When you ask around how to implement deep cloning in .NET, the first, second and third
recommendation you’ll hear will be to use the BinarySerializer to save your
object tree to memory and then load it again, thereby creating a copy. While simple to
implement, it’s incredibly slow, uses up several times the memory of the entire object
tree and most implementations I’ve seen even required all cloned classes to be serializable.
With .NET’s reflection capabilities, it’s pretty easy to traverse an object tree and build a deep clone of it, so I wrote a small class that does just that. Then I got the idea to use Linq Expression Trees to compile the instructions required for cloning a type at runtime…
"Cloners"
I implemented three different "cloners", all doing the exact same thing via different means. The only requirements were:
- A cloner needs to be able to clone any object, whether serializable or not
- A cloner must be able to clone objects that do not have a default constructor
- A cloner must be able to clone objects a) by their fields and b) by their properties
SerializationCloner
Uses the BinaryFormatter from the .NET Framework (equipped with a surrogate selector
that forces it to always use a surrogate for serialization which serializes all of a type’s fields,
similar to what is described A
Generic Method for Deep Cloning in C# 3.0).
ReflectionCloner
A cloner that traverses an object tree via .NET’s reflection API, creating copies of any object
it encounters. Accessing types via reflection is pretty slow, but still about an order of magnitude
faster than the BinaryFormatter. It also has the advantage of having no setup time
and working in Silverlight (and possibly the related .NET Compact Framework)
ExpressionTreeCloner
This is pretty advanced stuff. In C# 4.0, Linq Expression Trees can be used to represent any type of method or function as a tree of nodes. This cloner builds an expression tree to clone each type it encounters and then compiles it to a normal method which is kept in a cache. So once a type has been encountered by the cloner once, the cloning operation is as fast as assigning the fields in code by hand.
Results
As you can see, all of the cloners scale quite linearly and in a predictable manner. The expression
tree cloner suffers a bit from the overhead of compiling expressions at runtime, but boy does it
catch up once the number of clones exceeds 100. Given a large list (or deep object tree) with
10,000 objects to clone, the SerializationCloner would be busy for one and a half
seconds whereas the ExpressionTreeCloner would hand you the finished clone after just
45 milliseconds.
The SerializationCloner is still useful because it works on Silverlight and possibly
also on the .NET Compact Framework (I could only check it against Windows Phone 7 and the Xbox 360
via XNA). On the latter platforms, cloned objects need to supply a default constructor, though,
because the required methods to pull new objects from thin air are only provided by the full framework.
Cloning via BinarySerializer is definitely out. It has a setup time and there is no
case where the ReflectionCloner wouldn’t mop the floor with it. On average,
the ReflectionCloner is 6 times faster, while
the ExpressionTreeCloner is more than 60 times faster (once it gets
over its higher cold start time).
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8 comments
name says:
September 26, 2012 at 12:30 pm (UTC 1 )
Your reflection cloner fails on examples with circular references.
Cygon says:
September 26, 2012 at 4:30 pm (UTC 1 )
That’s right. Thanks, I hadn’t thought of those. The expression tree cloner should be affected as well.
I might update the code in the future, but at the moment I don’t have any circular references in the objects I’m using it with, so it’ll be a while until I get around to it.
Livia says:
November 13, 2012 at 2:29 pm (UTC 1 )
Good post and comparations.
But in my tests I’ve had same problem, circular references. Cygon, have you found a solution? I tried change the code and resulted in Stack Overflow Exception, as expected…
Cygon says:
November 14, 2012 at 2:31 pm (UTC 1 )
It’s not as much looking for a solution than just me needing to sit down and write some boring code
What needs to be done is simply to pass a dictionary (with
ReferenceEqualsbased comparer) around and use it look keep track of which reference types have already been cloned (and therefore need to be simply assigned instead of traversed).The project I’ve written this component for currently has no circular references, but I’ll see if I can get around to adding support for them, I think it would be useful to have a fast and complete object cloning facility for C# instead of all those crutches out there
Yasar says:
February 19, 2013 at 11:49 am (UTC 1 )
i have tried to use ReflectionCloner in silverlight but its giving an error “FormatterServices inaccessible due to its protection level”
can you please tell me any work around
Cygon says:
February 19, 2013 at 8:21 pm (UTC 1 )
That error you’ve been getting has pointed you to the following 5 lines:
#if (XBOX360 || WINDOWS_PHONE) object clone = Activator.CreateInstance(originalType); #else object clone = FormatterServices.GetUninitializedObject(originalType); #endifHave you tried the most obvious thing, aka just adding
SILVERLIGHTto the#if?Tim Herman says:
February 24, 2013 at 7:51 pm (UTC 1 )
I changed to the ReflectionCloner to handle circular references. It wasn’t much work really, maybe 45 mins of coding and testing. I’d like to tackle the expression tree cloner next, but the code is much harder to follow. I can post my change for the ReflectionCloner if you like.
Mhd Denno says:
March 19, 2013 at 4:54 pm (UTC 1 )
Can you please add the implementation that solve circular references, by the way very good code ,thanks