Beyond The Aspect

Political repression in Denver, USA, during the Democrat National Convention. Unbelievable.

A good e-book from vendors of a parallel computing toolkit. Marketing material, but of high value for everyone.

e-Book: How to Survive the Multicore Software Revolution (or at Least Survive the Hype)

Object cloning are one of the patterns we find in most software projects. It seems simple, so we easily run into a trivial implementation. But as code gets more complex, the cloning implementation rapidly gets larger and larger — and ugly. To keep cloning simple, I forced myself to follow a simple design pattern. Here is it:

   1:  class Cloneable : ICloneable

   2:  {

   3:     public object Clone()

   4:     {

   5:      Cloneable clone = this.MemberwiseClone();

   6:      this.CopyTo( clone );

   7:      return clone;

   8:     }

   9:  

  10:     protected virtual void CopyTo(Cloneable clone)

  11:     {

  12:        // Children should implement this method to

  13:        // clone cloneable properties.

  14:     }

  15:  }

The advantage of using MemberwiseClone is that I easily create a new instance. Additionally, it is really fast (the Rotor implementation uses memcpy). No need of reflection. It actually makes a shallow copy, so a big part of the job is already done:

• We have a shallow copy of fields of value types.
• Immutable reference types (like string) do not need to be cloned.

The goal of the CopyTo method is to take care of fields requiring special attention. The design pattern says that every class should call base.CopyTo() and make a deep copy of the fields it defines, for instance:

   1:  public class Address : Cloneable

   2:  {

   3:    // Immutables.

   4:     public string AddressLine1, AddressLine2,
                        AddressLine3, Zip, City;

   5:     // Value type.

   6:     public int CountryID;

   7:  

   8:     // No CopyTo: a shallow copy is sufficient.

   9:  

  10:  }

  11:  

  12:  public class Party : Cloneable

  13:  {

  14:       public int ID; // Value-type.

  15:       public string FirstName, LastName;  // Immutables.

  16:       public Address LegalAddress; // Cloneable.

  17:  

  18:      protected override void CopyTo(Cloneable clone)

  19:      {

  20:         base.CopyTo(clone);

  21:        ((Party) clone).LegalAddress =
                 (Address) this.LegalAddress.Clone();

  22:      }

  23:  }

  24:  

  25:  public class Customer : Party

  26:  {

  27:       public Address DeliveryAddress;

  28:  

  29:      protected override void CopyTo(Cloneable clone)

  30:      {

  31:         base.CopyTo(clone);

  32:        ((Customer) clone).DeliveryAddress =
                (Address) this.DeliveryAddress.Clone();

  33:      }

  34:  

  35:  }

This may looks trivial, and indeed it is, but it works so well!

Implementing CopyTo() Automatically

You could expect me on that point. CopyTo is basically plumbing code no one wants to write. There should be a way to implement it automatically.

Can you use PostSharp for this? Of course! The PostSharp implementation of this pattern is actually a part of the sample

PostSharp.Samples.Librarian, a sample released to the public domain (and packaged with PostSharp).

It generates code to clone cloneable fields, and raise a build-time error when it does not know how to cope with that field type. I won’t comment further on that until you ask; please refer to the sample.

However, I don’t use PostSharp for this pattern. The reason is that the PostSharp-based solution is implemented using the low-level APIes and I don’t want to use them unless it is really necessary. And since I don’t have any special requirement (like performance or CAS), I use… System.Reflection.

There is an old adage telling that when someone has a hammer, everything looks like a nail. I try not to fall in that trap! The implementation simply iterate over all fields (or properties) of the type, reads them from the source object and assigns the value to the clone.

Cloning Trees

What is a little more difficult is to clone trees when children have a relationship to the parent. I have defined an interface:

   1:  public interface ICloneableTreeItem : ICloneable

   2:  {

   3:       ICloneableTreeItem Clone(object parent);

   4:  }

The implementation pattern is of course much similar.

Summary

Simple patterns keep simple things simple, and Reflection is not Evil!