How to get the pointer of "this" ?

[Vinod K Chandran via Digitalmars-d-learn]

Hi all,
I have a class like this.
class Button : Control {
        ...
        HWND createButton(){
                ...
		SetWindowSubclass(this.mHandle, SUBCLASSPROC(&btnWndProc), 
UINT_PTR(subClsID), cast(DWORD_PTR) this);
        }
}
But compiler says that - "Error: 'this' is not an lvalue and 
cannot be modified"
I've seen many cpp code which uses pointer to 'this' as the last 
parameter in SetWindowSubclass(). Is there something wrong with 
my approach ?