Can I hide my ICollection<T> fields when I have a one-to-many mapping in EF4 code-only?

Question

My domain classes that have one-to-many mappings generally take the following form (untested code):

public Customer Customer
{
    // Public methods.

    public Order AddOrder(Order order)
    {
        _orders.Add(order);
    }

    public Order GetOrder(long id)
    {
        return _orders.Where(x => x.Id).Single();
    }

    // etc.

    // Private fields.

    private ICollection<Order> _orders = new List<Order>();
}

The EF4 code-only samples I've seen expose a public ICollection when dealing with one-to-many relationships.

Is there a way to persist and restore my collections with exposing them? If not, it would appear that my domain objects will be designed to meet the requirements of the ORM, which seems to go against the spirit of the endeavour. Exposing an ICollection (with it's Add, etc. methods) doesn't seem particularly clean, and wouldn't be my default approach.

Update

Found this post that suggests it wasn't possible in May. Of course, the Microsoft poster did say that they were "strongly considering implementing" it (I'd hope so) and we're half a year on, so maybe there's been some progress?

Answer 1

I found that whatever was done, EF requires the ICollection<T> to be public. I think this is because when the objects are loaded from the database, the mapping looks for a collection property, gets the collection and then calls the Add method of the collection to add each of the child objects.

I wanted to ensure that the addition was done through a method on the parent object so created a solution of wrapping the collection, catching the add and directing it to my preferred method of addition.

Extending a List and other collection types was not possible because the Add method is not virtual. One option is to extend Collection class and override the InsertItem method.

I have only focussed on the Add, Remove, and Clear functions of the ICollection<T> interface as those are the ones that can modify the collection.

First, is my base collection wrapper which implements the ICollection<T> interface The default behaviour is that of a normal collection. However, the caller can specify an alternative Add method to be called. In addition, the caller can enforce that the Add, Remove, Clear operations are not permitted by setting the alternatives to null. This results in NotSupportedException being thrown if anyone tries to use the method.

The throwing of an exception is not as good as preventing access in the first place. However, code should be tested (unit tested) and an exception will be found very quickly and a suitable code change made.

public abstract class WrappedCollectionBase<T> : ICollection<T>
{

    private ICollection<T> InnerCollection { get { return GetWrappedCollection(); } }

    private Action<T> addItemFunction;
    private Func<T, bool> removeItemFunction;
    private Action clearFunction;


    /// <summary>
    /// Default behaviour is to be like a normal collection
    /// </summary>
    public WrappedCollectionBase()
    {
        this.addItemFunction = this.AddToInnerCollection;
        this.removeItemFunction = this.RemoveFromInnerCollection;
        this.clearFunction = this.ClearInnerCollection;
    }

    public WrappedCollectionBase(Action<T> addItemFunction, Func<T, bool> removeItemFunction, Action clearFunction) : this()
    {
        this.addItemFunction = addItemFunction;
        this.removeItemFunction = removeItemFunction;
        this.clearFunction = clearFunction;
    }

    protected abstract ICollection<T> GetWrappedCollection();

    public void Add(T item)
    {
        if (this.addItemFunction != null)
        {
            this.addItemFunction(item);
        }
        else
        {
            throw new NotSupportedException("Direct addition to this collection is not permitted");
        }
    }

    public void AddToInnerCollection(T item)
    {
        this.InnerCollection.Add(item);
    }

    public bool Remove(T item)
    {
        if (removeItemFunction != null)
        {
            return removeItemFunction(item);
        }
        else
        {
            throw new NotSupportedException("Direct removal from this collection is not permitted");
        }
    }

    public bool RemoveFromInnerCollection(T item)
    {
        return this.InnerCollection.Remove(item);
    }

    public void Clear()
    {
        if (this.clearFunction != null)
        {
            this.clearFunction();
        }
        else
        {
            throw new NotSupportedException("Clearing of this collection is not permitted");
        }
    }

    public void ClearInnerCollection()
    {
        this.InnerCollection.Clear();
    }

    public bool Contains(T item)
    {
        return InnerCollection.Contains(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        InnerCollection.CopyTo(array, arrayIndex);
    }

    public int Count
    {
        get { return InnerCollection.Count; }
    }

    public bool IsReadOnly
    {
        get { return ((ICollection<T>)this.InnerCollection).IsReadOnly; }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return InnerCollection.GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return InnerCollection.GetEnumerator();
    }

}

Given that base class we can use it in two ways. Examples are using the original post objects.

