I have a my SQLiteOpenHelper class, which is written as a singleton. I should note that I am not doing this in Java, I am using Xamarin.Android C# to write this.
Here's a snippet from that class:
public class DatabaseHelper : SQLiteOpenHelper
{
private static readonly string TAG = typeof(DatabaseHelper).FullName;
private static readonly string _databaseName = "istockdb";
private static readonly int _databaseVersion = 32;
private static DatabaseHelper _instance;
private Context _context;
private DatabaseHelper(Context context) : base(context, _databaseName, null, _databaseVersion)
{
_context = context;
}
[MethodImpl(MethodImplOptions.Synchronized)]
public static DatabaseHelper Instance(Context context)
{
// *** Use the application context, which will ensure that ***
// *** the Activity's context is not accidentally leaked ***
return _instance ?? (_instance = new DatabaseHelper(context.ApplicationContext));
}
}
So I have my DatabaseHelper that is a singleton and is used like this within Activities and Services:
Service:
[Service(Name=Text.MobileBackgroundHbService, Enabled = true, Exported = true), IntentFilter(new []{Intents.SyncHeartbeats})]
public class BGHeartbeatService : BaseIntentService
{
public BGHeartbeatService()
{
this._database = DatabaseHelper.Instance(Application.Context);
}
protected override void OnHandleIntent(Intent intent)
{
if (this._database == null)
this._database = DatabaseHelper.Instance(Application.Context);
if (intent.Action.Equals(Intents.SyncHeartbeats)) SyncHeartbeatRecords();
var broadcastIntent = new Intent(Intents.MobileRefresh);
SendBroadcast(broadcastIntent);
}
}
Activity, actually a BaseActivity which all Activities inherit from:
[Activity(Label = "BaseActivity")]
public abstract class BaseActivity : AppCompatActivity
{
/// <summary>
/// Reference to the current context.
/// </summary>
protected Context _context { get; set; }
/// <summary>
/// "Tag" used for Log functionallity.
/// </summary>
protected string _tag { get; set; }
/// <summary>
/// Reference To <see cref="RemoteSyncServiceConnection"/>
/// </summary>
protected RemoteSyncServiceConnection _service_connection;
/// <summary>
/// Reference To The Current SessionState.
/// </summary>
protected SessionState _session_state;
/// <summary>
/// Reference To <see cref="SyncReceiver"/>
/// </summary>
protected SyncReceiver _sync_receiver;
/// <summary>
/// Base FloatingActionButton.
/// </summary>
protected FloatingActionButton _base_fab;
/// <summary>
/// Is the Fab Menu Shown / Hid.
/// </summary>
protected static bool _is_fab_open;
protected IConnection _printer_connection;
protected string _printer_address;
protected bool _service_required;
protected bool _receiver_required;
protected MediaPlayer _media_player;
protected DatabaseHelper _database;
/// <summary>
/// <see cref="IntentFilter"/> for the <see cref="SyncReceiver"/>
/// </summary>
protected readonly string[] _intent_filters =
{
Intents.AlarmCompleteOrders,
Intents.AlarmHeartbeats,
Intents.AlarmPkas,
Intents.AlarmTrackingScans,
Intents.MobileRefresh
};
#region Lifecycle Methods
/// <summary>
/// Application Lifecycle Method.
/// </summary>
/// <param name="savedInstanceState"></param>
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
// *** Initialize Xamarin.Essentials ***
Xamarin.Essentials.Platform.Init(this, savedInstanceState);
// *** Initialize the DatabaseHelper ***
if(this._database == null)
this._database = DatabaseHelper.Instance(this.ApplicationContext);
}
}
The DatabaseHelper instance is being disposed of frequently causing either services, or activities to try and access the disposed _database object.
How is this being disposed of and why?
I thought making the _instance static within the DatabaseHelper as well as making the constructor private and forcing the use of the DatabaseHelper.Instance method would keep a single instance of the DatabaseHelper that wouldn't be disposed of between activities and services?
Am I misunderstanding this?
EDIT logcat output from try/catch blocks showing the exception being thrown. The SaveHeartbeat method exists in the base activity.:
protected void SaveHeartbeat(DateTime time, string sourceActivity, [CallerMemberName] string sourceEvent = "")
{
try
{
var heartbeat = new SmartWarehouse.Shared.Models.Heartbeat(sourceActivity,
sourceEvent,
this._session_state.CurrentSession.ROWID.ToString());
this._database.InsertHeartbeat(heartbeat);
}
catch (Exception e)
{
// TODO: Document Exception
Util.Tools.Bark(e);
}
}
EDIT 2 DatabaseHelper.InsertHeartbeat():
/// <summary>
/// Inserts a Heartbeat record into local DB.
