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13.Interfaces


An interface defines a contract. A class or struct that implements an interface must adhere to its contract. An interface may inherit from multiple base interfaces, and a class or struct may implement multiple interfaces.

Interfaces can contain methods, properties, events, and indexers. The interface itself does not provide implementations for the members that it defines. The interface merely specifies the members that must be supplied by classes or structs that implement the interface.


13.1Interface declarations


An interface-declaration is a type-declaration (§9.6) that declares a new interface type.

interface-declaration:
attributesopt interface-modifiersopt partialopt interface identifier type-parameter-listopt
interface-baseopt type-parameter-constraints-clausesopt interface-body ;opt

An interface-declaration consists of an optional set of attributes (§17), followed by an optional set of interface-modifiers (§13.1.1), followed by an optional partial modifier, followed by the keyword interface and an identifier that names the interface, followed by an optional type-parameter-list specification (§10.1.3), followed by an optional interface-base specification (§13.1.2), followed by an optional type-parameter-constraints-clauses specification (§10.1.5), followed by a interface-body (§13.1.4), optionally followed by a semicolon.


13.1.1Interface modifiers


An interface-declaration may optionally include a sequence of interface modifiers:

interface-modifiers:
interface-modifier
interface-modifiers interface-modifier


interface-modifier:
new
public
protected
internal
private

It is a compile-time error for the same modifier to appear multiple times in an interface declaration.

The new modifier is only permitted on interfaces defined within a class. It specifies that the interface hides an inherited member by the same name, as described in §10.3.4.

The public, protected, internal, and private modifiers control the accessibility of the interface. Depending on the context in which the interface declaration occurs, only some of these modifiers may be permitted (§3.5.1).


13.1.2Partial modifier


The partial modifier indicates that this interface-declaration is a partial type declaration. Multiple partial interface declarations with the same name within an enclosing namespace or type declaration combine to form one interface declaration, following the rules specified in §10.2.

13.1.3Base interfaces


An interface can inherit from zero or more interface types, which are called the explicit base interfaces of the interface. When an interface has one or more explicit base interfaces, then in the declaration of that interface, the interface identifier is followed by a colon and a comma separated list of base interface types.

interface-base:
: interface-type-list

For a constructed interface type, the explicit base interfaces are formed by taking the explicit base interface declarations on the generic type declaration, and substituting, for each type-parameter in the base interface declaration, the corresponding type-argument of the constructed type.

The explicit base interfaces of an interface must be at least as accessible as the interface itself (§3.5.4). For example, it is a compile-time error to specify a private or internal interface in the interface-base of a public interface.

It is a compile-time error for an interface to directly or indirectly inherit from itself.

The base interfaces of an interface are the explicit base interfaces and their base interfaces. In other words, the set of base interfaces is the complete transitive closure of the explicit base interfaces, their explicit base interfaces, and so on. An interface inherits all members of its base interfaces. In the example

interface IControl


{
void Paint();
}

interface ITextBox: IControl


{
void SetText(string text);
}

interface IListBox: IControl


{
void SetItems(string[] items);
}

interface IComboBox: ITextBox, IListBox {}

the base interfaces of IComboBox are IControl, ITextBox, and IListBox.

In other words, the IComboBox interface above inherits members SetText and SetItems as well as Paint.

A class or struct that implements an interface also implicitly implements all of the interface’s base interfaces.

13.1.4Interface body


The interface-body of an interface defines the members of the interface.

interface-body:
{ interface-member-declarationsopt }

13.2Interface members


The members of an interface are the members inherited from the base interfaces and the members declared by the interface itself.

interface-member-declarations:
interface-member-declaration
interface-member-declarations interface-member-declaration


interface-member-declaration:
interface-method-declaration
interface-property-declaration
interface-event-declaration
interface-indexer-declaration

An interface declaration may declare zero or more members. The members of an interface must be methods, properties, events, or indexers. An interface cannot contain constants, fields, operators, instance constructors, destructors, or types, nor can an interface contain static members of any kind.

All interface members implicitly have public access. It is a compile-time error for interface member declarations to include any modifiers. In particular, interfaces members cannot be declared with the modifiers abstract, public, protected, internal, private, virtual, override, or static.

The example

public delegate void StringListEvent(IStringList sender);

public interface IStringList


{
void Add(string s);

int Count { get; }

event StringListEvent Changed;

string this[int index] { get; set; }


}

declares an interface that contains one each of the possible kinds of members: A method, a property, an event, and an indexer.

An interface-declaration creates a new declaration space (§3.3), and the interface-member-declarations immediately contained by the interface-declaration introduce new members into this declaration space. The following rules apply to interface-member-declarations:


  • The name of a method must differ from the names of all properties and events declared in the same interface. In addition, the signature (§3.6) of a method must differ from the signatures of all other methods declared in the same interface, and two methods declared in the same interface may not have signatures that differ solely by ref and out.

  • The name of a property or event must differ from the names of all other members declared in the same interface.

  • The signature of an indexer must differ from the signatures of all other indexers declared in the same interface.

The inherited members of an interface are specifically not part of the declaration space of the interface. Thus, an interface is allowed to declare a member with the same name or signature as an inherited member. When this occurs, the derived interface member is said to hide the base interface member. Hiding an inherited member is not considered an error, but it does cause the compiler to issue a warning. To suppress the warning, the declaration of the derived interface member must include a new modifier to indicate that the derived member is intended to hide the base member. This topic is discussed further in §3.7.1.2.

