csharplang/proposals/default-interface-methods.md

default interface methods

• Proposed
• Prototype: In progress
• Implementation: None
• Specification: In progress, below

Summary

Add support for virtual extension methods - methods in interfaces with concrete implementations. A class or struct that implements such an interface is required to have a single most specific implementation for the interface method, either implemented by the class or struct, or inherited from its base classes or interfaces. Virtual extension methods enable an API author to add methods to an interface in future versions without breaking source or binary compatibility with existing implementations of that interface.

These are similar to Java's "Default Methods".

(Based on the likely implementation technique) this feature requires corresponding support in the CLI/CLR. Programs that take advantage of this feature cannot run on earlier versions of the platform.

Motivation

The principal motivations for this feature are

• Default interface methods enable an API author to add methods to an interface in future versions without breaking source or binary compatibility with existing implementations of that interface.
• The feature enables C# to interoperate with APIs targeting Android (Java) and iOS (Swift), which support similar features.
• As it turns out, adding default interface implementations provides the elements of the "traits" language feature (https://en.wikipedia.org/wiki/Trait_(computer_programming)). Traits have proven to be a powerful programming technique (http://scg.unibe.ch/archive/papers/Scha03aTraits.pdf).

Detailed design

The syntax for an interface is extended to permit

• a body for a method or indexer, property, or event accessor (i.e. a "default" implementation)
• static methods, properties, indexers, and events.
• Explicit access modifiers (the default access is public)
• override modifiers

Members with bodies permit the interface to provide a "default" implementation for the method in classes and structs that do not provide an overriding implementation.

Interfaces may not contain instance state. While static fields are now permitted, instance fields are not permitted in interfaces. Instance auto-properties are not supported in interfaces, as they would implicitly declare a hidden field.

Static and private methods permit useful refactoring and organization of code used to implement the interface's public API.

Concrete methods in interfaces

The simplest form of this feature is the ability to declare a concrete method in an interface, which is a method with a body.

interface IA
{
    void M() { WriteLine("IA.M"); }
}

A class that implements this interface need not implement its concrete method.

class C : IA { } // OK

IA i = new C();
i.M(); // prints "IA.M"

The final override for IA.M in class C is the concrete method M declared in IA. Note that a class does not inherit members from its interfaces; that is not changed by this feature:

new C().M(); // error: class 'C' does not contain a member 'M'

Within an instance member of an interface, this has the type of the enclosing interface.

Modifiers in interfaces

The syntax for an interface is relaxed to permit modifiers on its members. The following are permitted: private, protected, internal, public, virtual, abstract, override, sealed, static, extern.

TODO: check what other modifiers exist.

An interface member whose declaration includes a body is a virtual member unless the sealed or private modifier is used. The virtual modifier may be used on a function member that would otherwise be implicitly virtual. Similarly, although abstract is the default on interface members without bodies, that modifier may be given explicitly. A non-virtual member may be declared using the sealed keyword.

It is an error for a private or sealed function member of an interface to have no body. A private function member may not have the modifier sealed.

Access modifiers may be used on interface members of all kinds of members that are permitted. The access level public is the default but it may be given explicitly.

Open Issue: We need to specify the precise meaning of the access modifiers such as protected and internal, and which declarations do and do not override them (in a derived interface) or implement them (in a class that implements the interface).

Interfaces may declare static members, including nested types, methods, indexers, properties, and events. The default access level for all interface members is public.

Interfaces may not declare constructors, destructors, or fields.

Open Issue: Should operator declarations be permitted in an interface? Probably not conversion operators, but what about others?

Open Issue: Should new be permitted on interface member declarations that hide members from base interfaces?

Open Issue: Should const declarations be permitted in an interface?

Open Issue: We do not currently permit partial on an interface or its members. That would require a separate proposal.