/// </summary>
/// <param name="heartbeat"></param>
/// <returns></returns>
public long InsertHeartbeat(Heartbeat heartbeat)
{
if (heartbeat == null) return -2L;
using (var db = this.WritableDatabase)
{
var id = -3L;
db.BeginTransactionNonExclusive();
try
{
var cv = GetContentValues(heartbeat);
id = db.Insert(DatabaseSchema.Heartbeat.TableName, null, cv);
db.SetTransactionSuccessful();
}
catch (Exception e)
{
// TODO: Document Exception
Util.Tools.Bark(e);
}
finally
{
db.EndTransaction();
}
return id;
}
}
Alright so my theory is that when I access the db object in the using() statement it is disposing of the database that the DatabaseHelper object uses. Also noticed that I'm not using db.InsertOrThrow() method which I should be.. Gonna do some re-working on my DatabaseHelper class to see if that resolves the issue.
It turns out that my singleton instance of the DatbaseHelper was not being disposed of.
Actually what was happening is I was disposing of the SQLiteDatabase object that was being used by the DatbaseHelper from within the helper methods.
All I had to do to actually resolve the issue was change:
/// <summary>
/// Inserts a Heartbeat record into local DB.
/// </summary>
/// <param name="heartbeat"></param>
/// <returns></returns>
public long InsertHeartbeat(Heartbeat heartbeat)
{
if (heartbeat == null) return -2L;
// This using() statement is causing the disposal
using (var db = this.WritableDatabase)
{
var id = -3L;
db.BeginTransactionNonExclusive();
try
{
var cv = GetContentValues(heartbeat);
id = db.Insert(DatabaseSchema.Heartbeat.TableName, null, cv);
db.SetTransactionSuccessful();
}
catch (Exception e)
{
// TODO: Document Exception
Util.Tools.Bark(e);
}
finally
{
db.EndTransaction();
}
return id;
}
}
TO:
/// <summary>
/// Inserts a Heartbeat record into local DB.
/// </summary>
/// <param name="heartbeat"></param>
/// <returns></returns>
public long InsertHeartbeat(Heartbeat heartbeat)
{
if (heartbeat == null) return -2L;
// This is no longer initialized in a using() statement
var db = this.WritableDatabase;
var id = -3L;
db.BeginTransactionNonExclusive();
try
{
var cv = GetContentValues(heartbeat);
id = db.Insert(DatabaseSchema.Heartbeat.TableName, null, cv);
db.SetTransactionSuccessful();
}
catch (Exception e)
{
// TODO: Document Exception
Util.Tools.Bark(e);
}
finally
{
db.EndTransaction();
}
return id;
}
SUMMARY:
By initializing my SQLiteDatabase db object inside of using() statements inside of my helper methods I was disposing of the SQLiteDatabase that my DatabaseHelper needed.
Today I've discovered that the FW 4.5 has their own undoredo manager (if I understood good) http://msdn.microsoft.com/en-us/library/System.ComponentModel.Design.UndoEngine%28v=vs.110%29.aspx
Well, I can't find any example about how to start using this class just to make a simple undo/redo of a text based control, I know other alternatives to do undoable things, but just I want to learn how to use this.
When I try to use the constructor it has any parameter to be passed, and also the Intellisense does not shows me any method for the System.ComponentModel.Design.UndoEngine class, really I don't know how I could use it.
Someone could illustrate ours with an example for C# or VBNET? (I preffer VBNET documentation if possibly, please)
UndoEngine is an abstract class, Visual Studio and Designers have implemented UndoEngine in their own way, and those must be private or not available for redistribution. You will not be able to use it, in fact abstract class is just an interface with little implementation, it is not at all an undo framework.
You still have to write your own undo management, however benefit of deriving your undo engine from UndoEngine class is, it can be easily hosted/integrated with VS and other MS based editors.
If you want to provide an editing experience inside Visual Studio document editor, then you have to derive your Undo framework class from UndoEngine, VS will automatically highlight disable undo/redo buttons and will call undo/redo methods on your class.
If you want to use UndoEngine inside your own application, UndoEngine will not help you for anything, you will have to write every functionality by yourself. UndoEngine just manages stack of Undo/Redo, real work is inside UndoUnit. It is based on Unit of Work concept, where your every action should actually represent a work that can be undone.
The simplest UndoEngine Implementation
Let us say you are changing a global variable,
// following code uses big UndoUnit
public void SetGlobalVariable(object v){
var oldValue = GlobalVariable;
GlobalVariable = v;
var action = new UndoUnit{
UndoAction = ()=>{
GlobalVariable = oldValue;
},
RedoAction = ()=>{
GlobalVariable = v;
}
};
UndoManager.Add(action);
}
/// <summary>
/// Used to indicates the designer's status
/// </summary>
public enum UndoUnitState
{
Undoing,
Redoing,
}
/// <summary>
/// A UndoUnitBase can be used as a IOleUndoUnit or just a undo step in
/// a transaction
/// </summary>
public class UndoUnitBase : IOleUndoUnit
{
public Action UndoAction {get;set;}
public Action RedoAction {get;set;}
private string name = null;
private Guid clsid = Guid.Empty;
private bool inDoAction = false;
private bool isStillAtTop = true;
private UndoUnitState unitState = UndoUnitState.Undoing;
protected UndoUnit UnitState
{
get { return unitState; }
set { unitState = value; }
}
/// <summary>
/// </summary>
/// <param name="undoManager"></param>
/// <param name="name"></param>
internal UndoUnitBase(string name)
{
this.name = name;
}
~UndoUnitBase()
{
}
/// <summary>
/// </summary>
protected bool InDoAction
{
get
{
return inDoAction;
}
}
public string UndoName
{
get
{
return name;
}
set
{
this.name = value;
}
}
public Guid Clsid
{
get { return clsid; }
set { clsid = value; }
}
/// <summary>
/// This property indicates whether the undo unit is at the top (most recently added to)
/// the undo stack. This is useful to know when deciding whether undo units for operations
/// like typing can be coallesced together.
