Develop Reference

Custom event and invocation on main thread

I was given a generic API class, that contains a custom event which always needs to be invoked by the main UI thread.
My job is to banish these invocation call from the custom class, to make it "painless".
It should be synchronized like the default events in WinForms (eg the Timer "Elapsed" event, which also needs no invocation when it published values to a text box)
Is it possible to solve this, since the custom class needs to know where to invoke?
Here's the (important part of the) code:
public class ContactSensorHelper
{
public event OnReleaseStateChanged ReleaseStateChanged;
public delegate void OnReleaseStateChanged(ContactSensorEventArgs e);
private ContactSensorEventArgs.ReleaseState recentReleaseState;
public void ReportStateChanged()
{
if (ReleaseStateChanged != null)
ReleaseStateChanged(new ContactSensorEventArgs()
{
State = recentReleaseState
});
}
public class ContactSensorEventArgs : EventArgs
{
//......
public ReleaseState State { get; set; }
//......
public enum ReleaseState
{
FullReleased,
PartlyReleased,
NotReleased
}
}
}
The call from main UI:
public void SensorInit()
{
//....
sensorHelper.ReleaseStateChanged += releaseStateChanged;
//....
}
private void releaseStateChanged(ContactSensorEventArgs e)
{
//example
textBox1.Text = e.State.ToString(); // Thread exception (obviously)
}
Does anybody have me a hint to start?

You could do this by using your own event calling, and storing a reference to the thread, when the event is attached.
With the event add/remove syntax, you can have the caller attach to the event like before, but internally you store a list, with a reference to the thread (using an AsyncOperation) and the delegate to be called (used a Tuple containing both in the example)
Below is an example. I tested it, and it worked as expected when testing, but you might have to add some locking of the list to make it thread safe in case events are added/removed simultaneously.
public class ContactSensorHelper:IDisposable
{
public delegate void OnReleaseStateChanged(ContactSensorEventArgs e);
private ContactSensorEventArgs.ReleaseState recentReleaseState;
public void ReportStateChanged()
{
if (statechangedList.Count > 0)
{
var e = new ContactSensorEventArgs()
{
State = recentReleaseState
};
statechangedList.ForEach(t =>
t.Item1.Post(o => t.Item2((ContactSensorEventArgs)o), e));
}
}
List<Tuple<AsyncOperation, OnReleaseStateChanged>> statechangedList = new List<Tuple<AsyncOperation,OnReleaseStateChanged>>();
public event OnReleaseStateChanged ReleaseStateChanged
{
add
{
var op = AsyncOperationManager.CreateOperation(null);
statechangedList.Add(Tuple.Create(op, value));
}
remove
{
var toremove = statechangedList.Where(t => t.Item2 == value).ToArray();
foreach (var t in toremove)
{
t.Item1.OperationCompleted();
statechangedList.Remove(t);
}
}
}
public void Dispose()
{
statechangedList.ForEach(t => t.Item1.OperationCompleted());
statechangedList.Clear();
}
public class ContactSensorEventArgs : EventArgs
{
//......
public ReleaseState State { get; set; }
//......
public enum ReleaseState
{
FullReleased,
PartlyReleased,
NotReleased
}
}
}

Avoiding null pointer exception in delegates

I'm using delegates
in my c# windows forms application project.Using that I'm trying to remove items in a list box. I'm getting this null pointer exception and can somebody suggest a way to avoid that?
Delegate
public delegate void OrderEventDelegate (Object sender, OrderEventArgs args);
OrderEventArgs class
public class OrderEventArgs
{
private String message;
public String Message
{
get { return message; }
set { message = value; }
}
private int tableNo;
public int TableNo
{
get { return tableNo; }
set { tableNo = value; }
}
}
Class 1
public partial class Class1 : Form
{
private event OrderEventDelegate readyEvent;
public Class1(HomeForm parent, int tableNo)
{
InitializeComponent();
readyEvent -= new OrderEventDelegate(parent.readyOrder);
}
public void button_click()
{
OrderEventArgs readyOrderArg = new OrderEventArgs();
readyOrderArg.TableNo = 1;
readyOrderArg.Message = "123";
readyEvent(this, readyOrderArg);
}
}
Here readyEvent -= new OrderEventDelegate(parent.readyOrder);readyOrder() is the method which remove items in the list, which is located in the 'Homeform'.
Exception

It is possible to initialize C# events with an empty delegate. This way it can always be called safely without a null pointer check. As shown in this answer: https://stackoverflow.com/a/340618/2404788.
public delegate void OrderEventDelegate (Object sender, OrderEventArgs args) = delegate {};

If there's a possibility of something being null and you can do something about it/don't want to critically fail when it is, then check for it:
if (readyEvent != null) {
readyEvent( ... );
}
But the point here, I suppose, is that you don't want this thing to be null; so you should subscribe a handler to the event. I'm not sure why you're trying to remove a new instance of the delegate handler, but to add one you would use +=.

