实体框架4.0和DDD模式

Question

I use EntityFramework as ORM and I have simple POCO Domain Model with two base classes that represent Value Object and Entity Object Patterns (Evans). These two patterns is all about equality of two objects, so I overrode Equals and GetHashCode methods. Here are these two classes:

 public abstract class EntityObject<T>{
        protected T _ID = default(T);

        public T ID {
            get { return _ID; }
            protected set { _ID = value; }
        }

        public sealed override bool Equals(object obj) {
            EntityObject<T> compareTo = obj as EntityObject<T>;
            return (compareTo != null) &&
            ((HasSameNonDefaultIdAs(compareTo) ||
            (IsTransient && compareTo.IsTransient)) &&
            HasSameBusinessSignatureAs(compareTo));
        }       

        public virtual void MakeTransient() {
            _ID = default(T);            

        }

        public bool IsTransient {
            get {
                return _ID == null || _ID.Equals(default(T));
            }
        }

        public override int GetHashCode() {
            if (default(T).Equals(_ID))
                return 0;
            return _ID.GetHashCode();
        }

        private bool HasSameBusinessSignatureAs(EntityObject<T> compareTo) {
            return ToString().Equals(compareTo.ToString());
        }

        private bool HasSameNonDefaultIdAs(EntityObject<T> compareTo) {
            return (_ID != null && !_ID.Equals(default(T))) &&
            (compareTo._ID != null && !compareTo._ID.Equals(default(T))) &&
            _ID.Equals(compareTo._ID);
        }

        public override string ToString() {
            StringBuilder str = new StringBuilder();
            str.Append(" Class: ").Append(GetType().FullName);
            if (!IsTransient)
                str.Append(" ID: " + _ID);
            return str.ToString();
        }
    }

public abstract class ValueObject<T, U> : IEquatable<T> where T : ValueObject<T, U> {
        private static List<PropertyInfo> Properties { get; set; }
        private static Func<ValueObject<T, U>, PropertyInfo, object[], object> _GetPropValue;

        static ValueObject() {
            Properties = new List<PropertyInfo>();           
            var propParam = Expression.Parameter(typeof(PropertyInfo), "propParam");
            var target = Expression.Parameter(typeof(ValueObject<T, U>), "target");
            var indexPar = Expression.Parameter(typeof(object[]), "indexPar");            
            var call = Expression.Call(propParam, typeof(PropertyInfo).GetMethod("GetValue", new[] { typeof(object), typeof(object[]) }),
                new[] { target, indexPar });
            var lambda = Expression.Lambda<Func<ValueObject<T, U>, PropertyInfo, object[], object>>(call, target, propParam, indexPar);
            _GetPropValue = lambda.Compile();                            
        }

        public U ID { get; protected set; }        

        public override Boolean Equals(Object obj) {
            if (ReferenceEquals(null, obj)) return false;
            if (obj.GetType() != GetType()) return false;
            return Equals(obj as T);
        }

        public Boolean Equals(T other) {
            if (ReferenceEquals(null, other)) return false;
            if (ReferenceEquals(this, other)) return true;
            foreach (var property in Properties) {
                var oneValue = _GetPropValue(this, property, null);
                var otherValue = _GetPropValue(other, property, null);
                if (null == oneValue && null == otherValue) return false;
                if (false == oneValue.Equals(otherValue)) return false;
            }
            return true;
        }

        public override Int32 GetHashCode() {
            var hashCode = 36;
            foreach (var property in Properties) {
                var propertyValue = _GetPropValue(this, property, null);               
                if (null == propertyValue)
                    continue;
                hashCode = hashCode ^ propertyValue.GetHashCode();
            }
            return hashCode;
        }

        public override String ToString() {
            var stringBuilder = new StringBuilder();
            foreach (var property in Properties) {
                var propertyValue = _GetPropValue(this, property, null); 
                if (null == propertyValue)
                    continue;
                stringBuilder.Append(propertyValue.ToString());
            }
            return stringBuilder.ToString();
        }

        protected static void RegisterProperty(Expression<Func<T, Object>> expression) {           
            MemberExpression memberExpression;
            if (ExpressionType.Convert == expression.Body.NodeType) {
                var body = (UnaryExpression)expression.Body;
                memberExpression = body.Operand as MemberExpression;
            }
            else 
                memberExpression = expression.Body as MemberExpression;
            if (null == memberExpression) 
                throw new InvalidOperationException("InvalidMemberExpression");           
            Properties.Add(memberExpression.Member as PropertyInfo);
        }
    }

Everything was OK until I tried to delete some related objects (aggregate root object with two dependent objects which was marked for cascade deletion): I've got an exception "The relationship could not be changed because one or more of the foreign-key properties is non-nullable". I googled this and found http://blog.abodit.com/2010/05/the-relationship-could-not-be-changed-because-one-or-more-of-the-foreign-key-properties-is-non-nullable/ I changed GetHashCode to base.GetHashCode() and error disappeared. But now it breaks all my code: I can't override GetHashCode for my POCO objects => I can't override Equals => I can't implement Value Object and Entity Object patters for my POCO objects. So, I appreciate any solutions, workarounds here etc.

Answer 1

If you want to override GetHashCode, you have to solve the problem directly. The problem sais:

"The relationship could not be changed because one or more of the foreign-key properties is non-nullable"

So,
1. Find the not nullable field that used as a foreign key and make it nullable (so when you delete the record - the fk can be null).
2. Don't mark the dependency as cascade deletion.