----------
Declared accessibility  Meaning  
public 			Access is not restricted.

protected		Access is limited to the containing class or types derived from the containing class.
                        存取只限於包含的類別或衍生自包含類別的型別.

internal		Access is limited to the current assembly.
                        存取只限於目前的組件

protected internal	Access is limited to the current assembly or types derived from the containing class.
                        存取只限於目前的組件或衍生自包含類別的型別

private			Access is limited to the containing type.
                        存取只限於包含類別



---------------	---------------------------------------	--------------------------------------------
Members of	Default member accessibility		Allowed declared accessibility of the member
成員所屬型別    成員預設存取範圍                        成員允許的宣告存取範圍
---------------	---------------------------------------	--------------------------------------------
enum		public					None
---------------	---------------------------------------	--------------------------------------------
class		private					public 
							protected
							internal
							private
							protected internal
---------------	---------------------------------------	--------------------------------------------
interface	public					None
---------------	---------------------------------------	--------------------------------------------
struct		private					public 
							internal 
							private 

----------
Internal 

The internal keyword is an access modifier for types and type members. 
Internal types or members are accessible only within files in the same assembly, as in this example:

public class BaseClass 
{
    // Only accessible within the same assembly
    internal static int x = 0;
}

----------
protected

The protected keyword is a member access modifier. 
A protected member is accessible within its class and by derived classes. 

----------
利用private constructor防止以new建立實體
public FOO
{
    static FOO() { ... }
    private FOO() {} // 利用private constructor防止以new建立實體

    public static ... Property { set{...} get{...} }
    public static void Method() { ... }
}