access modifiers

public – Access is unrestricted.
private – Access is limited to this type in this assembly only.
protected – Access is limited to this type or derivatives, or derivatives in a referenced assembly.
internal – Access is limited to this assembly only.
protected internal – Combination of protected + internal.
private protected – Combination of private + protected.

Modifier	This Assembly (Derived Type)	This Assembly (Other Type)	External Assembly (Derived Type)	External Assembly (Other Type)
public	yes	yes	yes	yes
private	no	no	no	no
protected	yes	no	yes	no
internal	yes	yes	no	no
protected (internal)	yes	yes	yes	no
private (internal)	yes	no	no	no

other modifiers

virtual – This member may be overridden in a derived type.
abstract – This type must be overridden in a derived type.
- It has no implementation.
- It cannot be instantiated.
override – This member provides a new implementation of the base type’s virtual or abstract member.
sealed – This type cannot be further overridden in a derived type (meaning that is also cannot be abstract).

access modifiers for members

The accessibility of the return type and parameter types of a method, indexer, or delegate must be >= that of the member itself:

For a public method M that returns class C, C must also be public.
For a protected property of Type A, A cannot be private.

struct members

Default: private
Options: public, internal, private

class members

Default: private
Options: public, protected, internal, protected internal, private protected, private

interface members

Default: public
Options: public, protected, internal, protected internal, private protected, private

enumeration members

Always public. No access modifiers allowed.

operators

Must always be public and static.

finalizers

Finalizers cannot have access modifiers.

`file` Modifier

Restricts top-level type’s scope and visibility to the file in which it is declared.
Declares a file-local type.
Generally applied to types written by a source generator.

Classes.cs:

// Only visible in Classes.cs
file class HiddenClass 
{
    // ...   
}