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

[Steven Schveighoffer]

On 5/22/20 9:10 AM, Vinod K Chandran wrote:
> On Friday, 22 May 2020 at 12:21:25 UTC, rikki cattermole wrote:
>> if (Child child = cast(Child)parent) {
>>     assert(child !is null);
>> }
> 
> Actually, problem occurs in addHandler function. It expects an argument 
> of type "EventArgs", not MouseEventArgs.

Yes, because what if you did this with your function:

fnp(new EventArgs(...));

It would be called with the type being implicitly cast to the child type 
without that being true.

What Rikki was recommending is that you write your handler like this:

void onClick(EventArgs e){
     if(auto me = cast(MouseEventArgs)e) {
     log("form clicked on x = ", me.x, ", and y = ", me.y);
     }
}

Actually, if you are certain it's a programming error for onClick to be 
called with a different type of event args, I'd do:

void onClick(EventArgs e){
     auto me = cast(MouseEventArgs)e;
     assert(me !is null, "Error, onClick called with invalid event type");
     log("form clicked on x = ", me.x, ", and y = ", me.y);
}