Overrides in interfaces

Override declarations (i.e. those containing the override modifier) allow the programmer to provide a most specific implementation of a virtual member in an interface where the compiler or runtime would not otherwise find one. It also allows turning an abstract member from a super-interface into a default member in a derived interface. An override declaration is permitted to explicitly override a particular base interface method by qualifying the declaration with the interface name (no access modifier is permitted in this case).

interface IA
{
    void M() { WriteLine("IA.M"); }
}
interface IB : IA
{
    override void IA.M() { WriteLine("IB.M"); } // explicitly named
}
interface IC : IA
{
    override void M() { WriteLine("IC.M"); } // implicitly named
}

If the interface is not named in the override declaration, then all matching methods (from direct or indirect base interfaces) are overridden. There must be at least one such method or the override declaration is an error.

Open issue: should that "direct and indirect" be "direct" here?

Overrides in interfaces are useful to provide a more specific (e.g. more efficient) implementation of a base interface's method. For example, a new First() method on IEnumerable may have a much more efficient implementation on the interface IList.

A method declared in an interface is never treated as an override of another method unless it contains he override modifier. This is necessary for compatibility.

interface IA
{
    void M();
}
interface IB : IA
{
    void M(); // not related to 'IA.M'; not an override
}

Override declarations in interfaces may not be declared sealed.

Public virtual function members in an interface may be overridden in a derived interface either implicitly (by using the override modifier in a public declaration in the derived interface) or explicitly (by qualifying the name in the override declaration with the interface type that originally declared the method, and omitting an access modifier).

virtual function members in an interface that are not public may only be overridden explicitly (not implicitly) in derived interfaces, and may only be implemented in a class or struct explicitly (not implicitly). In either case, the overridden or implemented member must be accessible where it is overridden.

Open Issue: Should we relax that to allow implicit override or implementation of a non-public function member? It would require a detailed proposal.

Reabstraction

A virtual (concrete) method declared in an interface may be overridden to be abstract in a derived interface

interface IA
{
    void M() { WriteLine("IA.M"); }
}
interface IB : IA
{
    override abstract void M();
}
class C : IB { } // error: class 'C' does not implement 'IA.M'.

The abstract modifier is not required in the declaration of IB.M (that is the default in interfaces), but it is probably good practice to be explicit in an override declaration.

This is useful in derived interfaces where the default implementation of a method is inappropriate and a more appropriate implementation should be provided by implementing classes.

Open Issue: Should reabstraction be permitted?

The most specific override rule

We require that every interface and class have a most specific override for every virtual member among the overrides appearing in the type or its direct and indirect interfaces. The most specific override is a unique override that is more specific than every other override. If there is no override, the member itself is considered the most specific override.

One override M1 is considered more specific than another override M2 if M1 is declared on type T1, M2 is declared on type T2, and either

1. T1 contains T2 among its direct or indirect interfaces, or
2. T2 is an interface type but T1 is not an interface type.

For example:

interface IA
{
    void M() { WriteLine("IA.M"); }
}
interface IB : IA
{
    override void IA.M() { WriteLine("IB.M"); }
}
interface IC : IA
{
    override void IA.M() { WriteLine("IC.M"); }
}
interface ID : IB, IC { } // error: no most specific override for 'IA.M'
abstract class C : IB, IC { } // error: no most specific override for 'IA.M'
abstract class D : IA, IB, IC // ok
{
    public abstract void M();
}

The most specific override rule ensures that a conflict (i.e. an ambiguity arising from diamond inheritance) is resolved explicitly by the programmer at the point where the conflict arises.

Because we support explicit abstract overrides in interfaces, we could do so in classes as well

abstract class E : IA, IB, IC // ok
{
    abstract void IA.M();
}

Open issue: should we support explicit interface abstract overrides in classes?

In addition, it is an error if in a class declaration the most specific override of some interface method is an an abstract override that was declared in an interface. This is an existing rule restated using the new terminology.

interface IF
{
    void M();
}
abstract class F : IF { } // error: 'F' does not implement 'IF.M'

It is possible for a virtual property declared in an interface to have a most specific override for its get accessor in one interface and a most specific override for its set accessor in a different interface. This is considered a violation of the most specific override rule.

static and private methods

Because interfaces may now contain executable code, it is useful to abstract common code into private and static methods. We now permit these in interfaces.