If a new modifier is included in a declaration that doesn’t hide an inherited member, a warning is issued to that effect. This warning is suppressed by removing the new modifier.

Note that the members in class object are not, strictly speaking, members of any interface (§13.2). However, the members in class object are available via member lookup in any interface type (§7.3).

13.2.1Interface methods


Interface methods are declared using interface-method-declarations:

interface-method-declaration:
attributesopt newopt return-type identifier type-parameter-list
( formal-parameter-listopt ) type-parameter-constraints-clausesopt ;

The attributes, return-type, identifier, and formal-parameter-list of an interface method declaration have the same meaning as those of a method declaration in a class (§10.6). An interface method declaration is not permitted to specify a method body, and the declaration therefore always ends with a semicolon.


13.2.2Interface properties


Interface properties are declared using interface-property-declarations:

interface-property-declaration:
attributesopt newopt type identifier { interface-accessors }


interface-accessors:
attributesopt get ;
attributesopt set ;
attributesopt get ; attributesopt set ;
attributesopt set ; attributesopt get ;

The attributes, type, and identifier of an interface property declaration have the same meaning as those of a property declaration in a class (§10.7).

The accessors of an interface property declaration correspond to the accessors of a class property declaration (§10.7.2), except that the accessor body must always be a semicolon. Thus, the accessors simply indicate whether the property is read-write, read-only, or write-only.

13.2.3Interface events


Interface events are declared using interface-event-declarations:

interface-event-declaration:
attributesopt newopt event type identifier ;

The attributes, type, and identifier of an interface event declaration have the same meaning as those of an event declaration in a class (§10.8).


13.2.4Interface indexers


Interface indexers are declared using interface-indexer-declarations:

interface-indexer-declaration:
attributesopt newopt type this [ formal-parameter-list ] { interface-accessors }

The attributes, type, and formal-parameter-list of an interface indexer declaration have the same meaning as those of an indexer declaration in a class (§10.9).

The accessors of an interface indexer declaration correspond to the accessors of a class indexer declaration (§10.9), except that the accessor body must always be a semicolon. Thus, the accessors simply indicate whether the indexer is read-write, read-only, or write-only.

13.2.5Interface member access


Interface members are accessed through member access (§7.5.4) and indexer access (§7.5.6.2) expressions of the form I.M and I[A], where I is an interface type, M is a method, property, or event of that interface type, and A is an indexer argument list.

For interfaces that are strictly single-inheritance (each interface in the inheritance chain has exactly zero or one direct base interface), the effects of the member lookup (§7.3), method invocation (§7.5.5.1), and indexer access (§7.5.6.2) rules are exactly the same as for classes and structs: More derived members hide less derived members with the same name or signature. However, for multiple-inheritance interfaces, ambiguities can occur when two or more unrelated base interfaces declare members with the same name or signature. This section shows several examples of such situations. In all cases, explicit casts can be used to resolve the ambiguities.

In the example

interface IList


{
int Count { get; set; }
}

interface ICounter


{
void Count(int i);
}

interface IListCounter: IList, ICounter {}

class C
{
void Test(IListCounter x) {
x.Count(1); // Error
x.Count = 1; // Error
((IList)x).Count = 1; // Ok, invokes IList.Count.set
((ICounter)x).Count(1); // Ok, invokes ICounter.Count
}
}

the first two statements cause compile-time errors because the member lookup (§7.3) of Count in IListCounter is ambiguous. As illustrated by the example, the ambiguity is resolved by casting x to the appropriate base interface type. Such casts have no run-time costs—they merely consist of viewing the instance as a less derived type at compile-time.

In the example

interface IInteger


{
void Add(int i);
}

interface IDouble


{
void Add(double d);
}

interface INumber: IInteger, IDouble {}

class C
{
void Test(INumber n) {
n.Add(1); // Invokes IInteger.Add
n.Add(1.0); // Only IDouble.Add is applicable
((IInteger)n).Add(1); // Only IInteger.Add is a candidate
((IDouble)n).Add(1); // Only IDouble.Add is a candidate
}
}

the invocation n.Add(1) selects IInteger.Add by applying the overload resolution rules of §7.4.3. Similarly the invocation n.Add(1.0) selects IDouble.Add. When explicit casts are inserted, there is only one candidate method, and thus no ambiguity.

In the example

interface IBase


{
void F(int i);
}

interface ILeft: IBase


{
new void F(int i);
}

interface IRight: IBase


{
void G();
}

interface IDerived: ILeft, IRight {}

class A
{
void Test(IDerived d) {
d.F(1); // Invokes ILeft.F
((IBase)d).F(1); // Invokes IBase.F
((ILeft)d).F(1); // Invokes ILeft.F
((IRight)d).F(1); // Invokes IBase.F
}
}

the IBase.F member is hidden by the ILeft.F member. The invocation d.F(1) thus selects ILeft.F, even though IBase.F appears to not be hidden in the access path that leads through IRight.

The intuitive rule for hiding in multiple-inheritance interfaces is simply this: If a member is hidden in any access path, it is hidden in all access paths. Because the access path from IDerived to ILeft to IBase hides IBase.F, the member is also hidden in the access path from IDerived to IRight to IBase.



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