IObserver and IObservable in C# for Observer vs Delegates, Events

All I am trying to do is implementing the observer pattern.
So, I came up with this solution:
We have a PoliceHeadQuarters whose primary job is to send notifications to all those who are subscribed to it. Consider that the DSP, Inspector and SubInspector classes are subscribed to PoliceHeadQuarters.
Using Events and Delegates I wrote
public class HeadQuarters
{
public delegate void NewDelegate(object sender, EventArgs e);
public event EventHandler NewEvent;
public void RaiseANotification()
{
var handler = this.NewEvent;
if (handler != null)
{
handler(this, new EventArgs());
}
}
}
public class SubInspector
{
public void Listen(object sender, EventArgs e)
{
MessageBox.Show(string.Format("Event Notification received by sender = {0} with eventArguments = {1}", sender, e.ToString()));
}
}
public class Inspector
{
public void Listen(object sender, EventArgs e)
{
MessageBox.Show(string.Format("Event Notification received by sender = {0} with eventArguments = {1}", sender, e.ToString()));
}
}
and this is how I invoked it
var headQuarters = new HeadQuarters();
var SubInspector = new SubInspector();
var Inspector = new Inspector();
headQuarters.NewEvent += Inspector.Listen;
headQuarters.NewEvent += SubInspector.Listen;
headQuarters.RaiseANotification();
so, both Inspector and SubInspector classes get notification whenever there the function RaiseANotification() is invoked.
It seems that the DotNet Framework 4, 4.5 supports a new way called IObserver and IObservable.
Can anyone give me a super simple example using IObservable and IObserver pattern for the above scenario? I googled only to find the available examples in the internet too bloated and difficult to understand.
My hinch: (probably i think it's wrong)
class DSP : IObserver //since it observes the headquarters ?
class PoliceHeadQuarters: IObservable // since here's where we send the notifications ?
Thanks in advance.
EDIT: Somebody also said that the MSDN documentation is also incorrect for IObservable #
IObservable vs Plain Events or Why Should I use IObservable?.

Here's a modification of MSDN example to fit your framework:
public struct Message
{
string text;
public Message(string newText)
{
this.text = newText;
}
public string Text
{
get
{
return this.text;
}
}
}
public class Headquarters : IObservable<Message>
{
public Headquarters()
{
observers = new List<IObserver<Message>>();
}
private List<IObserver<Message>> observers;
public IDisposable Subscribe(IObserver<Message> observer)
{
if (!observers.Contains(observer))
observers.Add(observer);
return new Unsubscriber(observers, observer);
}
private class Unsubscriber : IDisposable
{
private List<IObserver<Message>> _observers;
private IObserver<Message> _observer;
public Unsubscriber(List<IObserver<Message>> observers, IObserver<Message> observer)
{
this._observers = observers;
this._observer = observer;
}
public void Dispose()
{
if (_observer != null && _observers.Contains(_observer))
_observers.Remove(_observer);
}
}
public void SendMessage(Nullable<Message> loc)
{
foreach (var observer in observers)
{
if (!loc.HasValue)
observer.OnError(new MessageUnknownException());
else
observer.OnNext(loc.Value);
}
}
public void EndTransmission()
{
foreach (var observer in observers.ToArray())
if (observers.Contains(observer))
observer.OnCompleted();
observers.Clear();
}
}
public class MessageUnknownException : Exception
{
internal MessageUnknownException()
{
}
}
public class Inspector : IObserver<Message>
{
private IDisposable unsubscriber;
private string instName;
public Inspector(string name)
{
this.instName = name;
}
public string Name
{
get
{
return this.instName;
}
}
public virtual void Subscribe(IObservable<Message> provider)
{
if (provider != null)
unsubscriber = provider.Subscribe(this);
}
public virtual void OnCompleted()
{
Console.WriteLine("The headquarters has completed transmitting data to {0}.", this.Name);
this.Unsubscribe();
}
public virtual void OnError(Exception e)
{
Console.WriteLine("{0}: Cannot get message from headquarters.", this.Name);
}
public virtual void OnNext(Message value)
{
Console.WriteLine("{1}: Message I got from headquarters: {0}", value.Text, this.Name);
}
public virtual void Unsubscribe()
{
unsubscriber.Dispose();
}
}
public class Program
{
public static void Main(string[] args)
{
Inspector inspector1 = new Inspector("Greg Lestrade");
Inspector inspector2 = new Inspector("Sherlock Holmes");
Headquarters headquarters = new Headquarters();
inspector1.Subscribe(headquarters);
inspector2.Subscribe(headquarters);
headquarters.SendMessage(new Message("Catch Moriarty!"));
headquarters.EndTransmission();
Console.ReadKey();
}
}

