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Finalize, Dispose and Unmanaged Resources


There may be situations where some class has control over resources that are not managed by the runtime such as window handles, database connections, etc. and these resources need to be released as they occupy more memory or affect performance. It can be done explicitly by calling Dispose () to release these objects immediately when they are no more needed. Similarly these objects can also be released implicitly by implementing Finalize() so that they get collected by GC in case someone forgets to call dispose () on these objects. Also since calling Finalize impacts the performance, calls to Finalize can be prevented by invoking the GC.SuppressFinalize method but it should be made sure that it only gets called after Dispose method has completed successfully.

Public class Base:IDisposable
{
       Private bool disposed = false;
      //Implement IDisposable.
      Public void Dispose() 
     {
         Dispose(true);
         GC.SuppressFinalize(this);
      }

Protected virtual void Dispose(bool disposing)
{
    if (!disposed)
   {
       if (disposing)
      {
          // Free managed objects.
       }
       // Free unmanaged objects.
       disposed = true;
    }
}
     // using destructor for finalization.
    ~Base()
    {
        Dispose (false);
     }
}

Public class Derived: Base
{
     Private bool disposed = false;
     Protected override void Dispose(bool disposing)
     {
          if (!disposed)
         {
             if (disposing)
            {
                 // Release managed resources.
             }
             // Release unmanaged resources
             disposed = true;
        }
        base.Dispose(disposing);
     }
}

Public class Test
{
      Public static void Main()
      {
          //Scenario#1
          // Dispose is not called; Finalize will be called
          Derived a1 = new Derived();
          . . . . . . . .

          //Scenario#2
          //Dispose is implicitly called in the scope of the using statement.
          using (Derived a2 = new Derived())
          {
                . . . . . . . .
           }

          //Scenario #3
          // Dispose is explicitly called.
          Derived a3 = new Derived();
          . . . . . . . .
          a3.Dispose();
      }
 }

In the above example, object a2 and a3 are released by calling Dispose method of the base class as a result of which the overloaded dispose method of the derived class and then the base class dispose method gets called to release the managed and unmanaged resources .It can be also noted that the call to finalize() has been suppressed at the end of dispose. In case of object a1, since dispose() method is not called explicitly, finalize method gets called by GC which in turn calls the dispose method of derived class where the unmanaged resources gets released.

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