Open issue: Should we support private methods? Should we support static methods? YES

Open issue: should we permit interface methods to be protected or internal or other access? If so, what are the semantics? Are they virtual by default? If so, is there a way to make them non-virtual?

Open issue: If we support static methods, should we support (static) operators?

Base interface invocations

Code in a type that derives from an interface with a default method can explicitly invoke that interface's "base" implementation.

interface I0
{
   void M() { Console.WriteLine("I0"); }
}
interface I1 : I0
{
   override void M() { Console.WriteLine("I1"); }
}
interface I2 : I0
{
   override void M() { Console.WriteLine("I2"); }
}
interface I3 : I1, I2
{
   // an explicit override that invoke's a base interface's default method
   void I0.M() { I2.base.M(); }
}

An instance (nonstatic) method is permitted to invoke an accessible instance method override in a direct base interface nonvirtually by naming it using the syntax Type.base.M. This is useful when an override that is required to be provided due to diamond inheritance is resolved by delegating to one particular base implementation.

interface IA
{
    void M() { WriteLine("IA.M"); }
}
interface IB : IA
{
    override void IA.M() { WriteLine("IB.M"); }
}
interface IC : IA
{
    override void IA.M() { WriteLine("IC.M"); }
}

class D : IA, IB, IC
{
    void IA.M() { IB.base.M(); } 
}

Open issue: what syntax should we use for base invocation? Alternatives:

1. Interface.base.M()
2. base.M()

It is an error for an unqualified base (i.e. not specifying the base interface type) to appear in an interface.

Effect on existing programs

The rules presented here are intended to have no effect on the meaning of existing programs.

Example 1:

interface IA
{
    void M();
}
class C: IA // Error: IA.M has no concrete most specific override in C
{
    public static void M() { } // method unrelated to 'IA.M' because static
}

Example 2:

interface IA
{
    void M();
}
class Base: IA
{
    void IA.M() { }
}
class Derived: Base, IA // OK, all interface members have a concrete most specific override
{
    private void M() { } // method unrelated to 'IA.M' because private
}

The same rules give similar results to the analogous situation involving default interface methods:

interface IA
{
    void M() { }
}
class Derived: IA // OK, all interface members have a concrete most specific override
{
    private void M() { } // method unrelated to 'IA.M' because private
}

Open issue: confirm that this is an intended consequence of the specification. YES

Runtime method resolution

Open Issue: The spec should describe the runtime method resolution algorithm in the face of interface default methods. We need to ensure that the semantics are consistent with the language semantics, e.g. which declared methods do and do not override or implement an internal method.

CLR support API

In order for compilers to detect when they are compiling for a runtime that supports this feature, libraries for such runtimes are modified to advertise that fact through the API discussed in https://github.com/dotnet/corefx/issues/17116. We add

namespace System.Runtime.CompilerServices
{
    public static class RuntimeFeature
    {
        // Presence of the field indicates runtime support
        public const string DefaultInterfaceImplementation = nameof(DefaultInterfaceImplementation);
    }
}

Open issue: Is that the best name for the CLR feature? The CLR feature does much more than just that (e.g. relaxes protection constraints, supports overrides in interfaces, etc). Perhaps it should be called something like "concrete methods in interfaces", or "traits"?

Further areas to be specified

• It would be useful to catalog the kinds of source and binary compatibility effects caused by adding default interface methods and overrides to existing interfaces.

Drawbacks

This proposal requires a coordinated update to the CLR specification (to support concrete methods in interfaces and method resolution). It is therefore fairly "expensive" and it may be worth doing in combination with other features that we also anticipate would require CLR changes.

Alternatives

None.

