Q. Why would I choose this answer?
- Choose this answer if you want the fastest speed .NET is capable of.
- Ignore this answer if you want a really, really easy method of cloning.
In other words, go with another answer unless you have a performance bottleneck that needs fixing, and you can prove it with a profiler.
10x faster than other methods
The following method of performing a deep clone is:
- 10x faster than anything that involves serialization/deserialization;
- Pretty darn close to the theoretical maximum speed .NET is capable of.
And the method ...
For ultimate speed, you can use Nested MemberwiseClone to do a deep copy. Its almost the same speed as copying a value struct, and is much faster than (a) reflection or (b) serialization (as described in other answers on this page).
Note that if you use Nested MemberwiseClone for a deep copy, you have to manually implement a ShallowCopy for each nested level in the class, and a DeepCopy which calls all said ShallowCopy methods to create a complete clone. This is simple: only a few lines in total, see the demo code below.
Here is the output of the code showing the relative performance difference for 100,000 clones:
- 1.08 seconds for Nested MemberwiseClone on nested structs
- 4.77 seconds for Nested MemberwiseClone on nested classes
- 39.93 seconds for Serialization/Deserialization
Using Nested MemberwiseClone on a class almost as fast as copying a struct, and copying a struct is pretty darn close to the theoretical maximum speed .NET is capable of.
Demo 1 of shallow and deep copy, using classes and MemberwiseClone:
Create Bob
Bob.Age=30, Bob.Purchase.Description=Lamborghini
Clone Bob >> BobsSon
Adjust BobsSon details
BobsSon.Age=2, BobsSon.Purchase.Description=Toy car
Proof of deep copy: If BobsSon is a true clone, then adjusting BobsSon details will not affect Bob:
Bob.Age=30, Bob.Purchase.Description=Lamborghini
Elapsed time: 00:00:04.7795670,30000000
Demo 2 of shallow and deep copy, using structs and value copying:
Create Bob
Bob.Age=30, Bob.Purchase.Description=Lamborghini
Clone Bob >> BobsSon
Adjust BobsSon details:
BobsSon.Age=2, BobsSon.Purchase.Description=Toy car
Proof of deep copy: If BobsSon is a true clone, then adjusting BobsSon details will not affect Bob:
Bob.Age=30, Bob.Purchase.Description=Lamborghini
Elapsed time: 00:00:01.0875454,30000000
Demo 3 of deep copy, using class and serialize/deserialize:
Elapsed time: 00:00:39.9339425,30000000
To understand how to do a deep copy using MemberwiseCopy, here is the demo project that was used to generate the times above:
// Nested MemberwiseClone example.
// Added to demo how to deep copy a reference class.
[Serializable] // Not required if using MemberwiseClone, only used for speed comparison using serialization.
public class Person
{
public Person(int age, string description)
{
this.Age = age;
this.Purchase.Description = description;
}
[Serializable] // Not required if using MemberwiseClone
public class PurchaseType
{
public string Description;
public PurchaseType ShallowCopy()
{
return (PurchaseType)this.MemberwiseClone();
}
}
public PurchaseType Purchase = new PurchaseType();
public int Age;
// Add this if using nested MemberwiseClone.
// This is a class, which is a reference type, so cloning is more difficult.
public Person ShallowCopy()
{
return (Person)this.MemberwiseClone();
}
// Add this if using nested MemberwiseClone.
// This is a class, which is a reference type, so cloning is more difficult.
public Person DeepCopy()
{
// Clone the root ...
Person other = (Person) this.MemberwiseClone();
// ... then clone the nested class.
other.Purchase = this.Purchase.ShallowCopy();
return other;
}
}
// Added to demo how to copy a value struct (this is easy - a deep copy happens by default)
public struct PersonStruct
{
public PersonStruct(int age, string description)
{
this.Age = age;
this.Purchase.Description = description;
}
public struct PurchaseType
{
public string Description;
}
public PurchaseType Purchase;
public int Age;
// This is a struct, which is a value type, so everything is a clone by default.
public PersonStruct ShallowCopy()
{
return (PersonStruct)this;
}
// This is a struct, which is a value type, so everything is a clone by default.
public PersonStruct DeepCopy()
{
return (PersonStruct)this;
}
}
// Added only for a speed comparison.
