Title: TObject(Sender) vs. (Sender as TObject) - Differences Between a Hard Cast and an AS Cast in Delphi
In Delphi the AS keyword is used for casting an object (or interface) of one type to another.
For example if a class TDeveloper extends (derives from) TPerson as in:
type TDeveloper = class(TPerson)
you can safely cast an instance of type TDeveloper ("aDeveloper") to a TPerson using the (aDeveloper as TPerson) - this is called "as-cast".
What's more you can also use the hard-cast: TPerson(aDeveloper).
The question is: what's the difference between a hard-cast and an as-cast?
Hard-Cast vs. AS-Cast in Delphi
When several controls share an event handling procedure, for example a button and a menu item, you end up using the Sender parameter to identify the control that actually caused the event to be raised.
begin
if Sender is TButton then
begin
//do something if the button was clicked
end;
if Sender is TMenuItem then
begin
//do something if the menu item was clicked
end;
end;
Using the is keyword you can verify the actual runtime class of an object. The "Sender is TButton" checks whether the Sender parameter is a button or any type descending from TButton.
After you do the is-check you can cast an object to a type you need to operate with.
With a hard-cast, you are telling the compiler that you know what you expect the object to be. Hard-casts are therefore unsafe - Delphi will not complian if you hard-cast an instance of a TEdit to a TButton as in:
TButton(Edit1).Caption := 'junk code';
Will this work? Yes it will! Casting an edit box to a button, then chaning the Caption property of a "button" will result in changing the Text property of the Edit1 edit box.
On the other hand, the safe as-cast will raise the EInvalidCast exception when you try to use the cast value.
//raises "Invalid Type Cast"
(Edit1 as TButton).Caption := 'junk code';
Contrary to the hard-cast, when the as-cast is used with an object, additional code is generated to verify at run time that the new type is a base class of the referenced object.
Safe Hard-Cast with the Is-Checking
You'll often find Delphi code like :
if Sender is TButton then
begin
TButton(Sender).DoSomething;
end
We are combining the safe is-case with the unsafe hard-cast to produce a safe hard-cast.
You should generaly use the above code and not the is-check followed by the as-cast since as-casting is "time" consuming - compiler needs to traverse the RTL inheritance chain to see if an object can be casted to the class specified.
The Tricky Hard-Cast
A common scenario of storing integer values along strings inside a string list is a commonly used hard-casting trick in Delphi:
var
sl : TStringList;
begin
sl := TStringList.Create;
try
//add integer values as objects
sl.AddObject('One',TObject(1)) ;
sl.AddObject('Two',TObject(2)) ;
//hard cast an object to integer
int := Integer(sl.Objects[0]) ;
finally
sl.Free;
end;
end;