Another suggestion - you probably want to consider leveraging the reactive extensions library for any code using IObservable. The nuget package is Rx-Main and the homepage for it is here: http://msdn.microsoft.com/en-us/data/gg577609.aspx
Update: ReactiveX has been translated to many platforms and languages and is now managed as an open source project. Here is the landing page.
This will save you a lot of boilerplate code. Here's a super simple example:
var hq = new Subject<string>();
var inspectorSubscription = hq.Subscribe(
m => Console.WriteLine("Inspector received: " + m));
var subInspectorSubscription = hq.Subscribe(
m => Console.WriteLine("Sub Inspector received: " + m));
hq.OnNext("Catch Moriarty!");
It will output:
Inspector received: Catch Moriarty!
Sub Inspector received: Catch Moriarty!
Reactive Extensions is a big subject, and a very powerful library - worth investigating. I recommend the hands-on lab from the link above.
You would probably want to embed those subscriptions within your Inspector, SubInspector immplementatinos to more closely reflect your code. But hopefully this gives you an insight into what you can do with Rx.

RegisterCallback<T>(Action<T> func) , how do I store this function pointer in a class?

I'm trying to expose an API such that, I do the following
RegisterCallback<T>(Action<T> func)
{
someObj.FuncPointer = func;
}
Later on, I call func(obj) .. and the obj is of type T that the user said.
More concrete example:
var callbackRegistrar = new CBRegistrar();
callbackRegistrar.RegisterCallback<ISomeClass>(SomeFunc);
public static void SomeFunc(ISomeClass data)
{
//
}
EDIT: So I may not have been clear, so I'll add more code:
I want to make only "one" object of CBRegistrar, and connect it with many Callbacks, as such:
var callbackRegistrar = new CBRegistrar();
callbackRegistrar.RegisterCallback<ISomeClass>(SomeFunc);
callbackRegistrar.RegisterCallback<ISomeOtherClass>(SomeFunc2);
...
In fact the above code is called by reflecting over a directory of plugins.
The user puts this in their code -->
public static void SomeFunc(ISomeClass data)
{
//
}
public static void SumFunc2(ISomeOtherClass data)
{
//
}
It looks to me as if this is not possible using Generics, etc. What it looks like I might have to do is make an interface called IPlugin or something, and ask the user to do this ..
[PluginIdentifier(typeof(ISomeClass))]
public static void SomeFunc(IPluginData data)
{
var castedStuff = data as ISomeClass; // ISomeClass inherits from IPluginData
}
Seems like asking the user to do stuff that we should take care of, but anyway ...

You need a Action<T> func to store it in. There is a semantic check to make here: if someone calls RegisterCallback twice (with different values), do you want to replace the callback, or keep both ? Assuming the latter, someObj probably wants an event (indeed, this entire API could be exposed as an event), so - in the someObj class:
public event Action<T> FuncPointer;
private void InvokeCallback(T data) {
var handler = FuncPointer;
if(handler != null) handler(data);
}
Noting that RegisterCallback could be replaced entirely, still keeping the data on obj:
public event Action<T> Completed {
add { obj.FuncPointer += value; }
remove { obj.FuncPointer -= value; }
}
Then usage would be:
var callbackRegistrar = new CBRegistrar();
callbackRegistrar.Completed += SomeFunc;

Callback functions are not much used in C#. They've been replaced by events which are more elegant and easier to work with.
class CBRegistrar
{
public delegate void ActionRequiredEventHandler(object sender, ISomeClass e);
public event ActionRequiredEventHandler ActionRequired;
void RaiseActionRequiredEvent(ISomeClass parm)
{
if ( ActionRequired != null)
{
ActionRequired(this, parm);
}
}
}
class APIConsumer
{
var callbackRegistrar = new CBRegistrar();
public APIConsumer()
{
callbackRegistrar.ActionRequired += SomeFunc;
}
public void SomeFunc(object sender, ISomeClass data)
{
}
}
If you still want to use Callbacks, you can use Delegates which are more or less function pointer.

