public class Class1
    {
        int  i;				//No Access specifier means private
        public  int j;			// Public
        protected int k;			//Protected data
        internal int m;			// Internal means visible inside assembly
        protected internal int n;		//inside assembly as well as to derived classes outside assembly
        static int x;			// This is also private
        public static int y;		//Static means shared across objects
        [DllImport("MyDll.dll")]
        public static extern int MyFoo();  	//extern means declared in this assembly defined in some other assembly
        public void myFoo2()
        {
            //Within a class if you create an object of same class then you can access all data members through object reference even private data too
            Class1 obj = new Class1();
            obj.i =10;	//Error can’t access private data through object.But here it is accessible.:)
            obj.j =10;
            obj.k=10;
            obj.m=10;
            obj.n=10;
       //     obj.s =10;  //Errror Static data can be accessed by class names only
            Class1.x = 10;
         //   obj.y = 10; //Errror Static data can be accessed by class names only
            Class1.y = 10;
        }
    }

Now lets try to copy the same code inside Main method and try to compile
[STAThread]
        static void Main()
        {
           //Access specifiers comes into picture only when you create object of class outside the class
            Class1 obj = new Class1();
       //     obj.i =10;	//Error can’t access private data through object.
            obj.j =10;
      //      obj.k=10;     //Error can’t access protected data through object.
            obj.m=10;
            obj.n=10;
       //     obj.s =10;  //Errror Static data can be accessed by class names only
            Class1.x = 10;  //Error can’t access private data outside class
         //   obj.y = 10; //Errror Static data can be accessed by class names only
            Class1.y = 10;
        }
What if Main is inside another assembly
[STAThread]
        static void Main()
        {
           //Access specifiers comes into picture only when you create object of class outside the class
            Class1 obj = new Class1();
       //     obj.i =10;	//Error can’t access private data through object.
            obj.j =10;
      //      obj.k=10;     //Error can’t access protected data through object.
     //     obj.m=10; // Error can’t access internal data outside assembly
    //      obj.n=10; // Error can’t access internal data outside assembly

       //     obj.s =10;  //Errror Static data can be accessed by class names only
            Class1.x = 10;  //Error can’t access private data outside class
         //   obj.y = 10; //Errror Static data can be accessed by class names only
            Class1.y = 10;
        }