WTF abstract is?

[Zhouxuan]

////////////////////////////////////////////////////////////
//case 1

class A
{
     abstract void foo();
}

class B : A
{
     static if( __traits(isAbstractClass, typeof(this) ))
     {
     }

     override void foo()
     {
     }
}

void main()
{
     B b = new B();
}

//Error: cannot create instance of abstract class B
////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////
//case 2

abstract class A
{
     void foo();
}

class B : A
{
     static if( __traits(isAbstractClass, typeof(this) ))
     {
     }

     override void foo()
     {
     }
}

void main()
{
     B b = new B();
}

//Okay
////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////
//case 3

class A
{
     abstract void foo();
}

class B : A
{
     override void foo()
     {
     }
}

void main()
{
     B b = new B();
}

//Okay
////////////////////////////////////////////////////////////

How awkward it is given that "D is as old as C#"!

Do you guys never used abstract class?