public class A

{

    protected int x = 123;

    public static void Main()

    {

        var a = new A();

        var b = new B();

​

        // Error CS1540, because x can only be accessed by

        // classes derived from A.

        a.x = 10; 

        Console.WriteLine(a.x);

        // OK, because this class derives from A.

        b.x = 10;

    }

}

​

public class B : A

{

    public static void Main2()

    {

        var a = new A();

        var b = new B();

​

        // Error CS1540, because x can only be accessed by

        // classes derived from A.

        //a.x = 10; 

​

        // OK, because this class derives from A.

        b.x = 10;

    }

}