Default Access Modifiers in CSharp

Non-nested types, enumeration and delegate accessibilities (may only have internal or public accessibility)

accessibilities      | Default   | Permitted declared 
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namespace            | public    | none (always implicitly public)

enum                 | public    | none (always implicitly public)

interface            | internal  | public, internal

class                | internal  | public, internal

struct               | internal  | public, internal

delegate             | internal  | public, internal

Nested type and member accessiblities

accessibilities      | Default   | Permitted declared 
------------------------------------------------------------------
namespace            | public    | none (always implicitly public)

enum                 | public    | none (always implicitly public)

interface            | public    | none

class                | private   | All¹

struct               | private   | public, internal, private²

delegate             | private   | All¹

constructor          | private   | All¹

interface member     | public    | none (always implicitly public)

method               | private   | All¹

field                | private   | All¹

user-defined operator| none      | public (must be declared public)

¹ All === public, protected, internal, private, protected internal

² structs cannot inherit from structs or classes (although they can, interfaces), hence protected is not a valid modifier