As

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The reserved word as is used to cast (use one variable type as if it were another type)

procedure TmainForm.selectionGridDrawColumnCell(Sender: TObject; const Rect: TRect; DataCol: Integer; Column: TColumn; State: TGridDrawState);
begin
     (Sender As TDBGrid).DefaultDrawColumnCell(Rect,DataCol,Column,State);
end;

here the Sender variable of type TObject is being cast as type TDBGrid

As is an Object Pascal and a Borland Delphi extension.