对访问修饰关键字public, protected, internal and private的说明

对访问修饰关键字public, protected, internal and private的说明
1.
msdn: Internal types or members are accessible only within files in the same assembly.
只要在同一个assembly中就能访问，而不是命名空间。
如下面的例子，在同一个assembly ClassLibrary2.dll中：
namespace ClassLibrary2
{
    public class Class1
    {
        protected string TestString1 { get; set; }
        internal string TestString2 { get; set; }
        protected internal string TestString3 { get; set; }
    }
}

// 在子命名空间中访问
namespace ClassLibrary2.SubNamespace
{
    public class Class2
    {
        public Class2()
        {
            Class1 c1 = new Class1();
            //c1.TestString1 = "1"; // ‘ClassLibrary2.Class1.TestString1‘ is inaccessible due to its protection level
            c1.TestString2 = "2";
            c1.TestString3 = "3";
        }
    }
}

// 在另一个命名空间中访问
namespace AnotherNamespace
{
    public class Class3
    {
        public Class3()
        {
            ClassLibrary2.Class1 c1 = new ClassLibrary2.Class1();
            //c1.TestString1 = "1"; // ‘ClassLibrary2.Class1.TestString1‘ is inaccessible due to its protection level
            c1.TestString2 = "2";
            c1.TestString3 = "3";
        }
    }
}

2. 
protected表示只能在定义它的类的内部或者它的子类内部访问。
被protected internal两个关键字同时修饰的表示能被子类访问（子类可以在另一个assembly中）或者在同一个assembly中访问。
注意它们上并集，不是交集。
例如有另外一个assembly, ClassLibrary3.dll:
namespace ClassLibrary3
{
    public class A : ClassLibrary2.Class1
    {
        public A()
        {
            this.TestString1 = "1";
            //this.TestString2 = "2";  // cannot see it.
            this.TestString3 = "3";
        }
    }
}

3.
Top-level types, which are not nested in other types, can only have internal or public accessibility. The default accessibility for these

types is internal. 
如果尝试用private，protected, or protected internal修饰一个Top-level class,例如：
private class A
{
}
会报一个错：Error，Elements defined in a namespace cannot be explicitly declared as private, protected, or protected internal

但是可以修饰nested class, 如下，是不报错的。B缺省为internal。
class B
{
     private class B1 { }
     protected class B2 { }
     protected internal class B3 { }
}
nested type的缺省修饰如下：
Members of         Default member accessibility          Allowed declared accessibility of the member

enum              　　 public                                None
class                 　  private                              public, protected, internal, protected internal, pirvate
interface          　　public                                None

对访问修饰关键字public, protected, internal and private的说明

原文：http://www.cnblogs.com/0to9/p/5086578.html