What is the method MemberwiseClone() doing?

问题:

I am confused with this code below,

Developer devCopy = (Developer)dev.Clone();

Clone method of Developer class just creating a Employee clone, then how developer get another clone of developer.

public abstract class Employee
{
    public abstract Employee Clone();

    public string Name { get; set; }
    public string Role { get; set; }
}


public class Typist : Employee
{
    public int WordsPerMinute { get; set; }

    public override Employee Clone()
    {
        return (Employee)MemberwiseClone();
    }

    public override string ToString()
    {
        return string.Format("{0} - {1} - {2}wpm", Name, Role, WordsPerMinute);
    }
}


public class Developer : Employee
{
    public string PreferredLanguage { get; set; }

    public override Employee Clone()
    {
        return (Employee)MemberwiseClone();
    }

    public override string ToString()
    {
        return string.Format("{0} - {1} - {2}", Name, Role, PreferredLanguage);
    }
}


Developer dev = new Developer();
dev.Name = "Bob";
dev.Role = "Team Leader";
dev.PreferredLanguage = "C#";

Developer devCopy = (Developer)dev.Clone();
devCopy.Name = "Sue";

Console.WriteLine(dev);
Console.WriteLine(devCopy);

/* OUTPUT

Bob - Team Leader - C#
Sue - Team Leader - C#

*/

Typist typist = new Typist();
typist.Name = "Kay";
typist.Role = "Typist";
typist.WordsPerMinute = 120;

Typist typistCopy = (Typist)typist.Clone();
typistCopy.Name = "Tim";
typistCopy.WordsPerMinute = 115;

Console.WriteLine(typist);
Console.WriteLine(typistCopy);

/* OUTPUT

Kay - Typist - 120wpm
Tim - Typist - 115wpm

*/

回答1:

Because the method MemberwiseClone() is doing this for you. See the documentation

The MemberwiseClone method creates a shallow copy by creating a new object, and then copying the nonstatic fields of the current object to the new object. If a field is a value type, a bit-by-bit copy of the field is performed. If a field is a reference type, the reference is copied but the referred object is not; therefore, the original object and its clone refer to the same object.

Whenever you see a method you don't unerstand, you can trace who has declared it (in Visual Studio, I guess), and in turn see its documentation. That makes things pretty obvious most of the time.

回答2:

The function MemberwiseClone creates a new objects whose fields are bit-for-bit copies of those in the original structure. It is a necessary part of any inheritable class which allows cloning without use of Reflection or serialization, but it is only a small piece of the overall puzzle.

If you wish to allow cloning within an inheritable class, you should define a protected virtual T BaseClone<T>() cloning method; the base-level class which descends from Object should call base.MemberwiseClone; all other classes should use base.BaseClone<T> to get the new instance and then replace any mutable cloneable fields with clones of the ones in the original object.

I would also recommend defining the following interfaces:

interface ISelf<out T> {T Self();}
interface ICloneable<out T> : ISelf<T> {T Clone();}

That will allow for situations in which a class may have some descendants which can be cloned and some which cannot. Those which can be cloned can expose public cloning methods (which should chain to BaseClone<theirOwnType>). Methods which need cloneable derivatives of the base type can use parameters of type ICloneable<theBaseType>; this will allow them to accept any cloneable derivative of the base type, even if not all such derivatives share a common base class.