Inheritance

The CloneFromTo chain

Every [Cloneable] type contributes a helper that copies only the members it declares and chains to its base:

protected static void CloneFromTo(PhoneNumber from, PhoneNumber to)
{
    ContactMethod.CloneFromTo(from, to);
    to.CountryCode = from.CountryCode;
    to.LocalNumber = from.LocalNumber;
}

Every concrete type additionally gets a factory that news up the instance and runs the chain:

public MobileNumber CloneMobileNumber()
{
    var from = this;
    var to = new MobileNumber {Carrier = from.Carrier};
    CloneFromTo(from, to);
    return to;
}

public override PhoneNumber Clone() => CloneMobileNumber();

Each helper can copy private and protected state of the type that declares it. Where a base class provides no helper of its own, the nearest [Cloneable] descendant additionally copies that base's accessible members as part of its own helper (see below).

required and init-only members are collected from the whole inheritance chain and set in the leaf's object initializer.

Which type owns the Clone() name

The clone root is the topmost ancestor that declares a clone interface or a Clone() method of its own.

  • The root gets Clone(), returning the root type.
  • Everything below it gets Clone + its own type name, plus an override of Clone().
[Cloneable] public abstract partial class ContactMethod {}
[Cloneable] public abstract partial class PhoneNumber : ContactMethod, ICloneable<PhoneNumber> {}
[Cloneable] public partial class MobileNumber : PhoneNumber {}
Type Generated
ContactMethod CloneFromTo only
PhoneNumber public abstract PhoneNumber Clone(); + CloneFromTo
MobileNumber CloneMobileNumber(), public override PhoneNumber Clone() => CloneMobileNumber(); + CloneFromTo

A type that is [Cloneable] but declares no clone interface and no Clone() of its own — ContactMethod above — only contributes a helper, leaving the Clone() name to its subclasses. Such a type cannot be cloned polymorphically: a member typed ContactMethod is copied by reference, with a warning. Declare a clone interface on the base wherever you need that.

Use [Cloneable(MethodName = "...")] where the default name does not match an existing public method you want to keep.

Virtual by default

The generated root Clone() is virtual unless the class is sealed.

Absorbing plain base classes

A base class does not have to be [Cloneable]. If it provides no CloneFromTo helper of its own — a plain POCO base, or a base from another assembly — the nearest [Cloneable] descendant copies that base's members as part of its own CloneFromTo, walking up until it reaches a base that does provide a helper (which it then chains to as before).

The same deep-copy rules apply to absorbed members as to declared ones.

Annotating the base with [Cloneable] anyway is still preferable: only the base's own helper can reach its private state. When absorption cannot reach a member — a private field, or a cross-assembly internal one — that member's state is lost and CLONE004 warns about it. Annotate the base with [Cloneable], or put [IgnoreClone] on the member to silence the warning.

Edit this page