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ii.10.3.3-accessibility-and-overriding.md

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II.10.3.3 Accessibility and overriding

If the strict flag (§II.23.1.10) is specified then only accessible virtual methods can be overridden.

If a type overrides an inherited method through means other than a MethodImpl, it can widen, but it shall not narrow, the accessibility of that method. As a principle, if a client of a type is allowed to access a method of that type, then it should also be able to access that method (identified by name and signature) in any derived type. Table 7.1 specifies narrow and widen in this context—a "Yes" denotes that the derived class can apply that accessibility, a "No" denotes it is invalid.

If a type overrides an inherited method via a MethodImpl, it can widen or narrow the accessibility of that method.

Table II.1: Valid Widening of Access to a Virtual Method

Derived class\Base type Accessibility Compiler-controlled private family assembly famandassem famorassem public
Compiler-controlled See note 3 No No No No No No
private See note 3 Yes No No No No No
family See note 3 Yes Yes No Yes See note 1 No
assembly See note 3 Yes No See note 2 See note 2 No No
famandassem See note 3 Yes No No See note 2 No No
famorassem See note 3 Yes Yes See note 2 Yes Yes No
public See note 3 Yes Yes Yes Yes Yes Yes

  1. Yes, provided both are in different assemblies; otherwise, No.

  2. Yes, provided both are in the same assembly; otherwise, No.

  3. Yes, provided both are in the same module; otherwise, No.

[Note: A method can be overridden even if it might not be accessed by the derived class.

If a method has assembly accessibility, then it shall have public accessibility if it is being overridden by a method in a different assembly. A similar rule applies to famandassem, where also famorassem is allowed outside the assembly. In both cases assembly or famandassem, respectively, can be used inside the same assembly. end note]

A special rule applies to famorassem, as shown in the table. This is the only case where the accessibility is apparently narrowed by the derived class. A famorassem method can be overridden with family accessibility by a type in another assembly.

[Rationale: Because there is no way to specify "family or specific other assembly" it is not possible to specify that the accessibility should be unchanged. To avoid narrowing access, it would be necessary to specify an accessibility of public, which would force widening of access even when it is not desired. As a compromise, the minor narrowing of "family" alone is permitted. end rationale]