public class MyDeepCopy
{
public static T DeepCopy<T>(T obj)
{
object result = null;
using (var ms = new MemoryStream())
{
var formatter = new BinaryFormatter();
formatter.Serialize(ms, obj);
ms.Position = 0;
result = (T)formatter.Deserialize(ms);
ms.Close();
}
return (T)result;
}
}
Then, call the demo from main:
void MyMain(string[] args)
{
{
Console.Write("Demo 1 of shallow and deep copy, using classes and MemberwiseCopy:\n");
var Bob = new Person(30, "Lamborghini");
Console.Write(" Create Bob\n");
Console.Write(" Bob.Age={0}, Bob.Purchase.Description={1}\n", Bob.Age, Bob.Purchase.Description);
Console.Write(" Clone Bob >> BobsSon\n");
var BobsSon = Bob.DeepCopy();
Console.Write(" Adjust BobsSon details\n");
BobsSon.Age = 2;
BobsSon.Purchase.Description = "Toy car";
Console.Write(" BobsSon.Age={0}, BobsSon.Purchase.Description={1}\n", BobsSon.Age, BobsSon.Purchase.Description);
Console.Write(" Proof of deep copy: If BobsSon is a true clone, then adjusting BobsSon details will not affect Bob:\n");
Console.Write(" Bob.Age={0}, Bob.Purchase.Description={1}\n", Bob.Age, Bob.Purchase.Description);
Debug.Assert(Bob.Age == 30);
Debug.Assert(Bob.Purchase.Description == "Lamborghini");
var sw = new Stopwatch();
sw.Start();
int total = 0;
for (int i = 0; i < 100000; i++)
{
var n = Bob.DeepCopy();
total += n.Age;
}
Console.Write(" Elapsed time: {0},{1}\n\n", sw.Elapsed, total);
}
{
Console.Write("Demo 2 of shallow and deep copy, using structs:\n");
var Bob = new PersonStruct(30, "Lamborghini");
Console.Write(" Create Bob\n");
Console.Write(" Bob.Age={0}, Bob.Purchase.Description={1}\n", Bob.Age, Bob.Purchase.Description);
Console.Write(" Clone Bob >> BobsSon\n");
var BobsSon = Bob.DeepCopy();
Console.Write(" Adjust BobsSon details:\n");
BobsSon.Age = 2;
BobsSon.Purchase.Description = "Toy car";
Console.Write(" BobsSon.Age={0}, BobsSon.Purchase.Description={1}\n", BobsSon.Age, BobsSon.Purchase.Description);
Console.Write(" Proof of deep copy: If BobsSon is a true clone, then adjusting BobsSon details will not affect Bob:\n");
Console.Write(" Bob.Age={0}, Bob.Purchase.Description={1}\n", Bob.Age, Bob.Purchase.Description);
Debug.Assert(Bob.Age == 30);
Debug.Assert(Bob.Purchase.Description == "Lamborghini");
var sw = new Stopwatch();
sw.Start();
int total = 0;
for (int i = 0; i < 100000; i++)
{
var n = Bob.DeepCopy();
total += n.Age;
}
Console.Write(" Elapsed time: {0},{1}\n\n", sw.Elapsed, total);
}
{
Console.Write("Demo 3 of deep copy, using class and serialize/deserialize:\n");
int total = 0;
var sw = new Stopwatch();
sw.Start();
var Bob = new Person(30, "Lamborghini");
for (int i = 0; i < 100000; i++)
{
var BobsSon = MyDeepCopy.DeepCopy<Person>(Bob);
total += BobsSon.Age;
}
Console.Write(" Elapsed time: {0},{1}\n", sw.Elapsed, total);
}
Console.ReadKey();
}
Again, note that if you use Nested MemberwiseClone for a deep copy, you have to manually implement a ShallowCopy for each nested level in the class, and a DeepCopy which calls all said ShallowCopy methods to create a complete clone. This is simple: only a few lines in total, see the demo code above.
Value types vs. References Types
Note that when it comes to cloning an object, there is is a big difference between a "struct" and a "class":
- If you have a "struct", it's a value type so you can just copy it, and the contents will be cloned (but it will only make a shallow clone unless you use the techniques in this post).
- If you have a "class", it's a reference type, so if you copy it, all you are doing is copying the pointer to it. To create a true clone, you have to be more creative, and use differences between value types and references types which creates another copy of the original object in memory.
See differences between value types and references types.
Checksums to aid in debugging
- Cloning objects incorrectly can lead to very difficult-to-pin-down bugs. In production code, I tend to implement a checksum to double check that the object has been cloned properly, and hasn't been corrupted by another reference to it. This checksum can be switched off in Release mode.
- I find this method quite useful: often, you only want to clone parts of the object, not the entire thing.
Really useful for decoupling many threads from many other threads
One excellent use case for this code is feeding clones of a nested class or struct into a queue, to implement the producer / consumer pattern.
- We can have one (or more) threads modifying a class that they own, then pushing a complete copy of this class into a
ConcurrentQueue.
- We then have one (or more) threads pulling copies of these classes out and dealing with them.
This works extremely well in practice, and allows us to decouple many threads (the producers) from one or more threads (the consumers).
And this method is blindingly fast too: if we use nested structs, it's 35x faster than serializing/deserializing nested classes, and allows us to take advantage of all of the threads available on the machine.
Update
Apparently, ExpressMapper is as fast, if not faster, than hand coding such as above. I might have to see how they compare with a profiler.
IDeepCloneableis that not all collections of references to things that can be deep-cloned should be; the proper behavior when cloning aList<T>depends not just uponT, but also upon the purpose of the lists. If none of the items in the lists will ever be exposed to anything which would mutate them, then even if the items within the lists could be cloned, it would be better to copy the references directly. – Afterimageclonemethod on the class, then have it call an internal, private constructor that gets passedthis. So copying is turrible [sic], but copying carefully (and the article's definitely worth reading) isn't. ;^) – Remains