/// </summary>
public bool IsStillAtTop
{
get { return isStillAtTop; }
}
/// <summary>
/// This function do the actual undo, and then revert the action to be a redo
/// </summary>
/// <returns>objects that should be selected after DoAction</returns>
protected abstract void DoActionInternal();
/// <devdoc>
/// IOleUndoManager's "Do" action.
/// </devdoc>
void IOleUndoUnit.Do(IOleUndoManager oleUndoManager)
{
Do(oleUndoManager);
}
protected virtual int Do(IOleUndoManager oleUndoManager)
{
try
{
if(unitState== UndoUnitState.Undoing){
UndoAction();
}else{
RedoAction();
}
unitState = (unitState == UndoUnitState.Undoing) ? UndoUnitState.Redoing : UndoUnitState.Undoing;
if (oleUndoManager != null)
oleUndoManager.Add(this);
return VSConstants.S_OK;
}
catch (COMException e)
{
return e.ErrorCode;
}
catch
{
return VSConstants.E_ABORT;
}
finally
{
}
}
/// <summary>
/// </summary>
/// <returns></returns>
void IOleUndoUnit.GetDescription(out string pBstr)
{
pBstr = name;
}
/// <summary>
/// </summary>
/// <param name="clsid"></param>
/// <param name="pID"></param>
/// <returns></returns>
void IOleUndoUnit.GetUnitType(out Guid pClsid, out int plID)
{
pClsid = Clsid;
plID = 0;
}
/// <summary>
/// </summary>
void IOleUndoUnit.OnNextAdd()
{
// We are no longer the top most undo unit; another one was added.
isStillAtTop = false;
}
}
public class MyUndoEngine : UndoEngine, IUndoHandler
{
Stack<UndoEngine.UndoUnit> undoStack = new Stack<UndoEngine.UndoUnit>();
Stack<UndoEngine.UndoUnit> redoStack = new Stack<UndoEngine.UndoUnit>();
public ReportDesignerUndoEngine(IServiceProvider provider) : base(provider)
{
}
#region IUndoHandler
public bool EnableUndo {
get {
return undoStack.Count > 0;
}
}
public bool EnableRedo {
get {
return redoStack.Count > 0;
}
}
public void Undo()
{
if (undoStack.Count > 0) {
UndoEngine.UndoUnit unit = undoStack.Pop();
unit.Undo();
redoStack.Push(unit);
}
}
public void Redo()
{
if (redoStack.Count > 0) {
UndoEngine.UndoUnit unit = redoStack.Pop();
unit.Undo();
undoStack.Push(unit);
}
}
#endregion
protected override void AddUndoUnit(UndoEngine.UndoUnit unit)
{
undoStack.Push(unit);
}
}
If your question is how to use it at runtime, then the answer is in MSDN:
Specifies generic undo/redo functionality at design time.
So I doubt that it is easily usable at runtime.
If you meant an example of custom user control utilizing this class, I can't find any, sorry.
Find an implementation of the UndoEngine and how to use it right here:
https://github.com/icsharpcode/SharpDevelop/search?q=ReportDesignerUndoEngine&ref=cmdform
HTH
I want to make this method called RegisterCollection with the purpose of registering a collection of subobjects in a DomainObject. The idea is that i want to register collections in a list, so that when i call Save() on my DomainObject, it will call save on each of the registered collections sub domainobjects.
I've made this code, but i get this error when i build: Argument type 'OrderCollection' is not assignable to parameter type Collection.
I use C# with .Net 3.5. I've read somewhere that the type of conversion that fails, is supported in .NET 4.0. Not sure this is correctly understood, but anyway, i hope someone has some suggestions to what else to do or have a workaround.
Is this possible maybe with a CommandPattern of some sort?
public interface IDomainObject
{
void Save();
}
public class DomainObject : IDomainObject
{
private readonly IList<Collection<IDomainObject>> m_Collections = new List<Collection<IDomainObject>>();
protected void RegisterCollection(Collection<IDomainObject> collection)
{
m_Collections.Add(collection);
}
/// <summary>
/// Saves this instance collections.