The CBRegistrar will need to be generic (if it's OK to keep a single callback type) or it can do some internal casting (if several callback types need to be registered).
public class CBRegistrar<T>
{
private Action<T> callback;
private Dictionary<Type, object> callbackMap;
public CBRegistrar()
{
this.callbackMap = new Dictionary<Type, object>();
}
public void RegisterCallback(Action<T> func)
{
this.callback = func;
}
public void RegisterGenericCallback<U>(Action<U> func)
{
this.callbackMap[typeof(U)] = func;
}
public Action<U> GetCallback<U>()
{
return this.callbackMap[typeof(U)] as Action<U>;
}
}
public interface ISomeClass
{
string GetName();
}
public class SomeClass : ISomeClass
{
public string GetName()
{
return this.GetType().Name;
}
}
namespace ConsoleApplication
{
class Program
{
static void Main(string[] args)
{
var callbackRegistrar = new CBRegistrar<ISomeClass>();
callbackRegistrar.RegisterCallback(SomeFunc);
callbackRegistrar.RegisterGenericCallback<ISomeClass>(SomeFunc);
var someone = new SomeClass();
callbackRegistrar.GetCallback<ISomeClass>()(someone);
}
public static void SomeFunc(ISomeClass data)
{
// Do something
Console.WriteLine(data.GetName());
}
}
}

delegate or reflection?

I have a class with a method in which a string will be passed. That method will do some things to that string and it then passes the string to a certain object which can do other things with the string.
So it basically looks like this:
class Main
{
public Main()
{
strClass str = new strClass(this);
}
public function handler ( )
{
console.log("No string is passed yet, but this method is called from receiveData()");
}
}
class strClass
{
object handler;
public strClass ( handler )
{
// save the object
this.handler = handler;
}
public receiveData ( string str )
{
// This method does some stuff with the string
// And it then passes it on to the supplied object (handler) which will do
// the rest of the processing
// I'm calling the "handler" method in the object which got passed in the
// constructor
Type thisType = this.handler.GetType();
MethodInfo theMethod = thisType.GetMethod("handler");
theMethod.Invoke(this.handler, null);
}
}
Now this code works good, with the reflection stuff. But i was wondering, shouldn't this be possible (and maybe even better?) with delegates?? If so, how can i implement this by using a delegate instead?

Couldn't you use interfaces instead:
interface IStringHandler {
void HandleString(string s);
}
class strClass
{
IStringHandler handler = null;
public strClass(IStringHandler handler)
{
this.handler = handler;
}
public void ReceiveData(string s)
{
handler.HandleString(s);
}
}
class Main : IStringHandler
{
// Your code
}

A delegate is a better option here.
class Main
{
public Main()
{
StrClass str = new StrClass(this.Handler);
}
public void Handler ( )
{
//called from recieve data
}
}
class StrClass
{
readonly Action _handler;
public StrClass ( Action callback)
{
// save the object
this._handler = callback;
}
public void receiveData( string str )
{
this._handler();
}
}

You can do it with an Action like this:
class Main
{
public Main()
{
strClass str = new strClass(newString =>
{
console.log("This string I got back: " + newString);
});
}
}
class strClass
{
Action<string> callback;
public strClass (Action<string> callback)
{
// save the action
this.callback = callback;
}
public receiveData ( string str )
{
// Do something with the string
callback(str);
}
}

Even nicer than using delegates whould be using the
Chain of Responsibility design pattern, which does exactly what you need :).

Firstly, if you must call an unknown method by name, use dynamic - it is heavily optimised for this (although still not a great idea):
((dynamic)handler).handler(); // but please don't use this! see below
However, I would instead look at either an Action<string> (or maybe Func<string,string>), or an interface with a known method on it.

Basically, you want to change how your StrClass object react to data begin received. Sounds like events to me.
something like this, where you have handling methods both in the Main and in a generic HandlerObject:
class StrClass : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged = null;
public void OnPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (PropertyChanged != null)
PropertyChanged(this, e);
}
private string receivedString;
public string ReceivedString
{
get;
set
{
string oldStr = receivedString;
receivedString = value;
PropertyChanged(receivedString, new PropertyChangedEventArgs("ReceivedString"));
}
}
public void receiveData(string str)
{
//event fires here
ReceivedString = str;
}
}
class HandlerObject
{
public void HandlerMethod1(string s)
{
//magic
}
public void HandlerMethod2(string s)
{
//different kind of magic
}
}
class Program
{
static void HandlerMethod3(string s)
{
//another kind of magic!
}
static void Main(string[] args)
{
StrClass class1 = new StrClass();
StrClass class2 = new StrClass();
StrClass class3 = new StrClass();
HandlerObject handler = new HandlerObject();
class1.PropertyChanged += (s, e) => { handler.HandlerMethod1(s.ToString()); };
class2.PropertyChanged += (s, e) => { handler.HandlerMethod2(s.ToString()); };
class3.PropertyChanged += (s, e) => { HandlerMethod3(s.ToString()); };
}
}