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TObject(Sender) vs. (Sender as TObject) Differences Between a Hard Cast and an AS Cast in Delphi

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;