1) Create a specific type of wrapped collection (For example, List) public class WrappedListCollection : WrappedCollectionBase, IList { private List innerList;

    public WrappedListCollection(Action<T> addItemFunction, Func<T, bool> removeItemFunction, Action clearFunction)
        : base(addItemFunction, removeItemFunction, clearFunction)
    { 
    this.innerList = new List<T>();
    }

    protected override ICollection<T> GetWrappedCollection()
    {
        return this.innerList;
    }
 <...snip....> // fill in implementation of IList if important or don't implement IList
    }

This can then be used:

 public Customer Customer
 {
 public ICollection<Order> Orders {get { return _orders; } }
 // Public methods.

 public void AddOrder(Order order)
 {
    _orders.AddToInnerCollection(order);
 }

// Private fields.

private WrappedListCollection<Order> _orders = new WrappedListCollection<Order>(this.AddOrder, null, null);
}

2) Give a collection to be wrapped using

 public class WrappedCollection<T> : WrappedCollectionBase<T>
{
    private ICollection<T> wrappedCollection;

    public WrappedCollection(ICollection<T> collectionToWrap, Action<T> addItemFunction, Func<T, bool> removeItemFunction, Action clearFunction)
        : base(addItemFunction, removeItemFunction, clearFunction)
    {
        this.wrappedCollection = collectionToWrap;
    }

    protected override ICollection<T> GetWrappedCollection()
    {
        return this.wrappedCollection;
    }
}

which can be used as follows:

{ public ICollection Orders {get { return _wrappedOrders; } } // Public methods.

 public void AddOrder(Order order)
 {
    _orders.Add(order);
 }

// Private fields.
private ICollection<Order> _orders = new List<Order>();
private WrappedCollection<Order> _wrappedOrders = new WrappedCollection<Order>(_orders, this.AddOrder, null, null);
}

There are some other ways to call the WrappedCollection constructors For example, to override add but keep remove and clear as normal

private WrappedListCollection<Order> _orders = new WrappedListCollection(this.AddOrder,  (Order o) => _orders.RemoveFromInnerCollection(o), () => _orders.ClearInnerCollection());

I agree that it would be best if EF would not require the collection to be public but this solution allows me to control the modification of my collection.

For the problem of preventing access to the collection for querying you can use approach 2) above and set the WrappedCollection GetEnumerator method to throw a NotSupportedException. Then your GetOrder method can stay as it is. A neater method however may be to expose the wrapped collection. For example:

 public class WrappedCollection<T> : WrappedCollectionBase<T>
 {
    public ICollection<T> InnerCollection { get; private set; }

    public WrappedCollection(ICollection<T> collectionToWrap, Action<T> addItemFunction, Func<T, bool> removeItemFunction, Action clearFunction)
        : base(addItemFunction, removeItemFunction, clearFunction)
    {
        this.InnerCollection = collectionToWrap;
    }


    protected  override ICollection<T> GetWrappedCollection()
    {
        return this.InnerCollection;
    }
}

Then the call in the GetOrder method would become

_orders.InnerCollection.Where(x => x.Id == id).Single();

Answer 2

Another way to accomplish this would be to create an associated interface for each of your POCOs to expose only what you want outside of the persistence/domain layers. You can also interface your DbContext class to also hide and control access to the DbSet collections. As it turns out, the DbSet properties can be protected, and the model builder will pick them up when it's creating tables, but when you try to access the collections they will be null. A factory method (in my example, CreateNewContext) can be used instead of the constructor to get the interfaced DbContext to conceal the DbSet collections.

There's quite a bit of extra effort in coding, but if hiding implementation details within the POCOs is important, this will work.

UPDATE: It turns out you CAN populate DBSets if they are protected, but not directly in the DBContext. They can't be aggregate roots (i.e. accessibility of the entity has to be through a collection in one of the public DBSet entities). If hiding the implementation of DBSet is important, the interface pattern I've described is still relevant.

public interface ICustomer
{
   void AddOrder(IOrder order);
   IOrder GetOrder(long id);
}

public Customer : ICustomer
{
   // Exposed methods:
   void ICustomer.AddOrder(IOrder order)
   {
      if (order is Order)
         orders.Add((Order)order);
      else
         throw new Exception("Hey! Not a mapped type!");
   }

   IOrder ICustomer.GetOrder(long id)
   {
      return orders.Where(x => x.Id).Single();
   }

   // public collection for EF
   // The Order class definition would follow the same interface pattern illustrated 
   // here for the Customer class.
   public ICollection<Order> orders = new List<Order>();
}

public interface IMyContext
{
   IEnumerable<ICustomer> GetCustomers();
   void AddCustomer(ICustomer customerObject);
   ICustomer CreateNewCustomer()
}


public class MyContext : DbContext, IMyContext
{
   public static IMyContext CreateNewContext() { return new MyContext(); }

   public DbSet<Customer> Customers {get;set;}
   public  DbSet<Order> Orders {get;set;}

   public IEnumerable<ICustomer> GetCustomers()
   {
      return Customers;
   }

   public void AddCustomer(ICustomer customerObject)
   {
      if (customerObject is Customer)
         Customers.Add((Customer)customerObject);
      else
         throw new Exception("Hey! Not a mapped type");
   }

   public ICustomer CreateNewCustomer()
   {
      return Customers.Create();
   }

   // wrap the Removes, Finds, etc as necessary. Remember to add these to the 
   // DbContext's interface

   // Follow this pattern also for Order/IOrder

}