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;

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» An Invalid Hard-Cast is NIL in Delphi for .NET. Find out why TButton(Edit1) is NIL in Delphi for .NET
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