C++/CLI, Finalize and Dispose

原文地址

http://weblogs.thinktecture.com/cnagel/2006/04/ccli-finalize-and-dispose.html

C++/CLI has a great syntax for dealing with resources. The syntax of C++/CLI is simpler than C#.

To implement the interface IDisposable, with C# this syntax is needed:

// C#
public class Resource : IDisposable
{
   public void Dispose()
   {
      Console.WriteLine("release resource");
   }
}

C++/CLI is using the destructor syntax to implement the interface IDisposable:

// C++/CLI
public ref class Resource
{
public:
   ~Resource()
   {
      Console::WriteLine("release resource");
   }
};

If the Resource class allocates native resources, the Finalize method of the base class should be overridden. With C# the Finalize method is overridden using a destructor syntax:

// C#
public class Resource : IDisposable
{
   ~Resource()
   {
      Dispose();
   }

   public void Dispose()
   {
      Console.WriteLine("release resource");
   }
}

C++/CLI already uses the destructor syntax to implement IDisposable. To implement the Finalize method, a new syntax is available:

// C++/CLI
public ref class Resource
{
public:
   ~Resource()
   {
      this->!Resource();
   }
protected:
   !Resource()
   {
      Console::WriteLine("release resource");
   }
};

If the Resource class contains embedded objects that should be disposed, this pattern is needed with C#:

// C#
public class Resource : IDisposable
{
   private EmbeddedResource embedded;

   public Resource()
   {
      Console.WriteLine("allocating resource");
      embedded = new EmbeddedResource();
   }

   ~Resource()
   {
      Dispose(false);
   }

   protected virtual void Dispose(bool disposing)
   {
      Console.WriteLine("release resource");
      if (disposing)
      {
         embedded.Dispose();
      }
   }

   public void Dispose()
   {
      Dispose(true);
   }
}

This pattern is already implemented with ~Resource and !Resource:

// C++/CLI
public ref class Resource
{
private:
   EmbeddedResource^ embedded;
public:
   Resource()
   {
      embedded = gcnew EmbeddedResource();
   }
   ~Resource()
   {
      delete embedded;
      this->!Resource();
   }
protected:
   !Resource()
   {
      Console::WriteLine("release resource");
   }
};

C++/CLI also has advantages using and releasing the Resource class. With C#, the Dispose method can be invoked directly:

// C#
Resource r = new Resource();
r.Foo();
r.Dispose();

This is similar to the following C++/CLI constructs. The C++ delete gets compiled to invoking the Dispose method:

// C++/CLI
Resource^ r = gcnew Resource();
r->Foo();
delete r;

With both of these usages it was not taken into account that the Foo method may throw an exception, and thus the Dispose method is not invoked. C# has the using statement for dealing with this easily. At the end of the using scope, Dispose is invoked.

// C#
using (Resource r = new Resource())
{
   r.Foo();
}

Instead of the using statement, C++/CLI allows declaring the object locally. The object is still allocated on the managed heap. However, when the object gets out of scope, the Dispose method is invoked. This is simpler than the C# using statement.

// C++/CLI
Resource r;
r.Foo();