/// </summary>
public virtual void Save()
{
SaveCollections();
}
private void SaveCollections()
{
foreach (var itemCollection in m_Collections)
{
foreach (var item in itemCollection)
{
item.Save();
}
}
}
}
public class OrderCollection : Collection<IOrder>
{
}
public interface IOrder : IDomainObject
{
}
public class Customer : DomainObject
{
private readonly OrderCollection m_OrderCollection = new OrderCollection();
public Customer()
{
// Throws: Argument type 'OrderCollection' is not assignable to parameter type Collection<IDomainObject>
RegisterCollection(m_OrderCollection);
}
}
Your problem is that Collection<IOrder>is not compatable with Collection<IDomainObject> , this is down to Covariance and Contravariance .
Imagine your collection of IOrder, if it was treated as a list of IDomainObject, then that would allow you to add any IDomainObject to the list, this means i could add an ISomethingElse as long as it derived from IDomainObject, clearly this would be a bad thing..
Do you have to maintain a List of Collections or can you replace it with a simple List of IDomainObject?
If so, you could just make RegisterCollection generic:
private readonly IList<IDomainObject> m_Collections = new List<IDomainObject>();
protected void RegisterCollection<T>(Collection<T> collection) where T : IDomainObject {
foreach (var obj in collection)
m_Collections.Add(obj);
}
Thanks to type inference, you won't have to specify T and can call RegisterCollection with any Collection of classes that derive from IDomainObject.
EDIT: Ok, I understand now why you want to keep the list. Another solution I can think of is to create a custom Collection class with a non-generic base class:
public abstract class DomainObjectCollection {
public abstract void Save();
}
public class DomainObjectCollection<T> : DomainObjectCollection where T : IDomainObject {
private readonly Collection<T> collection;
public DomainObjectCollection(Collection<T> collection) {
this.collection = collection;
}
public static implicit operator DomainObjectCollection<T> (Collection<T> collection) {
return new DomainObjectCollection<T>(collection);
}
public override void Save() {
foreach (var obj in collection)
obj.Save();
}
}
public class DomainObject : IDomainObject {
private readonly IList<DomainObjectCollection> m_Collections = new List<DomainObjectCollection>();
protected void RegisterCollection<T>(Collection<T> collection) where T : IDomainObject {
m_Collections.Add((DomainObjectCollection<T>)collection);
}
/// <summary>
/// Saves this instance collections.
/// </summary>
public virtual void Save() {
SaveCollections();
}
private void SaveCollections() {
foreach (var itemCollection in m_Collections) {
itemCollection.Save();
}
}
}
I think i've found a way around. It's not pretty, but it's working. Let me know what you think. Suggestions are most welcome.
Notice i've used some of your suggestion Enzi for RegisterCollection.
This allows me to keep the SaveCollections generic and add more collections at runtime.
public class DomainObject : IDomainObject
{
private readonly IList<IList> m_Collections = new List<IList>();
protected void RegisterCollection<T>(List<T> collection) where T : IDomainObject
{
m_Collections.Add(collection);
}
public int Id { get; set; }
/// <summary>
/// Saves this instance collections.
/// </summary>
public virtual void Save()
{
SaveCollections();
}
private void SaveCollections()
{
foreach (var itemCollection in m_Collections)
{
foreach (var item in itemCollection))
{
((IDomainObject)item).Save();
}
}
}
}
public interface IDomainObject
{
/// <summary>
/// Gets or sets the id.
/// </summary>
/// <value>
/// The id.
/// </value>
int Id { get; set; }
/// <summary>
/// Saves this instance.
/// </summary>
void Save();
/// <summary>
/// Deletes this instance.
/// </summary>
void Delete();
}
Is it safe to access asp.net session variables through static properties of a static object?
Here is what I mean:
public static class SessionHelper
{
public static int Age
{
get
{
return (int)HttpContext.Current.Session["Age"];
}
set
{
HttpContext.Current.Session["Age"] = value;
}
}
public static string Name
{
get
{
return (string)HttpContext.Current.Session["Name"];
}
set
{
HttpContext.Current.Session["Name"] = value;
}
}
}
Is it possible that userA could access userB's session data this way?
Yes, that way is fine - just make sure you don't do this:
public static class SessionHelper
{
private static HttpSession sess = HttpContext.Current.Session;
public static int Age
{
get
{
return (int)sess["Age"];
}
set
{
sess["Age"] = value;
}
}
}
As ive seen this way show one user's session data to another user. (Albeit in ASP.NET 1.1)
IMHO, this is actually a good approach. It is type safe, add that level abstraction that could allow you to change things with minimal impact.
An example of something you might change, if you decided some state should move to the cache or even the database combined with caching, these would require additional thread synchronization, but could all be handled by the internals of this class. You might consider changing name of the class to something less session specific.
The one comment I would have on your particular example is that you should check that the Session variable is not null and either return an appropriate default, assert or raise an informative exception if it is. Just in case the property is read before it is being set.
