Question

I have quite a few problems, probably misunderstandings, about the new keyword with properties in derived classes. Most of the documentation deals with new on derived methods. For instance:

[DataContract]
[KnownType(typeof(PostCategory))]
public class BaseCategory : IEquatable<BaseCategory>
{
//...

    /// <summary>
    /// Enables access to child categories (immediate ones). Filled in by data access provider. Not serializable.
    /// </summary>
    public SortedDictionary<string, BaseCategory> Children { get; set; }
//...
}

[DataContract]
[KnownType(typeof(BaseCategory))]
public class PostCategory : BaseCategory
{
//...
    public new SortedDictionary<string, PostCategory> Children { get; set; }
//...
}

Then in my data repository implementation I have a generic implemenation for all the classes that derive from BaseCategory. I do this:

    public class CachedXmlCategorizationProvider<T> : ICategorizationRepository<T> where T : BaseCategory, new()
    {
        private readonly ICacheProvider _cacheProvider = AppServices.Cache;
        private readonly string _file = Path.Combine(XmlProvider.DataStorePhysicalPath, typeof (T).Name, "categorization.xml");
        private SortedDictionary<string, T> _dictionary;

    //...
        private SortedDictionary<string, T> LoadFromDatastore()
        {
            if (!DefaultsExist())
            {
                return CreateDefaults();
            }

            var dcs = new DataContractSerializer(typeof (SortedDictionary<string, T>));
            XmlDictionaryReader reader =
                XmlDictionaryReader.CreateTextReader(new FileStream(_file, FileMode.Open, FileAccess.Read),
                                                     new XmlDictionaryReaderQuotas());

            _dictionary = (SortedDictionary<string, T>) dcs.ReadObject(reader, true);
/* HERE! Apparently this fills in the Children property of the base class */
            // fill in Children
            foreach (var pair in _dictionary)
            {
                pair.Value.Children = GetChildren(pair.Value.Id);
            }
            reader.Close();
            return _dictionary;
        }
    //...
    }

and when I try to access the Children property of a specific PostCategory instance at runtime it is null and the base.Children is set to the correct dictionary of Children as set by the GetChildren() method.

The thing is, my code relies on checking the PostCategory.Children instance and because it’s null my code is failing.

What is the correct way to achieve what I want (have one base class and change the type of a property in derived class)?

Answer 1

Inheritance for the new keyword does not function in the same manner as virtual override members, in that it does not function at all. If you access the member via a reference to a base class, the base class’s member is accessed and the hidden member is completely bypassed. In the case of proper inheritance, this sort of call would go down the derived classes had any of them overridden the base member.

If you want proper behaviour you will need to make the base property virtual and override it in a derived class.

It appears as though you are trying to make the derived type more specific. This could possibly be achieved using interfaces and co-variance.