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

[Vinod K Chandran]

On Friday, 22 May 2020 at 16:12:12 UTC, Steven Schveighoffer 
wrote:
> 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);
> }

Thanks for the answer. I understand that, in D, derived class and 
base class are not the same as in vb.net or any other language. 
(Please correct me if i am wrong).
In vb.net, assume that i have a class setup like this--
Public Class Base
     Public Property SampleInt As Integer
End Class

Public Class Child : Inherits Base
     Public Property SampleDouble As Double
End Class
//Assume that i have a list of Base class like this--
Dim sampleList As New List(Of Base)
// Now, i can use this list like this--
sampleList.Add(New Child(10.5)) Is this possible in D without 
casting ?