Visibility Modifiers

We have already encountered quite a number of so-called modifiers-keywords that can be applied to a type or to a member. Modifiers can indicate the visibility of a method, such as public or private, or the nature of an item, such as whether a method is virtual or abstract. C# has a number of modifiers, and at this point it’s worth taking a minute to provide the complete list.

• Visibility Modifiers

These modifiers indicate which other code items can view an item.

Modifier	Applies To	Description
public	Any types or members	The item is visible to any other code.
protected	Any member of a type ,aslo any nested type	The item is visible only to any derived type.
internal	Any member of a type ,aslo any nested type	The item is visible only within its containing assembly.
private	Any types or members	The item is visible only inside the type to which it belongs.
protected internal	Any member of a type ,aslo any nested type	The item is visible to any code within its containing assembly and aslo to any code inside a derived type.

Note that type definitions can be public or private, depending on whether you want the type to be visible outside its containing assembly. 

public class mClass

{

    // etc.

}

• You cannot define types as protected, internal, or protected internal, as these visibility levels would be meaningless for a type contained in a namespace. Hence these visibilities can only be applied to members.
• However, you can define nested types (that is, types contained within other types) with these visibilities, since in this case the type also has the status of a member. Hence the following code is correct:

 public class OutClass

 {

    protected class InClass

    {

        // etc.

    }

       // etc.

  }

• If you have a nested type, the inner type is always able to see all members of the outer type.
•  Hence with the above code, any code inside InClass always has access to all members of OutClass, even where those members are private.

• Other Modifiers

These modifiers can be applied to members of types, and have various uses. A few of these modifiers also make sense when applied to types.

Modifier	Applies To	Description
new	Function Members	The member hides an inherited member with the same signature.
static	All members	The member does not operate on a specific instance of the class.
virtual	Classes and function members only	The member can be overridden by derived class.
abstract	Function members only	A virtual member that defines the signature of the member, but doesn’t provide an implementation.
override	Function members only	The member overrides an inherited virtual or abstract member.
sealed	Classes	The member overrides an inherited virtual member, but cannot be overridden by any classes that inherit from this class. Must be used in conjunction with override.