C# 3.0 includes a new feature known as extension methods, and fiddling with it triggered the idea of creating a mechanism for copying or cloning (virtually) any .NET object or graph of objects. The manifestation of that idea has become a rather decent little framework for copying objects. It performs a deep copy as automatically as it possibly can, and provides mechanisms to easily solve many of the cases which cannot be covered automatically. It is great for copying your custom object hierarchies, and saves you the pain of a solution like implementing
ICloneable for an entire hierarchy of objects. Click here to grab it now, and read on for a presentation.
Let’s start off with a few words on extension methods. They are best explained through an example. Let’s say we want to be able to calculate area given size. Wouldn’t it be nice to be able to add
GetArea to the already existing
Size class? Well, let’s do so!
public static class ExtensionMethods
public static int GetArea(this Size size)
return size.Width * size.Height;
As you can see, the new syntax simply allows you to tell the compiler that the
of this method is a
. This means that the method is an extension of the
As mentioned, I had the idea of extending the very base of the C# class hierarchy (
System.Object) with a method for copying or cloning “any” object. Obviously, the method cannot automatically copy any object, since it cannot possibly know how to construct an object from an arbitrary class. Hence, a small framework needed to be created. The goals were to:
- Enable copying of many objects automatically.
- Enable copying of virtually any object with very little effort.
- Automate and hide away as much as possible (The KISS Principle).
The result is Copyable (pun intended).
The Copyable framework
Copyable is a small framework for copying (or cloning, if you will) objects. The straightforward way of using it is to just reference the assembly it’s in from your project, and start copying!
SomeType instance = new SomeType();
// ...do lots of stuff to the object...
SomeType copy = instance.Copy(); // Create a deep copy
copy is now a deep copy of
instance, no matter how complex the object graph for
instance is. The relations in the
copy graph is the same as in
instance, but all objects in the
copy object graph are copies of those in
For the automated copy to work, though, one of the following statements must hold for
- Its type must have a parameterless constructor, or
- It must be a
- It must have an
IInstanceProvider registered for its type.
Copy method, The
Copyable class and
IInstanceProvider interface are the two major building blocks of the Copyable framework. Each of these blocks enable copying of objects that cannot automatically be copied.
The Copyable base class
Copyable is an abstract base class for objects that can be copied. To create a copyable class, you simply subclass
Copyable and call its constructor with the arguments of your constructor.
class MyClass : Copyable
public MyClass(int a, double b, string c)
: base(a, b, c)
This code above makes
MyClass a copyable class. Note that if
MyClass had had a parameterless constructor, subclassing
Copyable would not be necessary.
MyClass can now be copied just like the previous example.
MyClass a = new MyClass(1, 2.0, "3");
MyClass b = a.Copy();
The introduction of the
Copyable base class solves many problems, but not all. Let’s say you wanted to copy a
System.Drawing.SolidBrush. This class does not have a parameterless constructor, which means it cannot be copied “automatically” by the framework. Also, you cannot alter it so that it subclasses
Copyable. So, what do you do? You create an instance provider.
The IInstanceProvider interface
An instance provider is defined by the interface
IInstanceProvider. As the name clearly states, the implementation is a provider of instances. One instance provider can provide instances of one given type. The Copyable framework automatically detects
IInstanceProvider implementations in all assembies in its application domain, so all you need to do to create a working instance provider is to define it. No registration or other additional operations are required. To simplify the implementation of instance providers and the
IInstanceProvider interface, an abstract class
InstanceProvider is included in the framework.
public class SolidBrushProvider</dt>
public override SolidBrush CreateTypedCopy(SolidBrush s)
return new SolidBrush(s.Color);
This implementation will be used automatically by the Copyable framework. NOTE: To be usable, the instance provider MUST have a parameterless constructor.
The instance provider pattern does not solve the case where you want different initial states for your SolidBrush instances depending on which context you use them for copying. For those cases, an overload of
Copy() exists which takes an already created instance as an argument. This argument will become the copy.
SolidBrush instance = new SolidBrush(Color.Red);
instance.Color = Color.Black;
SolidBrush copy = new SolidBrush(Color.Red);
instance.Copy(copy); // Create a deep copy
In this example,
copy is now of the color
Limitations and pitfalls
Although this solution works in most cases, it’s not a silver bullet. Be aware when you copy classes that hold unmanaged resources such as handles. If these classes are designed on the premise that their resources are exclusive to them, they will manage them as they see fit. Imagine if you copied a class which holds a handle, disposed one of the instances, and continued using the copy. The handle will (probably) be freed by the original instance, and the copy will generate an access violation by attempting reading or writing freed memory.
That’s it! The Copyable framework can be downloaded from here. For those interested in reading more on extension methods, For additional information, MSDN provides an excellent explanation in the C# Programming Guide, and Scott Guthrie has an introduction article here.
Enjoy Copyable, and please let me know if you find it useful or come across any problems with it.
UPDATE 2009-12-11: Due to popular demand, I have made the source code for Copyable available under the MIT license. The source can be downloaded here.
UPDATE 2010-01-31: The requirement of parameterless constructors has been removed in the latest version of Copyable available on GitHub. A new release will follow soon.