I am developing a Silverlight 3 application and I would like to delegate all unexpected error handling in a single instance of a class I have named ErrorHandler. This class has one method named HandleApplicationException, plus a couple of other methods to handle more specialized errors.
In my application I am using Unity for dependency injection, but since I want the error handling object to be available even when the Unity container is not yet set up, I register the object as AppDomain global data in the App class constructor, this way:
public App()
{
this.Startup += this.Application_Startup;
this.Exit += this.Application_Exit;
this.UnhandledException += this.Application_UnhandledException;
AppDomain.CurrentDomain.SetData("ErrorHandler", new ErrorHandler());
InitializeComponent();
}
And in case of unhandled exception, I retrieve the error handler object and use it this way:
private void Application_UnhandledException(object sender, ApplicationUnhandledExceptionEventArgs e)
{
e.Handled = true;
var errorHandler = AppDomain.CurrentDomain.GetData("ErrorHandler") as ErrorHandler;
Debug.Assert(errorHandler != null, "No ErrorHandler registered.");
errorHandler.HandleApplicationException(e.ExceptionObject);
}
The problem is that the AppDomain.GetData method in the Application_UnhandledException method is throwing a MethodAccessException. I don't understand why, as I am just invoking a public method on the AppDomain class. I have used a similar approach in other applications and it worked fine (anyway these were not Silverlight applications).
So, what's going on? Am I doing something wrong?
Ok, I got it. From MSDN documentation:
This member has a
SecurityCriticalAttribute attribute,
which restricts it to internal use by
the .NET Framework for Silverlight
class library. Application code that
uses this member throws a
MethodAccessException.
I have resorted to storing the error handler in a public property in the App class, then I access it using ((App)Application.Current).ErrorHandler. I don't like doing things this way but I suppose it is ok in this special case.
Why can't you just use a static instance of ErrorHandler? I.e. have something like ErrorHandler.Current?
It looks like you're trying to manually construct a poor man's IoC framework to be honest.
Consider doing some research on Unity/Ninject and see for yourself why strongly-typed decoupling is better.
Related
I'm trying to follow along with Jonathan Worthington's airport announcement example in An Event-driven and Reactive Future
It compiles.
The problem: SayGateChange is never called. I'm new to Rx. I must be leaving something out. What I have here is his code as exactly as I could transcribe it. Sadly, there is no source available online.
AddGateChange is supposed to push a new item onto EventStreams.GateChanged, which in turn is supposed to be watched by Announcer.Announcements, which is supposed to be watched by SayGateChange.
I'm in Windows forms, not WPF, if that makes a difference.
I will gladly put it into a console app or LinqPad if that will make it work.
using Microsoft.VisualBasic;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
using System.Collections.ObjectModel;
using System.Collections.Specialized;
using System.Reactive.Concurrency;
using System.Reactive.Linq;
using System.Reactive.Subjects;
using System.Threading;
public class frmAnnouncements
{
Announcer _Announcer = new Announcer();
ObservableCollection<string> Announcements = new ObservableCollection<string>();
private void frmRx_Load(System.Object sender, System.EventArgs e)
{
PopulateAnnouncements();
AddGateChange();
}
private void AddGateChange()
{
EventStreams.GateChanged.OnNext(new GateChanged {
Destination = "DAL",
FlightCode = 1503
});
}
private void PopulateAnnouncements()
{
_Announcer.Announcements.ObserveOnDispatcher().Subscribe(SayGateChange);
}
private void SayGateChange(string Message)
{
Interaction.MsgBox(Message);
}
public class GateChanged
{
public string FlightCode;
public string Destination;
}
public class EventStreams
{
public static Subject<GateChanged> GateChanged = new Subject<GateChanged>();
}
public class Announcer
{
public Announcer()
{
this.Announcements = EventStreams.GateChanged.Select(e => string.Format("gate change {0} to {1} ", e.FlightCode, e.Destination));
}
public IObservable<string> Announcements;
}
public frmAnnouncements()
{
Load += frmRx_Load;
}
}
As #Enigmativity stated, using ObserveOnDispatcher() is a problem - although without looking at Interaction.MsgBox its hard to be 100% certain it's the whole story - I guess it may be in the video, but it's rather long and I didn't watch it all.
The use of ObservableOnDispatcher() suggests you have pulled in the wrong nuget package for Rx:
For WPF applications, use rx-xaml (deprecated synonym rx-wpf), which provides the extension method ObserveOnDispatcher()
For Winforms applications, use rx-winforms, which provides the extension method overload ObserveOn(Control)
Both Winforms and WPF have a similar design where the user interface runs on a dedicated thread. In Winforms this is known as the "UI Thread" and in WPF as the "Dispatcher". Although the concept is very similar, the implementation is quite different.
