AppSettingWrapperAttribute.cs

    /// Ruthlessly inspired by from http://kozmic.net/2014/03/22/strongly-typed-app-settings-with-castle-dictionaryadapter/

    public class AppSettingRequiredAttribute : Attribute
    {
    }

    public class AppSettingWrapperAttribute : DictionaryBehaviorAttribute, IDictionaryPropertyGetter, IPropertyDescriptorInitializer
    {
        object IDictionaryPropertyGetter.GetPropertyValue(IDictionaryAdapter dictionaryAdapter, string key, object storedValue, PropertyDescriptor property, bool ifExists)
        {
            if (storedValue != null) return storedValue;
            if (property.Property.HasAttribute<AppSettingRequiredAttribute>())
                throw new InvalidOperationException(string.Format("Required app setting '{0}' not found!", key));
            return null;
        }

        // enable this to validate property values that were read and set
        void IPropertyDescriptorInitializer.Initialize(PropertyDescriptor propertyDescriptor, object[] behaviors)
        {
            propertyDescriptor.Fetch = true;
        }
    }