In general I think the use of property setters on classes is a sign of bad class design and a lack of OO principles - in most cases when you modify the 'state' of a class some 'behaviour' is invoked whether it be implicit or explicit;
e.g. when I change the address on a user account I want to make sure the address object has been populated (at least).
So these days I've started to design classes that have public getters & private setters, and if you want to modify state you are required to call a method, e.g. 'ChangeAddress'. This is nothing new, there are plenty of examples out there.
So in the example below 'Token' is property that's defined when the object is constructed and 'Path' is a property which is modified by calling the method 'ChangePath'.
Now this attitude has been going well as it's made my classes more behaviour rich with better encapsulation & abstraction.
The only problem I've come across is when I'm trying to re-hydrate objects from the database where the properties I'm attempt to set have some kind of special behaviour - properties representing timestamps & versions that are historically set in the database via some function.
So in the following example I've got 3 properties - 'CreatedOn' 'LastModified' & 'Version'. I don't want to expose a method to set this values - I want them to be immutable...
But this is going to be a problem with an ORM, how am I going to update these values when saving or updating - I want them appear to be auto-updating from a users perspective.
Now when using nHibernate as your ORM this becomes very easy because you can attach Listeners and do custom actions.
So I've got a custom action when NH saves or updates an entity to set these properties with the correct values.
Now if you follow this code, when an entity is saved for the first time then the 'CreatedOn' & 'LastModified' property values are set to the current date & time and the 'Version' property is incremented (the initial value is 0) via the private backing properties. When the entity is updated only the 'LastModified' & 'Version' properties are set via the private backing properties.
This event listener is then instantiated via the fluent NH config.
Awkward Coder
e.g. when I change the address on a user account I want to make sure the address object has been populated (at least).
So these days I've started to design classes that have public getters & private setters, and if you want to modify state you are required to call a method, e.g. 'ChangeAddress'. This is nothing new, there are plenty of examples out there.
So in the example below 'Token' is property that's defined when the object is constructed and 'Path' is a property which is modified by calling the method 'ChangePath'.
public class File : ObservableEntity<int>
{
private string _path;
private Guid _token;
private readonly IExtractFileProperties _propertiesExtractor = new FilePropertiesExtractor();
public File()
{
_token = Guid.NewGuid();
}
public virtual string Path
{
get { return _path; }
private set { ChangePropertyAndNotify(ref _path, value, x => Path); }
}
public virtual Guid Token
{
get { return _token; }
private set { ChangePropertyAndNotify(ref _token, value, x => Token); }
}
public File ChangePath(string path)
{
Path = path;
Size = _propertiesExtractor.Size(Path);
Format = _propertiesExtractor.Format(Path);
FileCreatedOn = _propertiesExtractor.CreatedOn(Path);
FileLastModified = _propertiesExtractor.LastModified(Path);
return this;
}
}
Now this attitude has been going well as it's made my classes more behaviour rich with better encapsulation & abstraction.
The only problem I've come across is when I'm trying to re-hydrate objects from the database where the properties I'm attempt to set have some kind of special behaviour - properties representing timestamps & versions that are historically set in the database via some function.
So in the following example I've got 3 properties - 'CreatedOn' 'LastModified' & 'Version'. I don't want to expose a method to set this values - I want them to be immutable...
But this is going to be a problem with an ORM, how am I going to update these values when saving or updating - I want them appear to be auto-updating from a users perspective.
public abstract class ObservableEntity<T> : IEntity<T>, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged = delegate { };
private T _id;
private DateTime? _createdOn;
private DateTime? _lastModified;
private int? _version;
public virtual T Id
{
get { return _id; }
private set { ChangePropertyAndNotify(ref _id, value, x => Id); }
}
public virtual DateTime? CreatedOn
{
get { return _createdOn; }
private set { ChangePropertyAndNotify(ref _createdOn, value, x => CreatedOn); }
}
public virtual DateTime? LastModified
{
get { return _lastModified; }
private set { ChangePropertyAndNotify(ref _lastModified, value, x => LastModified); }
}
public virtual int? Version
{
get { return _version; }
private set { ChangePropertyAndNotify(ref _version, value, x => Version); }
}
protected void ChangePropertyAndNotify<T2>(ref T2 value, T2 newValue, Expression<Func<object, T2>> expression)
{
value = newValue;
Notify(expression);
}
protected void Notify<T2>(Expression<Func<object, T2>> expression)
{
var propertyName = ((MemberExpression)expression.Body).Member.Name;
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Now when using nHibernate as your ORM this becomes very easy because you can attach Listeners and do custom actions.