ObserveOnDispatcher in WPF will cause the observer notifications OnXXX to be invoked on the dispatcher thread.
In WinForms, where you use ObserveOn(this), the this will generally be the form itself. For any WinForms control, this will locate the control's SynchronizationContext and Post OnXXX notifications to that.
Both overloads are smart in that invocations are direct if you happen to be on the correct Dispatcher thread or UI thread already.
I do seem to remember that WinForms is a lot more tolerant of updating UI off the UI thread - although this problem occurs in WPF too. This isn't a good thing, since it can lead to unpredictable results that are hard to debug. I note that the WinForms MessageBox.Show method, for example, doesn't care which thread it is invoked on since it creates it's own window. In general, use of some form of ObserveOn/ObserveOnDispatcher is always recommended in UI scenarios.
For this reason, it's a good idea to understand how these work in detail. For this, and to learn about the related SubscribeOn, have a look at this question.
I am surprised that you didn't get an informative InvalidOperationException stating that "The current thread has no Dispatcher associated with it." I can only think some other part of your code is swallowing exceptions, or you are using WPF code in your app as well and a Dispatcher had been created associated with the Winforms UI thread. That code behind Interaction.MsgBox is probably to blame for swallowing an error. Either way, I suggest removing rx-xaml to avoid confusion.
I am using entity framework to persist data in a N-tier Wpf application. My dbcontext is shared amongst all repositories and is never disposed. When I persist data I mark an object as modifed and try to save changes. If an error accurs while persisting the object the object is still markted as modifed and if the user aborts the current opperation he will get the same error when saving another object.
I have solved this by overriding SaveChanges in my dbcontext and if any error accurs I accept all changes(see code below). So if an error accurs the object and all objects are marked unchanged even if theyr'e not persited.
This doesn't feel right...
Does anyone agree with this solution?
Another solution would be to new the dbcontext in each method in my repositores and disposing of them right away. That will make my repositories more complicated and "noicy" and I will also lose the ability to lazy load data...
Does anyone have a different solution for me?
//In my repositories
public void UpdateObject(Object object)
{
dbContext.Entry(object).State = EntityState.Modified;
dbContext.SaveChanges();
}
//In my dbcontext class
private ObjectContext ObjectContext()
{
return (this as IObjectContextAdapter).ObjectContext;
}
public override int SaveChanges()
{
try
{
return base.SaveChanges();
}
catch (Exception)
{
ObjectContext().AcceptAllChanges();
throw;
}
}
Our team uses an approach similar to below:
Repository:
public class StudentRepository
{
private readonly MyEntities _context;
public StudentRepository(MyEntities context)
{
_context = context;
}
// Basic CRUD methods etc
}
Business Logic:
public AddStudent(Student student)
{
using( var context = new MyEntities())
{
var studentrepo = new StudentRepository(context);
studentrepo.Add(student);
context.SaveChanges();
}
}
This is an oversimplified example, but should give you an idea. To reduce code, we also use a base generic repository class for the CRUD methods.
If the project we are working on includes a web service, we instantiate the dbcontext in the API Controller and override the Dispose method to get rid of it.
Having such a long lived context is not a good idea. It will get large and slow with all the entities and changes being tracked, concurrency related issues may arise and exceptions thrown by your context can impact your entire application.
http://msdn.microsoft.com/en-us/data/jj729737
Another solution would be to new the dbcontext in each method in my
repositores and disposing of them right away. That will make my
repositories more complicated and "noicy" and I will also lose the
ability to lazy load data
In a disconnected scenario I would create and dispose with each request/unit of work. Concerned about your repos getting complicated? Then don't use this extra layer of abstraction. Are the repos really necessary? What do you gain over using the DbContext directly?
As for lazy loading I think in a disconnected n-tier scenario that lazy loading is not really appropriate. You should probably use eager loading of required data for your view or have separate method calls to get the related data.
I wrote an overload for my DomainService class. Problem is, when I recompile, it's not showing up as an overload for my DomainContext. What's wrong? Here is a code sample:
[EnableClientAccess]
public class FoodDomainService : LinqToEntitiesDomainService<FoodEntities>
{
public FoodDomainService(CultureInfo cultureInfo)
{
Thread.CurrentThread.CurrentCulture = cultureInfo;
}
}
And this doesn't work:
FoodDomainContext _foodContext = new FoodDomainContext(Thread.CurrentThread.CurrentCulture);
I get an error that there is no overload matching that. Am I not allowed to do this? Do I need an attribute of some kind?
