I am going to expand on last week’s Simple Custom Validation Attributes with custom validation attributes that need to look at more than a single property of a model. Again using the contact application example, lets say that if a contact has a country of USA then state and zip code are required.

As before we have class which inherits from ValidationAttribute. IsValid is still overridden, but it is a different overload than the previous example that provides access to the ValidationContext, which in our case is the contact model that is being validated.

using System;
using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;

public class ValidCountryRequired : ValidationAttribute
{
    private readonly string _countryProperty;
    private readonly string[] _requriedByCountries;

    public ValidCountryRequired(string countryProperty,
                                string[] requriedByCountries)
    {
        _countryProperty = countryProperty;
        _requriedByCountries = requriedByCountries;
    }

    protected override ValidationResult IsValid(object value,
                                                ValidationContext context)
    {
        PropertyInfo propertyInfo;
        propertyInfo = context.ObjectType.GetProperty(_countryProperty);
        if (propertyInfo == null)
        {
            return new ValidationResult(_countryProperty +
                                        " does not exist");
        }

        var country = propertyInfo.GetValue(context.ObjectInstance, null);

        if (_requriedByCountries.Contains(country.ToString()) &&
            (value == null ||
             String.IsNullOrWhiteSpace(value.ToString())))
        {
            return new ValidationResult(context.DisplayName +
                                        " is requried for contacts from " +
                                        country);
        }

        return null;
    }
}

For this example I have added a constructor that takes the name of the country property in the model being validated and a string array of the countries that need to make sure the state and zip code are filled in. Both parameters are stored in class level variables for use in the IsValid function.

The IsValid function is still where all the work takes place. The overload of IsValid we are working with provides the value of the property being validated and the validation context that validation is running for. The validation context will allow us to access other properties on the model being validated.

The first step to checking a different property than the one being validated is to get the property info from the validation context by using context.ObjectType.GetProperty and passing the property name that was passed in via the constructor. If GetProperty returns null then the context did not contain the property that was expected. In this case a new ValidationResult is returned with a message that the expected country property was not found.

Using the property info for the country property we can call GetValue with context.ObjectInstance to get the contact’s country. If that country is found in the string array of countries that require the property being validated then a check is run to make sure the property has a value and is not blank.

If the property does not have a value then a new ValidationResult is returned with an error message. If the property does have a value then null is returned indicating that the property is valid.

Example usage from my contact model:

[ValidCountryRequired("Country", new[] { "USA" })]
public string State { get; set; }
[Display(Name = "Zip Code")]
[ValidCountryRequired("Country", new[] { "USA" })]
public string ZipCode { get; set; }

One other thing to note is the usage of context.DisplayName when creating the ValidationResult. By using display name in the message the ZipCode property will show as “Zip Code” in the message returned.

Last week I did an overview of  Data Annotations. This week I am going to expand on data annotations by giving an example of a very simple custom validation attribute.

Sticking with my contact application theme lets say that I only wanted to allow contacts from a very small list of countries. The .Net framework does not have a built in validation attribute to cover this scenario which means I would need to create my own custom validation attribute to fill this need.

To create a custom validation attribute all you need is a class that inherits from ValidationAttribute and overrides the IsValid function.

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;

public class ValidCountry : ValidationAttribute
{
    private readonly List<string> validCountries = new List<string>
    {
        "USA",
        "Canada",
        "Mexico"
    }; 

    public override bool IsValid(object value)
    {
        return value != null &&
               !validCountries.Contains((string)value);
    }
}

When validation for a property is performed the IsValid function of all the property’s validation attributes are run. The IsValid function is called with the value of the property being validated which will have to be cast to the type needed for validation. In my country example value is being cast to string and used see if the value is found in a list of valid countries.

Any property with the ValidCountry attribute would not validate unless the property was set to USA, Canada or Mexico. This is a contrived example, but it does give you an idea of how easy it is to add your own custom validation attributes.