ECMA-334: 10.5.4 Accessibility constraints

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C# Language Specification

10. Basic concepts 10.5 Member access 10.5.1 Declared accessibility 10.5.2 Accessibility domains 10.5.3 Protected access for instance members 10.5.4 Accessibility constraints

© 2006 ECMA International

10.5.4 Accessibility constraints

Several constructs in the C# language require a type to be at least as accessible as a member or another type. A type T is said to be at least as accessible as a member or type M if the accessibility domain of T is a superset of the accessibility domain of M. In other words, T is at least as accessible as M if T is accessible in all contexts in which M is accessible.

The following accessibility constraints exist:

• The direct base class of a class type shall be at least as accessible as the class type itself.

• The explicit base interfaces of an interface type shall be at least as accessible as the interface type itself.

• The return type and parameter types of a delegate type shall be at least as accessible as the delegate type itself.

• The type of a constant shall be at least as accessible as the constant itself.

• The type of a field shall be at least as accessible as the field itself.

• The return type and parameter types of a method shall be at least as accessible as the method itself.

• The type of a property shall be at least as accessible as the property itself.

• The type of an event shall be at least as accessible as the event itself.

• The type and parameter types of an indexer shall be at least as accessible as the indexer itself.

• The return type and parameter types of an operator shall be at least as accessible as the operator itself.

• The parameter types of an instance constructor shall be at least as accessible as the instance constructor itself.

[Example: In the following code

class A {…}
public class B: A {…}

the B class results in a compile-time error because A is not at least as accessible as B. end example]

[Example: Likewise, in the following code

class A {…}
public class B
{
  A F() {…}
  internal A G() {…}
  public A H() {…}
}

the H method in B results in a compile-time error because the return type A is not at least as accessible as the method. end example]