8.8 Destroying Objects

Unlike many other programming languages (C, C++, Pascal, etc.), C# provides garbage collection. Your objects are automatically destroyed when you are done with them. You do not need to worry about cleaning up after your objects unless you use unmanaged resources. An unmanaged resource is an operating-system feature outside of the .NET Framework, such as a file handle or a database connection.

If you do control an unmanaged resource, you need to explicitly free that resource when you are done with it. Implicit control over this resource is provided with a destructor, which is called by the garbage collector when your object is destroyed. Note that this material is fairly advanced; it is included here for completeness.

You declare a C# destructor with a tilde as follows:

~MyClass(){}

It is not legal to call a destructor explicitly — your destructor will be called by the garbage collector. If you do handle precious unmanaged resources (such as file handles) that you want to close and dispose of as quickly as possible, you ought to implement the IDisposable interface. (You will learn more about interfaces in Chapter 14.)

The IDisposable interface requires you to create a method named Dispose(), which will be called by your clients.

If you provide a Dispose() method, you should stop the garbage collector from calling your object's destructor. To stop the garbage collector, call the static method GC.SuppressFinalize(), passing in the this reference for your object. Your destructor can then call your Dispose() method. Thus, you might write:

using System;
class Testing : IDisposable
{
  bool is_disposed = false;
  protected virtual void Dispose(bool disposing)
  {
    if (!is_disposed) // only dispose once!
    {
      if (disposing)
      {
        Console.WriteLine("Not in destructor, OK to reference other objects");
      }
      // perform cleanup for this object
      Console.WriteLine("Disposing...");
    }
    this.is_disposed = true;
  }
  
  public void Dispose()
  {
    Dispose(true);
    // tell the GC not to finalize
    GC.SuppressFinalize(this);
  }
  
  ~Testing()
  {
    Dispose(false);
    Console.WriteLine("In destructor.");
  }
}

For some objects, you'd rather have your clients call the Close() method. (For example, Close makes more sense than Dispose() for file objects.) You can implement this by creating a private Dispose() method and a public Close() method and having your Close() method invoke Dispose().

Because you cannot be certain that your user will call Dispose() reliably, and because finalization is nondeterministic (i.e., you can't control when the GC will run), C# provides a using statement to ensure that Dispose() is called at the earliest possible time. The idiom is to declare which objects you are using and then to create a scope for these objects with curly braces. When the close brace is reached, the Dispose() method will be called on the object automatically, as illustrated here:

using System.Drawing;
class Tester
{
   public static void Main()
   {
      using (Font theFont = new Font("Arial", 10.0f))
      {
        // use the font
      }
   }
}

The Font object is created within the using statement. When the using statement ends, Dispose() is called on the Font object.