In fact, here is my "base" SessionClass.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
public static class CSession
{
private static readonly string zE = "";
private static readonly string CrLF = Environment.NewLine;
private static bool bStopHere = true;
/// <summary>
/// Get a session variable
/// </summary>
/// <param name="pSessionKey"></param>
/// <returns></returns>
public static object Get(string pSessionKey)
{
object t = null;
if (HttpContext.Current.Session[pSessionKey] != null) { t = (object)HttpContext.Current.Session[pSessionKey]; }
return t;
}//object Get(string pSessionKey)
/// <summary>
/// Set a session variable
/// </summary>
/// <param name="pSessionKey"></param>
/// <param name="pObject"></param>
public static void Set(string pSessKey, object pObject)
{
HttpContext.Current.Session.Remove(pSessKey);
HttpContext.Current.Session.Add(pSessKey, pObject);
}//void Set(string pSessionKey, object pObject)
public static string GetString(string pSessKey)
{
string sTemp = zE;
object t = Get(pSessKey);
if (t != null) { sTemp = (string)t; } else { sTemp = zE; }
return sTemp;
}//string GetString(string pSessionKey)
public static int GetInt(string pSessKey)
{
int s = 0;
object t = Get(pSessKey);
if (t != null) { s = (int)t; }
return s;
}//int GetInt(string pSessionKey)
public static Int32 GetInt32(string pSessKey)
{
Int32 s = 0;
object t = Get(pSessKey);
if (t != null) { s = (Int32)t; }
return s;
}//Int32 GetInt32(string pSessionKey)
public static bool GetBool(string pSessKey)
{
bool s = false;
object t = Get(pSessKey);
if (t != null) { s = (bool)t; }
return s;
}//bool GetBool(string pSessionKey)
}//static class CSession
I've created a new class called Actor which processes messages passed to it.
The problem I am running into is figuring out what is the most elegant way to pass related but different messages to the Actor. My first idea is to use inheritance but it seems so bloated but it is strongly types which is a definite requirement.
Have any ideas?
Example
private abstract class QueueMessage { }
private class ClearMessage : QueueMessage
{
public static readonly ClearMessage Instance = new ClearMessage();
private ClearMessage() { }
}
private class TryDequeueMessage : QueueMessage
{
public static readonly TryDequeueMessage Instance = new TryDequeueMessage();
private TryDequeueMessage() { }
}
private class EnqueueMessage : QueueMessage
{
public TValue Item { get; private set; }
private EnqueueMessage(TValue item)
{
Item = item;
}
}
Actor Class
/// <summary>Represents a callback method to be executed by an Actor.</summary>
/// <typeparam name="TReply">The type of reply.</typeparam>
/// <param name="reply">The reply made by the actor.</param>
public delegate void ActorReplyCallback<TReply>(TReply reply);
/// <summary>Represents an Actor which receives and processes messages in concurrent applications.</summary>
/// <typeparam name="TMessage">The type of message this actor accepts.</typeparam>
/// <typeparam name="TReply">The type of reply made by this actor.</typeparam>
public abstract class Actor<TMessage, TReply> : IDisposable
{
/// <summary>The default total number of threads to process messages.</summary>
private const Int32 DefaultThreadCount = 1;
/// <summary>Used to serialize access to the message queue.</summary>
private readonly Locker Locker;
/// <summary>Stores the messages until they can be processed.</summary>
private readonly System.Collections.Generic.Queue<Message> MessageQueue;
/// <summary>Signals the actor thread to process a new message.</summary>
private readonly ManualResetEvent PostEvent;
/// <summary>This tells the actor thread to stop reading from the queue.</summary>
private readonly ManualResetEvent DisposeEvent;
/// <summary>Processes the messages posted to the actor.</summary>
private readonly List<Thread> ActorThreads;
/// <summary>Initializes a new instance of the Genex.Concurrency<TRequest, TResponse> class.</summary>
public Actor() : this(DefaultThreadCount) { }
/// <summary>Initializes a new instance of the Genex.Concurrency<TRequest, TResponse> class.</summary>
/// <param name="thread_count"></param>
public Actor(Int32 thread_count)
{
if (thread_count < 1) throw new ArgumentOutOfRangeException("thread_count", thread_count, "Must be 1 or greater.");
Locker = new Locker();
MessageQueue = new System.Collections.Generic.Queue<Message>();
EnqueueEvent = new ManualResetEvent(true);
PostEvent = new ManualResetEvent(false);
DisposeEvent = new ManualResetEvent(true);
ActorThreads = new List<Thread>();
for (Int32 i = 0; i < thread_count; i++)
{
var thread = new Thread(ProcessMessages);
thread.IsBackground = true;
thread.Start();
ActorThreads.Add(thread);
}
}
/// <summary>Posts a message and waits for the reply.</summary>
/// <param name="value">The message to post to the actor.</param>
/// <returns>The reply from the actor.</returns>
public TReply PostWithReply(TMessage message)
{
using (var wrapper = new Message(message))
{
lock (Locker) MessageQueue.Enqueue(wrapper);
PostEvent.Set();
wrapper.Channel.CompleteEvent.WaitOne();
return wrapper.Channel.Value;
}
}
/// <summary>Posts a message to the actor and executes the callback when the reply is received.</summary>
/// <param name="value">The message to post to the actor.</param>
/// <param name="callback">The callback that will be invoked once the replay is received.</param>
public void PostWithAsyncReply(TMessage value, ActorReplyCallback<TReply> callback)
{
if (callback == null) throw new ArgumentNullException("callback");
ThreadPool.QueueUserWorkItem(state => callback(PostWithReply(value)));
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
if (DisposeEvent.WaitOne(10))
{
DisposeEvent.Reset();
PostEvent.Set();
foreach (var thread in ActorThreads)
{
thread.Join();
}
((IDisposable)PostEvent).Dispose();
((IDisposable)DisposeEvent).Dispose();
}
}
/// <summary>Processes a message posted to the actor.</summary>
/// <param name="message">The message to be processed.</param>
protected abstract void ProcessMessage(Message message);
/// <summary>Dequeues the messages passes them to ProcessMessage.</summary>
private void ProcessMessages()
{
while (PostEvent.WaitOne() && DisposeEvent.WaitOne(10))
{
var message = (Message)null;
while (true)
{
lock (Locker)
{
message = MessageQueue.Count > 0 ?