Unresolved questions

• Open questions are called out throughout the proposal, above.
• See also https://github.com/dotnet/csharplang/issues/406 for a list of open questions.
• The detailed specification must describe the resolution mechanism used at runtime to select the precise method to be invoked.
• The interaction of metadata produced by new compilers and consumed by older compilers needs to be worked out in detail. For example, we need to ensure that the metadata representation that we use does not cause the addition of a default implementation in an interface to break an existing class that implements that interface when compiled by an older compiler. This may affect the metadata representation that we can use.
• The design must consider interoperation with other languages and existing compilers for other languages.

Resolved Questions

Abstract Override

The earlier draft spec contained the ability to "reabstract" an inherited method:

interface IA
{
    void M();
}
interface IB : IA
{
    override void M() { }
}
interface IC : IB
{
    override void M(); // make it abstract again
}

My notes for 2017-03-20 showed that we decided not to allow this. However, there are at least two use cases for it:

1. The Java APIs, with which some users of this feature hope to interoperate, depend on this facility.
2. Programming with traits benefits from this. Reabstraction is one of the elements of the "traits" language feature (https://en.wikipedia.org/wiki/Trait_(computer_programming)). The following is permitted with classes:

public abstract class Base
{
    public abstract void M();
}
public abstract class A : Base
{
    public override void M() { }
}
public abstract class B : A
{
    public override abstract void M(); // reabstract Base.M
}

Unfortunately this code cannot be refactored as a set of interfaces (traits) unless this is permitted. By the Jared principle of greed, it should be permitted.

Closed issue: Should reabstraction be permitted? [YES] My notes were wrong. The LDM notes say that reabstraction is permitted in an interface. Not in a class.

Virtual Modifier vs Sealed Modifier

From Aleksey Tsingauz:

We decided to allow modifiers explicitly stated on interface members, unless there is a reason to disallow some of them. This brings an interesting question around virtual modifier. Should it be required on members with default implementation?

We could say that:

• if there is no implementation and neither virtual, nor sealed are specified, we assume the member is abstract.
• if there is an implementation and neither abstract, nor sealed are specified, we assume the member is virtual.
• sealed modifier is required to make a method neither virtual, nor abstract.

Alternatively, we could say that virtual modifier is required for a virtual member. I.e, if there is a member with implementation not explicitly marked with virtual modifier, it is neither virtual, nor abstract. This approach might provide better experience when a method is moved from a class to an interface:

• an abstract method stays abstract.
• a virtual method stays virtual.
• a method without any modifier stays neither virtual, nor abstract.
• sealed modifier cannot be applied to a method that is not an override.

What do you think?

Closed Issue: Should a concrete method (with implementation) be implicitly virtual? [YES]

Decisions: Made in the LDM 2017-04-05:

1. non-virtual should be explicitly expressed through sealed or private.
2. sealed is the keyword to make interface instance members with bodies non-virtual
3. We want to allow all modifiers in interfaces
4. Default accessibility for interface members is public, including nested types
5. private function members in interfaces are implicitly sealed, and sealed is not permitted on them.
6. Private classes (in interfaces) are permitted and can be sealed, and that means sealed in the class sense of sealed.
7. Absent a good proposal, partial is still not allowed on interfaces or their members.

Binary Compatibility 1

When a library provides a default implementation

interface I1
{
    void M() { Impl1 }
}
interface I2 : I1
{
}
class C : I2
{
}

We understand that the implementation of I1.M in C is I1.M. What if the assembly containing I2 is changed as follows and recompiled

interface I2 : I1
{
    override void M() { Impl2 }
}

but C is not recompiled. What happens when the program is run? An invocation of (C as I1).M()

1. Runs I1.M
2. Runs I2.M
3. Throws some kind of runtime error

Decision: Made 2017-04-11: Runs I2.M, which is the unambiguously most specific override at runtime.

Design meetings

2017-03-08 LDM Meeting Notes 2017-03-21 LDM Meeting Notes 2017-03-23 meeting "CLR Behavior for Default Interface Methods" 2017-04-05 LDM Meeting Notes 2017-04-11 LDM Meeting Notes 2017-04-18 LDM Meeting Notes 2017-04-19 LDM Meeting Notes