C# Language Specification

Interface methods

Interface methods are declared using interface-method-declarations:

interface-method-declaration:
attributes_opt new_opt return-type identifier type-parameter-list
( formal-parameter-list_opt ) type-parameter-constraints-clauses_opt ;

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.

Each formal parameter type of an interface method must be input-safe (§13.1.3.1), and the return type must be either void or output-safe. Furthermore, each class type constraint, interface type constraint and type parameter constraint on any type parameter of the method must be input-safe.

These rules ensure that any covariant or contravariant usage of the interface remains typesafe. For example,

interface I { void M() where U : T; }

is illegal because the usage of T as a type parameter constraint on U is not input-safe.

Were this restriction not in place it would be possible to violate type safety in the following manner:

class B {}

class D : B {}
class E : B {}
class C : I { public void M() {…} }
…
I b = new C();
b.M();

This is actually a call to C.M. But that call requires that E derive from D, so type safety would be violated here.

2.