MessageQueue.Dequeue() :
null;
if (message == null)
{
PostEvent.Reset();
break;
}
}
try
{
ProcessMessage(message);
}
catch
{
}
}
}
}
/// <summary>Represents a message that is passed to an actor.</summary>
protected class Message : IDisposable
{
/// <summary>The actual value of this message.</summary>
public TMessage Value { get; private set; }
/// <summary>The channel used to give a reply to this message.</summary>
public Channel Channel { get; private set; }
/// <summary>Initializes a new instance of Genex.Concurrency.Message class.</summary>
/// <param name="value">The actual value of the message.</param>
public Message(TMessage value)
{
Value = value;
Channel = new Channel();
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
Channel.Dispose();
}
}
/// <summary>Represents a channel used by an actor to reply to a message.</summary>
protected class Channel : IDisposable
{
/// <summary>The value of the reply.</summary>
public TReply Value { get; private set; }
/// <summary>Signifies that the message has been replied to.</summary>
public ManualResetEvent CompleteEvent { get; private set; }
/// <summary>Initializes a new instance of Genex.Concurrency.Channel class.</summary>
public Channel()
{
CompleteEvent = new ManualResetEvent(false);
}
/// <summary>Reply to the message received.</summary>
/// <param name="value">The value of the reply.</param>
public void Reply(TReply value)
{
Value = value;
CompleteEvent.Set();
}
/// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
public void Dispose()
{
((IDisposable)CompleteEvent).Dispose();
}
}
}
Steve Gilham summarized how the compiler actually handles discriminated unions. For your own code, you could consider a simplified version of that. Given the following F#:
type QueueMessage<T> = ClearMessage | TryDequeueMessage | EnqueueMessage of T
Here's one way to emulate it in C#:
public enum MessageType { ClearMessage, TryDequeueMessage, EnqueueMessage }
public abstract class QueueMessage<T>
{
// prevents unwanted subclassing
private QueueMessage() { }
public abstract MessageType MessageType { get; }
/// <summary>
/// Only applies to EnqueueMessages
/// </summary>
public abstract T Item { get; }
public static QueueMessage<T> MakeClearMessage() { return new ClearMessage(); }
public static QueueMessage<T> MakeTryDequeueMessage() { return new TryDequeueMessage(); }
public static QueueMessage<T> MakeEnqueueMessage(T item) { return new EnqueueMessage(item); }
private sealed class ClearMessage : QueueMessage<T>
{
public ClearMessage() { }
public override MessageType MessageType
{
get { return MessageType.ClearMessage; }
}
/// <summary>
/// Not implemented by this subclass
/// </summary>
public override T Item
{
get { throw new NotImplementedException(); }
}
}
private sealed class TryDequeueMessage : QueueMessage<T>
{
public TryDequeueMessage() { }
public override MessageType MessageType
{
get { return MessageType.TryDequeueMessage; }
}
/// <summary>
/// Not implemented by this subclass
/// </summary>
public override T Item
{
get { throw new NotImplementedException(); }
}
}
private sealed class EnqueueMessage : QueueMessage<T>
{
private T item;
public EnqueueMessage(T item) { this.item = item; }
public override MessageType MessageType
{
get { return MessageType.EnqueueMessage; }
}
/// <summary>
/// Gets the item to be enqueued
/// </summary>
public override T Item { get { return item; } }
}
}
Now, in code that is given a QueueMessage, you can switch on the MessageType property in lieu of pattern matching, and make sure that you access the Item property only on EnqueueMessages.
