c# - Copy constructor versus Clone()

Question

In C#, what is the preferred way to add (deep) copy functionality to a class? Should one implement the copy constructor, or rather derive from ICloneable and implement the Clone() method?

Remark: I wrote "deep" within brackets because I thought it was irrelevant. Apparently others disagree, so I asked whether a copy constructor/operator/function needs to make clear which copy variant it implements.

Answer 1

You should not derive from ICloneable.

The reason is that when Microsoft designed the .net framework they never specified whether the Clone() method on ICloneable should be a deep or shallow clone, thus the interface is semantically broken as your callers won't know whether the call will deep or shallow clone the object.

Instead, you should define your own IDeepCloneable (and IShallowCloneable) interfaces with DeepClone() (and ShallowClone()) methods.

You can define two interfaces, one with a generic parameter to support strongly typed cloning and one without to keep the weakly typed cloning ability for when you are working with collections of different types of cloneable objects:

public interface IDeepCloneable
{
    object DeepClone();
}
public interface IDeepCloneable<T> : IDeepCloneable
{
    T DeepClone();
}

Which you would then implement like this:

public class SampleClass : IDeepCloneable<SampleClass>
{
    public SampleClass DeepClone()
    {
        // Deep clone your object
        return ...;
    }
    object IDeepCloneable.DeepClone()   
    {
        return this.DeepClone();
    }
}

Generally I prefer to use the interfaces described as opposed to a copy constructor it keeps the intent very clear. A copy constructor would probably be assumed to be a deep clone, but it's certainly not as much of a clear intent as using an IDeepClonable interface.

This is discussed in the .net Framework Design Guidelines and on Brad Abrams' blog

_{(I suppose if you are writing an application (as opposed to a framework/library) so you can be sure no one outside of your team will be calling your code, it doesn't matter so much and you can assign a semantic meaning of "deepclone" to the .net ICloneable interface, but you should make sure this is well documented and well understood within your team. Personally I'd stick to the framework guidelines.)}