How to validate one field related to another's value in ASP .NET MVC 3

Question

I had two fields some thing like phone number and mobile number. Some thing like..

    [Required]
    public string Phone { get; set; }

    [Required]
    p         


        

Answer 1

One possibility is to write a custom validation attribute:

public class RequiredIfOtherFieldIsNullAttribute : ValidationAttribute, IClientValidatable
{
    private readonly string _otherProperty;
    public RequiredIfOtherFieldIsNullAttribute(string otherProperty)
    {
        _otherProperty = otherProperty;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var property = validationContext.ObjectType.GetProperty(_otherProperty);
        if (property == null)
        {
            return new ValidationResult(string.Format(
                CultureInfo.CurrentCulture, 
                "Unknown property {0}", 
                new[] { _otherProperty }
            ));
        }
        var otherPropertyValue = property.GetValue(validationContext.ObjectInstance, null);

        if (otherPropertyValue == null || otherPropertyValue as string == string.Empty)
        {
            if (value == null || value as string == string.Empty)
            {
                return new ValidationResult(string.Format(
                    CultureInfo.CurrentCulture,
                    FormatErrorMessage(validationContext.DisplayName),
                    new[] { _otherProperty }
                ));
            }
        }

        return null;
    }

    public IEnumerable GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = FormatErrorMessage(metadata.GetDisplayName()),
            ValidationType = "requiredif",
        };
        rule.ValidationParameters.Add("other", _otherProperty);
        yield return rule;
    }
}

which you would apply to one of the properties of your view model:

public class MyViewModel
{
    [RequiredIfOtherFieldIsNull("Mobile")]
    public string Phone { get; set; }

    public string Mobile { get; set; }
}

then you could have a controller:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        return View(new MyViewModel());
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        return View(model);
    }
}

and finally a view in which you will register an adapter to wire the client side validation for this custom rule:

@model MyViewModel





@using (Html.BeginForm())
{
    

        @Html.LabelFor(x => x.Phone)
        @Html.EditorFor(x => x.Phone)
        @Html.ValidationMessageFor(x => x.Phone)
    

    

        @Html.LabelFor(x => x.Mobile)
        @Html.EditorFor(x => x.Mobile)
        @Html.ValidationMessageFor(x => x.Mobile)
    

    OK
}

Pretty sick stuff for something so extremely easy as validation rule that we encounter in our everyday lives. I don't know what the designers of ASP.NET MVC have been thinking when they decided to pick a declarative approach for validation instead of imperative.

Anyway, that's why I use FluentValidation.NET instead of data annotations to perform validations on my models. Implementing such simple validation scenarios is implemented in a way that it should be - simple.