EDIT
Here's another alternative, based on Juliet's code. I've tried to streamline things so that it's got a more usable interface from C#, though. This is preferable to the previous version in that you can't get a MethodNotImplemented exception.
public abstract class QueueMessage<T>
{
// prevents unwanted subclassing
private QueueMessage() { }
public abstract TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase);
public static QueueMessage<T> MakeClearMessage() { return new ClearMessage(); }
public static QueueMessage<T> MakeTryDequeueMessage() { return new TryDequeueMessage(); }
public static QueueMessage<T> MakeEnqueueMessage(T item) { return new EnqueueMessage(item); }
private sealed class ClearMessage : QueueMessage<T>
{
public ClearMessage() { }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return clearCase();
}
}
private sealed class TryDequeueMessage : QueueMessage<T>
{
public TryDequeueMessage() { }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return tryDequeueCase();
}
}
private sealed class EnqueueMessage : QueueMessage<T>
{
private T item;
public EnqueueMessage(T item) { this.item = item; }
public override TReturn Match<TReturn>(Func<TReturn> clearCase, Func<TReturn> tryDequeueCase, Func<T, TReturn> enqueueCase)
{
return enqueueCase(item);
}
}
}
You'd use this code like this:
public class MessageUserTest
{
public void Use()
{
// your code to get a message here...
QueueMessage<string> msg = null;
// emulate pattern matching, but without constructor names
int i =
msg.Match(
clearCase: () => -1,
tryDequeueCase: () => -2,
enqueueCase: s => s.Length);
}
}
In your example code, you implement PostWithAsyncReply in terms of PostWithReply. That isn't ideal, because it means that when you call PostWithAsyncReply and the actor takes a while to handle it, there are actually two threads tied up: the one executing the actor and the one waiting for it to finish. It would be better to have the one thread executing the actor and then calling the callback in the asynchronous case. (Obviously in the synchronous case there's no avoiding the tying up of two threads).
Update:
More on the above: you construct an actor with an argument telling it how many threads to run. For simplicity suppose every actor runs with one thread (actually quite a good situation because actors can then have internal state with no locking on it, as only one thread accesses it directly).
Actor A calls actor B, waiting for a response. In order to handle the request, actor B needs to call actor C. So now A and B's only threads are waiting, and C's is the only one actually giving the CPU any work to do. So much for multi-threading! But this is what you get if you wait for answers all the time.
Okay, you could increase the number of threads you start in each actor. But you'd be starting them so they could sit around doing nothing. A stack uses up a lot of memory, and context switching can be expensive.
So it's better to send messages asynchronously, with a callback mechanism so you can pick up the finished result. The problem with your implementation is that you grab another thread from the thread pool, purely to sit around and wait. So you basically apply the workaround of increasing the number of threads. You allocate a thread to the task of never running.
It would be better to implement PostWithReply in terms of PostWithAsyncReply, i.e. the opposite way round. The asynchronous version is low-level. Building on my delegate-based example (because it involves less typing of code!):
private bool InsertCoinImpl(int value)
{
// only accept dimes/10p/whatever it is in euros
return (value == 10);
}
public void InsertCoin(int value, Action<bool> accepted)
{
Submit(() => accepted(InsertCoinImpl(value)));
}
So the private implementation returns a bool. The public asynchronous method accepts an action that will receive the return value; both the private implementation and the callback action are executed on the same thread.
Hopefully the need to wait synchronously is going to be the minority case. But when you need it, it could be supplied by a helper method, totally general purpose and not tied to any specific actor or message type:
public static T Wait<T>(Action<Action<T>> activity)
{
T result = default(T);
var finished = new EventWaitHandle(false, EventResetMode.AutoReset);
activity(r =>
{
result = r;
finished.Set();
});
finished.WaitOne();
return result;
}
So now in some other actor we can say:
bool accepted = Helpers.Wait<bool>(r => chocMachine.InsertCoin(5, r));
The type argument to Wait may be unnecessary, haven't tried compiling any of this. But Wait basically magics-up a callback for you, so you can pass it to some asynchronous method, and on the outside you just get back whatever was passed to the callback as your return value. Note that the lambda you pass to Wait still actually executes on the same thread that called Wait.
We now return you to our regular program...
As for the actual problem you asked about, you send a message to an actor to get it to do something. Delegates are helpful here. They let you effectively get the compiler to generate you a class with some data, a constructor that you don't even have to call explicitly and also a method. If you're having to write a bunch of little classes, switch to delegates.
abstract class Actor
{
Queue<Action> _messages = new Queue<Action>();
protected void Submit(Action action)
{
// take out a lock of course
_messages.Enqueue(action);
}
// also a "run" that reads and executes the
// message delegates on background threads
}
Now a specific derived actor follows this pattern:
class ChocolateMachineActor : Actor
{
private void InsertCoinImpl(int value)
{
// whatever...
}
public void InsertCoin(int value)
{
Submit(() => InsertCoinImpl(value));
}
}
So to send a message to the actor, you just call the public methods. The private Impl method does the real work. No need to write a bunch of message classes by hand.
