Skip to main content

[Code refactoring] Releasing resources that you still need

Let’s look over the following code and see if we can find the bug. When the Callback method is called, the _callbackAction field is null all the time (except 2-3 exception).
public class FooReader: IDisposable
{
  private ExternalComResource _externalResource;

  private Action<Foo> _callbackAction;
  
  public void Start(Action<Foo> callbackAction)
  {            
    lock (this)
    {
      _callbackAction = callbackAction;                               

      _externalResource = new ExternalComResource();
      ...
      _externalResource.Callback += Callback;
      _externalResource.Start(1);      
    }
  }
  
  public void Stop()
  {
    StopAndReleaseResources();
  }

  private void Callback(object source, EventArgs e)
  {
    ...
    StopCapture();            
    ...
    if (_callbackAction != null)
    {
      _callbackAction.Invoke(someData);
    }
    ...
  }       

  public void Dispose()
  {
    StopAndReleaseResources();
  }

  private void StopAndReleaseResources()
  {
    if (_externalResource != null)
    {
      lock (this)
      {
        if (_externalResource != null)
        {
          _externalResource.Stop();
          _externalResource.Dispose();
          _externalResource = null;
          _callbackAction = null;
        }
      }
    }
  }
}
Did you spotted the problem?
Well, the root cause of the problem is ‘Stop’ method. If we look closely you will see that the callback field is set to NULL when the results comes.
This is the kind of bug where you can spend 2-3 hours, trying to understand why you cannot access local fields from other thread, looking over AppDomain, MarshalByRefObject and what the hell is panning.
What you should never do: You should never use the action that is executed in the Dispose in other method because you know that resources are disposed. You could dispose more than you want.
The code should look like this this (but we still use the 'Dispose' method for Stop):
public class FooReader: IDisposable
{
  private ExternalComResource _externalResource;

  private Action<Foo> _callbackAction;
  
  public void Start(Action<Foo> callbackAction)
  {            
    lock (this)
    {
      _callbackAction = callbackAction;                               

      _externalResource = new ExternalComResource();
      ...
      _externalResource.Callback += Callback;
      _externalResource.Start(1);      
    }
  }
  
  public void Stop()
  {
    StopAndReleaseResources();
  }

  private void Callback(object source, EventArgs e)
  {
    ...
    StopCapture();            
    ...
    if (_callbackAction != null)
    {
      _callbackAction.Invoke(someData);
    }
    ...
  }       

  public void Dispose()
  {
    StopAndReleaseResources();
  }

  private void StopAndReleaseResources()
  {
    if (_externalResource != null)
    {
      lock (this)
      {
        if (_externalResource != null)
        {
          _externalResource.Stop();
          _externalResource.Dispose();
          _externalResource = null;  
          //_callbackAction = null;        
        }
      }
    }
  }
}

Comments

Popular posts from this blog

How to check in AngularJS if a service was register or not

There are cases when you need to check in a service or a controller was register in AngularJS.
For example a valid use case is when you have the same implementation running on multiple application. In this case, you may want to intercept the HTTP provider and add a custom step there. This step don’t needs to run on all the application, only in the one where the service exist and register.
A solution for this case would be to have a flag in the configuration that specify this. In the core you would have an IF that would check the value of this flag.
Another solution is to check if a specific service was register in AngularJS or not. If the service was register that you would execute your own logic.
To check if a service was register or not in AngularJS container you need to call the ‘has’ method of ‘inhector’. It will return TRUE if the service was register.
if ($injector.has('httpInterceptorService')) { $httpProvider.interceptors.push('httpInterceptorService&#…

ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded

Today blog post will be started with the following error when running DB tests on the CI machine:
threw exception: System.InvalidOperationException: The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer' registered in the application config file for the ADO.NET provider with invariant name 'System.Data.SqlClient' could not be loaded. Make sure that the assembly-qualified name is used and that the assembly is available to the running application. See http://go.microsoft.com/fwlink/?LinkId=260882 for more information. at System.Data.Entity.Infrastructure.DependencyResolution.ProviderServicesFactory.GetInstance(String providerTypeName, String providerInvariantName) This error happened only on the Continuous Integration machine. On the devs machines, everything has fine. The classic problem – on my machine it’s working. The CI has the following configuration:

TeamCity.NET 4.51EF 6.0.2VS2013
It seems that there …

Fundamental Books of a Software Engineer (version 2018)

More then six years ago I wrote a blog post about fundamental books that any software engineer (developer) should read. Now it is an excellent time to update this list with new entries.

There are 5 different categories of books, that represent the recommended path. For example, you start with Coding books, after that, you read books about Programming, Design and so on.
There are some books about C++ that I recommend not because you shall know C++, only because the concepts that you can learn from it.

Coding

Writing solid codeCode completeProgramming Pearls, more programming pearls(recommended)[NEW] Introduction to Algorithms

Programming

Refactoring (M. Fowler)Pragmatic ProgrammerClean code[NEW] Software Engineering: A Practitioner's Approach[NEW] The Mythical Man-Month[NEW] The Art of Computer Programming

Design

Applying UML and Patterns (GRASP patterns)C++ coding standards (Sutter, Alexandrescu)The C++ programming language (Stroustrup, Part IV)Object-oriented programming (Peter Coad)P…