as

     The As keyword is used for casting objects or interfaces of one type to another. Casting allows an object to be referenced by a parent class type. For example, all objects may be referred to as a TObject class type:

     button1 := Button1 As TObject;

     If the object has already been cast to a parent class type, then casting to a valid child class type is allowed. Use the Is keyword to check for castability before attemting a cast. Invalid casting gives EInvalidCast when you try to use the cast value.

Example

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{$MODE DELPHI} // Support "Class"

uses
   Classes; // so we have something to refer to

type
   myStringList= class(TStringList)
   public
      procedure WriteLnList;
   end;

var
  myByte  : Byte;
  myChar  : Char;
  StrList1: TStringList;
  StrList2: myStringList;

procedure myStringList.WriteLnList;
Var
   Loop:Integer;

begin
   For Loop:=0 to Count-1 do
      Writeln(Loop,' contains ',Self[Loop]);
end;
  
begin
  myByte := 65;

  // Cast this Byte to Char using the standard casting method
  myChar := Char(myByte);
  Writeln('myByte standard casting to Char = ',myChar);

  StrList1 := TStringList.Create;
  StrList1.Add('Apple');
  StrList1.Add('Banana');
  StrList1.Add('Orange');
  // Standard casting does not check if we change classes:
  myStringList(StrList1).WritelnList;

  // Casting using 'as' rejects the invalid casting
  StrList2 := StrList1 as myStringList;

  // The following yields the EInvalidCast error
  StrList2.WritelnList;

  StrList1.Free;
end.

Output

myByte standard casting to Char = A
0 contains Apple
1 contains Banana
2 contains Orange
An unhandled exception occurred at $08048263 :
EInvalidCast : Invalid type cast

See Also

Is.

Matias Vara
toro.sourceforge.net