How to use base class & child class as parameter in one function ?

[Vinod K Chandran]

Hi all,
I have a windows gui setup like this;

class EventArgs {} \\ Base class for all messages
class MouseEventArgs : EventArgs {	// child class for handling 
mouse messages
     ...
     int x;
     int y;
     this(WPARAM wpm, LPARAM lpm){
	this.x = xFromLparam(lpm);
	this.y = yFromLparam(lpm);
     }
}

alias EvtFuncPtr = void function(EventArgs); // function pointer

struct MsgHandler {
     uint message ;
     HWND handle ;
     bool isActive ;
     EvtFuncPtr fnPtr;
}
// Then in my window class...
void addHandler(uint we, EvtFuncPtr fnp){ // This is error point.
	auto mh = MsgHandler();
	mh.handle = this.mHandle;
	mh.message = we;
	mh.fnPtr = fnp;
	mh.isActive = true;
	this.msgHandlerList ~= mh;  // This is a list<MsgHandler> in 
window class.	
}

// And in the WndProc...
auto thisWin = findWindowClass(hWnd);  // get the window class 
with hWnd.
auto mh = thisWin.findHandler(hWnd, message);    // get the event 
handler for this message & hWnd
if(mh.isActive) { // if there is an event handler fixed,
	switch (message){
	   case 512 : .. case 526 :  // if it's a Mouse messages
		auto ea = new MouseEventArgs(wParam, lParam);
		mh.fnPtr(ea);   // execute that event handler function
		break;
	   default : break;
	}
}

// And this is the window creation site...
auto app = new Application() ;
auto frm = new Window(app) ;
frm.createWindow() ;

frm.addHandler(frm.load, &onLoad);
frm.addHandler(frm.Click, &onClick);

void onLoad(EventArgs e){
	log("form is loaded...");
}
void onClick(MouseEventArgs e){	 // Compiler wants to change the 
MouseEventArgs with EventArgs.
	log("form clicked on x = ", e.x, ", and y = ", e.y);
}

I wrote  EventArgs as the parameter type in function pointer but 
i want to use it's child classes also. But i can't.