So I've got a custom action when NH saves or updates an entity to set these properties with the correct values.
public class EntitySaveOrUpdateEventListener : IPreInsertEventListener, IPreUpdateEventListener
{
public bool OnPreInsert(PreInsertEvent @event)
{
var hasType = (@event.Entity.GetType().GetInterface(typeof(IVersionable).Name, true) != null);
if (!hasType)
return false;
var now = DateTime.Now;
var createdOnIndex = new List<string>(@event.Persister.PropertyNames).FindIndex(x => x == "CreatedOn");
@event.State[createdOnIndex] = now;
SetPrivateVariable("CreatedOn", now, @event.Entity);
var lastModifiedIndex = new List<string>(@event.Persister.PropertyNames).FindIndex(x => x == "LastModified");
@event.State[lastModifiedIndex] = now;
SetPrivateVariable("LastModified", now, @event.Entity);
var versionIndex = new List<string>(@event.Persister.PropertyNames).FindIndex(x => x == "Version");
var version = GetPrivateVariable<int?>("Version", @event.Entity);
version = version.GetValueOrDefault() + 1;
@event.State[versionIndex] = version;
SetPrivateVariable("Version", version, @event.Entity);
return false;
}
public bool OnPreUpdate(PreUpdateEvent @event)
{
var hasType = (@event.Entity.GetType().GetInterface(typeof(IVersionable).Name, true) != null);
if (!hasType)
return false;
var now = DateTime.Now;
var lastModifiedIndex = new List<string>(@event.Persister.PropertyNames).FindIndex(x => x == "LastModified");
@event.State[lastModifiedIndex] = now;
SetPrivateVariable("LastModified", now, @event.Entity);
var versionIndex = new List<string>(@event.Persister.PropertyNames).FindIndex(x => x == "Version");
var version = GetPrivateVariable<int?>("Version", @event.Entity);
version = version.GetValueOrDefault() + 1;
@event.State[versionIndex] = version;
SetPrivateVariable("Version", version, @event.Entity);
return false;
}
public void SetPrivateVariable(string name, object value, object entity)
{
var entityType = entity.GetType();
var pi = entityType.GetProperty(name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
pi.ReflectedType.BaseType.InvokeMember(name,
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.SetProperty | BindingFlags.Instance,
null,
entity,
new[] { value });
}
public T GetPrivateVariable<T>(string name, object entity)
{
var entityType = entity.GetType();
var pi = entityType.GetProperty(name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
return (T)pi.GetValue(entity, null);
}
}
Now if you follow this code, when an entity is saved for the first time then the 'CreatedOn' & 'LastModified' property values are set to the current date & time and the 'Version' property is incremented (the initial value is 0) via the private backing properties. When the entity is updated only the 'LastModified' & 'Version' properties are set via the private backing properties.
This event listener is then instantiated via the fluent NH config.
private static ISessionFactory CreateSessionFactoryImpl()
{
return Fluently.Configure()
.Database(MsSqlConfiguration.MsSql2005.ConnectionString(c => c.FromConnectionStringWithKey("Pronunciations")).ShowSql())
.Mappings(m => m.FluentMappings.AddFromAssemblyOf<IMap>())
.ExposeConfiguration(c =>
{
c.EventListeners.PreInsertEventListeners = new IPreInsertEventListener[] { new EntitySaveOrUpdateEventListener() };
c.EventListeners.PreUpdateEventListeners = new IPreUpdateEventListener[] { new EntitySaveOrUpdateEventListener() };
})
.BuildSessionFactory();
}
Awkward Coder
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