You are not allowed to do this. When newing up the context from your Silverlight client, you are not directly intantiating your service. Instead, you instantiate a proxy class that was generated by RIA Services, and that proxy class will then call your service. This is why you don't see your constructor: because RIA did not generate it in your proxy.
Doing what you're trying to do would also implicate that there is a round-trip to the server at the time of newing up that FoodDomainContext class, which is not going to happen, because you need to complete the initialisation of that object before you can do so.
Anyway, instead of that you can create a method called SetCurrentCulture() and then call it after initializing the proxy.
This will not work because DomainContext is generated on client code of silverlight, click on view all folders or jump to definition and you will see that code generated will not contain your extra constructor.
Instead you will have to create a method in your domain service and pass information to server.
public SetCultreInfo(int lang,...)
{
.. set culture info
}
On your client, inside constructor you should call,
public MyDomainContext()
{
this.SetCulture(....);
}
I am using Ninject as DI container in a Silverlight application. Now I am extending the application to support interception and started integrating DynamicProxy2 extension for Ninject. I am trying to intercept call to properties on a ViewModel and ending up getting following exception:
“Attempt to access the method failed: System.Reflection.Emit.DynamicMethod..ctor(System.String, System.Type, System.Type[], System.Reflection.Module, Boolean)”
This exception is thrown when invocation.Proceed() method is called. I tried two implementations of the interceptor and they both fail
public class NotifyPropertyChangedInterceptor: SimpleInterceptor
{
protected override void AfterInvoke(IInvocation invocation)
{
var model = (IAutoNotifyPropertyChanged)invocation.Request.Proxy;
model.OnPropertyChanged(invocation.Request.Method.Name.Substring("set_".Length));
}
}
public class NotifyPropertyChangedInterceptor: IInterceptor
{
public void Intercept(IInvocation invocation)
{
invocation.Proceed();
var model = (IAutoNotifyPropertyChanged)invocation.Request.Proxy;
model.OnPropertyChanged(invocation.Request.Method.Name.Substring("set_".Length));
}
}
I want to call OnPropertyChanged method on the ViewModel when property value is set.
I am using Attribute based interception.
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class NotifyPropertyChangedAttribute : InterceptAttribute
{
public override IInterceptor CreateInterceptor(IProxyRequest request)
{
if(request.Method.Name.StartsWith("set_"))
return request.Context.Kernel.Get<NotifyPropertyChangedInterceptor>();
return null;
}
}
I tested the implementation with a Console Application and it works alright.
I also noted in Console Application as long as I had Ninject.Extensions.Interception.DynamicProxy2.dll in same folder as Ninject.dll I did not have to explicitly load DynamicProxy2Module into the Kernel, where as I had to explicitly load it for Silverlight application as follows:
IKernel kernel = new StandardKernel(new DIModules(), new DynamicProxy2Module());
Could someone please help? Thanks
Reflection can be really tricky in silverlight because of security issues.
Check Gabe's answer for this question, it's the same problem.
The good news is that you can achieve the same functionality you want using dynamic instead of proxies. Just extend your ViewModel from DynamicObject and override the TrySetMember method.
I hope it helps :)
I'm looking for a way to persist Silverlight objects to a user's PC, then re-hydrate them so the user can finish editing them.
Serialising with DataContractSerializer and persisting to IsolatedStorageFile works fine. However, deserialising causes a problem. Here's the code that causes the failure:
private string _FirstNames = string.Empty;
public string FirstNames
{
get { return _FirstNames; }
set
{
new PersonNameValidator().Validate(value); //<-- BOOM 8(
Set(ref _FirstNames, value, () => this.FirstNames);
}
}
The deserialiser calls the property setter, which in turn throws an exception and aborts the deserialisation.
I've tried explicitly applying DataContract/DataMember/IgnoreDataMember attributes, but then it doesn't play nicely with private fields:
System.Security.SecurityException
occurred Message="The data contract
type
'Trident.Model.Journey.JourneyApplication'
cannot be serialized because the
member '_TravellerSavingsAmount' is
not public. Making the member public
will fix this error. Alternatively,
you can make it internal, and use the
InternalsVisibleToAttribute attribute
on your assembly in order to enable
serialization of internal members -
see documentation for more details. Be
aware that doing so has certain
security implications."
How can I bypass the property setters during deserialisation?
I'd like to keep my classes focused on the domain, and not too polluted with infrastructure concerns.
A couple of ideas:
serialize a property that is used only for serialization thereby bypassing any validation
serialize a parent class and use a derived class for validation