] = options.message;
});