Why doesn't the .NET framework provide a method to deep copy objects? [closed]

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Closed 7 years ago.

I was using a custom method to deep clone objects the other day, and I know you can deep clone in different ways (reflection, binary serialization, etc), so I was just wondering:

What is/are the reason(s) that Microsoft does not include a deep copy method in the framework?

回答1:

The problem is substantially harder than you seem to realize, at least in the general case.

For starters, a copy isn't just deep or shallow, it's a spectrum.

Let's imagine for a second that we have a list of arrays of strings, and we want to make a copy of it.

• We start out at the shallowest level, we just copy the reference of the whole thing to another variable. Any changes to the list referenced from either variable is seen by the other.

• So now we go and create a brand new list to give to the second variable. For each item in the first list we add it to the second list. Now we can modify the list referenced from either variable without it being seen by the other one. But what if we grab the first item of a list and change the string that's in the first array, it will be seen by both lists!

• Now we're going through and creating a new list, and for each array in the first list we're creating a new array, adding each of the strings in the underlying array to the new array, and adding each of those new arrays to the new list. Now we can mutate any of the arrays in either list without seeing the changes. But wait, both lists are still referencing the same strings (which are value types after all; they internally have a character array for their data). What if some mean person were to come along and mutate one of the strings (using unsafe code you could actually do this)! So now you're copying all of the strings with a deep copy. But what if we don't need to do that? What if we know that nobody is so mean that they would mutate the string? Or, for that matter, what if we know that none of the arrays will be mutated (or that if they will be, that they're supposed to be reflected by both lists).

Then of course there are problems such as circular references, fields in a class that don't really represent it's state (i.e. possibly cached values or derived data that could just be re-calculated as-needed by a clone).

Realistically you'd need to have every type implement IClonable or some equivalent, and have it's own custom code for how to clone itself. This would be a lot of work to maintain for a language, especially since there are so many ways that complex objects could possibly be cloned. The cost would be quite high, and the benefits (outside of a handful of objects that it is deemed worthwhile to implement clone methods for) are generally not worth it. You, as a programmer, and write your own logic for cloning a type based on how deep you know you need to go.

回答2:

It's similar to how it works (or doesn't work) in C and C++:

To do a deep copy, you actually have to know how different data is interpreted. In trivial cases, a shallow copy (which is provided) is the same as a deep copy. But once this is no longer true, it really depends on the implementation and interpretation. There's no general rule of thumb.

Let's use a game as a simple example:

• A NPC object has two integers as members. One integer represents its health points, the other one is its unique ID.
• If you clone the NPC, you have to keep the amount of health, while changing the unique ID. This is something the compiler/runtime can't determine on their own. You have to code this, essentially telling the program "how to copy".

I can think of two possible solutions:

• Add a keyword to denote things that can't be copied. While this sounds like a good idea, it doesn't really solve the issue. You can tell the compiler that UniqueID must not copied, but at the same time you can't define how this should happen. And even if you could, you could just...
• Create a copy constructor (C++) or a method to copy/clone the object (C#, e.g. CopyTo()).

回答3:

Hmm.. My view is that:

A) because very rarely you want to have the copy really deep
B) because the framework cannot guarantee to know how to truly and meaningfully CLONE an object
C) because implementing deep-cloning in a naiive way is simple and takes one method and several lines of code using reflection and recursion

but I'll try to find an old MSDN article that covered that

edit: I've not found :( I'm still sure that I saw it somewhere, but I cannot google-it-out now.. However some useful links about related ICloneable and derived:

http://blogs.msdn.com/b/brada/archive/2004/05/03/125427.aspx http://blogs.msdn.com/b/mrtechnocal/archive/2009/10/19/why-not-icloneable-t.aspx https://stackoverflow.com/a/3712564/717732

So, as I've not found the author's words, let me expand the points:

A: because very rarely you want to have the copy really deep

You see, how can the framework guess how deep should it be in general? Let's assume that completely-deep and let's assume it has been implemented. Now we have memberwise-clone and total-clone methods. Still, there are some cases when people will need clone-me-but-not-the-root-base. So they post another questions why the total-clone has no way of cutting off the raw base. Or second-to-raw. Etc. Providing deep-clone solves almost nothing from the .Net team's point of view, as we, the users, will still rant about that just because we see some partial tools and are lazy and want to have everything:)

B) because the framework cannot guarantee to know how to truly and meaningfully CLONE an object

Especially with some special objects with handles or native-like IDs like those from Entity Framework, .Net Remoting Proxies, COM-wrappers etc: You might sucessfully read and clone the upper class hierarchy layers, but eventually, somewhere below you find some arcane thingies like IntPtrs that you just know that you should not copy. Most of the times. But sometimes you can. But the framework's code must be universal. Deep-cloning would either have to be harshly complicated with many sanity checks against specially-looking class members, or it would produce dangerous results if the programmer would invoke it on something that has base classes that the programmer did not care to analyze.

B+) Also, please note that the more base classes you have in your tree, the more probably is that they will have some parameterized constructors, which might indicate that direct-copying is not a good idea. Direct-copiable classes usually have parameterless constructors and all the copiable data accessible by properties..

B++) From the framework's designer point of view, taking memory and speed concerns, shallow copying is almost always very fast, while deep copying is just the opposite. It is beneficial to the framework's and platform's reputation to NOT allow the developers to freely deep-copy huge objects. Anyways, would you need a deep-copy if your object was lightweight and simple, huh? :) Not providing a deep-copy encourages the developers to think around the need of deep-copy, what usually makes the application lighter and faster.

C) because implementing deep-cloning in a naiive way is simple and takes one method and several lines of code using reflection and recursion

Having a shallow copy, how hard it is to actually write a deep copy? Not so hard! Just implement a method that is given an object 'obj':

pseudocode:
object deepcopier(object obj)
    newobject = obj.shallowcopy()
    foreach(field in newobject.fields)
        newobject.field = deepcopier(newobject.field)
    return newobject

and well, that's all. Of course the field enumeration must be performed by Reflection, and also reading/writing the fields - too.

However, this way is very naiive. It this has a serious flaw: what if some object has two fields that point to the same another object? We should detect it and do the cloning once then assign both fields to that one clone. Also if an object pointed by some field has reference to some object that is also pointed by another object (...) - that may also need to be tracked and cloned only once. Also, how about cycles? if somewhere there deep in the tree, an object has a reference back to the root? Such algo like above would happily descent and would re-copy everything once again, then again, and eventually would choke with StackOverflow.

This makes the cloning quite hard to be tracked and starts to look more like serialization. In fact if your class is a DataContract or Serializable, you can simply serialize it and deserialize to get a perfect deep copy :)

Deep-cloning is hard to do in an universal way, unless you know what the object means and what all its fields mean and know which ones should really be cloned and which should be unified. If you, as developer, know that this is just a data-object that is perfectly safe to deep-clone, so whydontya just make it Serializable? If you can't make it Serializable, then probably you also can't deep-clone it!

来源：https://stackoverflow.com/questions/12733571/why-doesnt-the-net-framework-provide-a-method-to-deep-copy-objects