Obviously I've left out the stuff about replying, but that can all be done with more parameters. (See update above).
Union types and pattern matching map pretty directly to the visitor pattern, I've posted about this a few times before:
What task is best done in a functional programming style?
https://stackoverflow.com/questions/1883246/none-pure-functional-code-smells/1884256#1884256
So if you want to pass messages with lots of different types, you're stuck implementing the visitor pattern.
(Warning, untested code ahead, but should give you an idea of how its done)
Let's say we have something like this:
type msg =
| Add of int
| Sub of int
| Query of ReplyChannel<int>
let rec counts = function
| [] -> (0, 0, 0)
| Add(_)::xs -> let (a, b, c) = counts xs in (a + 1, b, c)
| Sub(_)::xs -> let (a, b, c) = counts xs in (a, b + 1, c)
| Query(_)::xs -> let (a, b, c) = counts xs in (a, b, c + 1)
You end up with this bulky C# code:
interface IMsgVisitor<T>
{
T Visit(Add msg);
T Visit(Sub msg);
T Visit(Query msg);
}
abstract class Msg
{
public abstract T Accept<T>(IMsgVistor<T> visitor)
}
class Add : Msg
{
public readonly int Value;
public Add(int value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
class Sub : Msg
{
public readonly int Value;
public Add(int value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
class Query : Msg
{
public readonly ReplyChannel<int> Value;
public Add(ReplyChannel<int> value) { this.Value = value; }
public override T Accept<T>(IMsgVisitor<T> visitor) { return visitor.Visit(this); }
}
Now whenever you want to do something with the message, you need to implement a visitor:
class MsgTypeCounter : IMsgVisitor<MsgTypeCounter>
{
public readonly Tuple<int, int, int> State;
public MsgTypeCounter(Tuple<int, int, int> state) { this.State = state; }
public MsgTypeCounter Visit(Add msg)
{
Console.WriteLine("got Add of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(1 + State.Item1, State.Item2, State.Item3));
}
public MsgTypeCounter Visit(Sub msg)
{
Console.WriteLine("got Sub of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(State.Item1, 1 + State.Item2, State.Item3));
}
public MsgTypeCounter Visit(Query msg)
{
Console.WriteLine("got Query of " + msg.Value);
return new MsgTypeCounter(Tuple.Create(State.Item1, 1 + State.Item2, State.Item3));
}
}
Then finally you can use it like this:
var msgs = new Msg[] { new Add(1), new Add(3), new Sub(4), new ReplyChannel(null) };
var counts = msgs.Aggregate(new MsgTypeVisitor(Tuple.Create(0, 0, 0)),
(acc, x) => x.Accept(acc)).State;
Yes, its as obtuse as it seems, but that's how you pass multiple messages a class in a type-safe manner, and that's also why we don't implement unions in C# ;)
A long shot, but anyway..
I am assuming that discriminated-union is F# for ADT (Abstract Data Type). Which means the type could be one of several things.
In case there are two, you could try and put it in a simple generic class with two type parameters:
public struct DiscriminatedUnion<T1,T2>
{
public DiscriminatedUnion(T1 t1) { value = t1; }
public DiscriminatedUnion(T2 t1) { value = t2; }
public static implicit operator T1(DiscriminatedUnion<T1,T2> du) {return (T1)du.value; }
public static implicit operator T2(DiscriminatedUnion<T1,T2> du) {return (T2)du.value; }
object value;
}
To make it work for 3 or more, we need to replicate this class a number of times.
Any one has a solution for function overloading depending on the runtime type?
If you have this
type internal Either<'a, 'b> =
| Left of 'a
| Right of 'b
in F#, then the C# equivalent of the CLR generated for class Either<'a, 'b> has inner types like
internal class _Left : Either<a, b>
{
internal readonly a left1;
internal _Left(a left1);
}
each with a tag, a getter and a factory method
internal const int tag_Left = 0;
internal static Either<a, b> Left(a Left1);
internal a Left1 { get; }
plus a discriminator
internal int Tag { get; }
and a raft of methods to implement interfaces IStructuralEquatable, IComparable, IStructuralComparable
There is a compile-time checked discriminated union type at Discriminated union in C#
private class ClearMessage
{
public static readonly ClearMessage Instance = new ClearMessage();
private ClearMessage() { }
}
private class TryDequeueMessage
{
public static readonly TryDequeueMessage Instance = new TryDequeueMessage();
private TryDequeueMessage() { }
}
private class EnqueueMessage
{
public TValue Item { get; private set; }
private EnqueueMessage(TValue item) { Item = item; }
}
Using the discriminated union could be done as follows:
// New file
// Create an alias
using Message = Union<ClearMessage, TryDequeueMessage, EnqueMessage>;
int ProcessMessage(Message msg)
{
return Message.Match(
clear => 1,
dequeue => 2,
enqueue => 3);
}