Skip to main content

Field like events and polymorphic invocation

Ce vi se pare gresit in codul de mai jos:
public class Base
{
public virtual event EventHandler EventOne;
}
public class Derived : Base
{
public override event EventHandler EventOne;
public void DoAction()
{
...
EventOne(this, args);
}
}
In clasa de baza avem un eveniment la care se poate face override in orice clasa derivata. Desi la prima vedere totul pare sa fie in ordine daca ne uitam mai atent, putem sa observam o problema care poate sa apara din cauza la override.
EventOne o sa existe in doua locatii diferite, atat in clasa de baza cat si in clasa derivata. Din aceasta cauza o sa fie momente cand EventOne din clasa de baza nu o sa fie niciodata instantiat.
Din aceasta cauza, in cazul in care evenimentul este apelat din clasa de baza ne putem trezi cu un comportament ciudat, precum NullReferenceException sau sa se execute cod la aruncarea unui eveniment pe care noi nu ne-am astepta sa se execute.
Ce putem sa facem ca sa rezolvam aceasta problema?
Avem la indemana doua solutii. Prima varianta este sa eliminam virtual si override. O sa fie nevoie sa ne declaram o metode de tip "event-firing" in clasa de baza. Aceasta metoda o sa fie adaugata in clasa de baza si tot ce va face este sa arunce evenimentul nostru. In clasa derivata nu se va folosii direct evenimentul ci se va apela metoda pe care noi am declarato in clasa de baza. Iar aceata la randul ei o sa arunce evenimentul. Mai jos puteti sa gasiti implementarea propusa:
public class Base
{
public event EventHandler EventOne;
public void FireEventOne(object sender, EventArgs args)
{
if (EventOne != null)
{
EventOne(sender, args);
}
}
}
public class Derived
{
public void DoAction()
{
...
FireEventOne(this, args);
}
}

In varianta data mai sus, clasa derivata nu mai poate sa faca override la eveniment, aceasta putand doar sa se inscrie la acest eveniment. Field-ul EventOne nu o sa mai fie duplicat in clasa de baza. Prim aceasta metoda am eliminat orice comportament ciudat care ar fi putut sa apara.
Daca avem nevoie sa facem override, atunci va recomand sa mergeti pe varianta in care in clasa de baza evenimentul sa fie declarat ca si abstract( nu consider aceasta variata ca fiind cea mai sanatoasa).
public class Base
{
public abstract event EventHandler EventOne;
}
Va recomand sa va ganditi de doua ori daca chiar aveti nevoie de un eveniment abstract. S-ar putea ca design-ul pe care l-ati ales sa nu fie cel mai bun.

Comments

  1. Pai cred ca prima intrebare ar fi: de ce ar avea nevoie cineva sa declare un event .. virtual si sa faca override la el?

    La urma urmei, event EventHandler nu e decat o colectie de delegates..

    Eventual daca vrea sa customizeze modul in care sunt stocati acei delegates folosing add/remove accessors sau sa fie thread-safe, da' ..

    ReplyDelete
  2. Si mie mi se pare un pic trasa de par. Iar FireEventOn - numita , de obicei, OnEventOne e ceea ce fac de obicei.

    ReplyDelete
  3. In doua proiecte diferite am vazut problema asta. Partea buna ii ca ReSharper-ul urla destul de tare cand vede probleme de genul acesta.

    ReplyDelete
  4. La http://blogs.msdn.com/b/samng/archive/2007/11/26/virtual-events-in-c.aspx se pare ca explica un nene de la Microsoft mai in detaliu care e problema, si de ce nu o pot fixa cum ar vrea..

    ReplyDelete

Post a Comment

Popular posts from this blog

Windows Docker Containers can make WIN32 API calls, use COM and ASP.NET WebForms

After the last post , I received two interesting questions related to Docker and Windows. People were interested if we do Win32 API calls from a Docker container and if there is support for COM. WIN32 Support To test calls to WIN32 API, let’s try to populate SYSTEM_INFO class. [StructLayout(LayoutKind.Sequential)] public struct SYSTEM_INFO { public uint dwOemId; public uint dwPageSize; public uint lpMinimumApplicationAddress; public uint lpMaximumApplicationAddress; public uint dwActiveProcessorMask; public uint dwNumberOfProcessors; public uint dwProcessorType; public uint dwAllocationGranularity; public uint dwProcessorLevel; public uint dwProcessorRevision; } ... [DllImport("kernel32")] static extern void GetSystemInfo(ref SYSTEM_INFO pSI); ... SYSTEM_INFO pSI = new SYSTEM_INFO(

Azure AD and AWS Cognito side-by-side

In the last few weeks, I was involved in multiple opportunities on Microsoft Azure and Amazon, where we had to analyse AWS Cognito, Azure AD and other solutions that are available on the market. I decided to consolidate in one post all features and differences that I identified for both of them that we should need to take into account. Take into account that Azure AD is an identity and access management services well integrated with Microsoft stack. In comparison, AWS Cognito is just a user sign-up, sign-in and access control and nothing more. The focus is not on the main features, is more on small things that can make a difference when you want to decide where we want to store and manage our users.  This information might be useful in the future when we need to decide where we want to keep and manage our users.  Feature Azure AD (B2C, B2C) AWS Cognito Access token lifetime Default 1h – the value is configurable 1h – cannot be modified

What to do when you hit the throughput limits of Azure Storage (Blobs)

In this post we will talk about how we can detect when we hit a throughput limit of Azure Storage and what we can do in that moment. Context If we take a look on Scalability Targets of Azure Storage ( https://azure.microsoft.com/en-us/documentation/articles/storage-scalability-targets/ ) we will observe that the limits are prety high. But, based on our business logic we can end up at this limits. If you create a system that is hitted by a high number of device, you can hit easily the total number of requests rate that can be done on a Storage Account. This limits on Azure is 20.000 IOPS (entities or messages per second) where (and this is very important) the size of the request is 1KB. Normally, if you make a load tests where 20.000 clients will hit different blobs storages from the same Azure Storage Account, this limits can be reached. How we can detect this problem? From client, we can detect that this limits was reached based on the HTTP error code that is returned by HTTP