Delphi in a Nutshell

Name

As Keyword

Syntax

Object reference as Class type
Object or interface reference as Interface type

Description

The as operator converts an object reference to a different class type or converts an interface reference to a different interface type. The type of the expression is the class or interface type on the right-hand side of the as operator.

Tips and Tricks

• If the object or interface reference is nil, the result is nil.

• The object’s declared class must be a descendant or ancestor of the class type. If the object reference is not of a compatible type, the compiler issues an error. If the declared type is compatible, but the object’s true type at runtime is not the class type or a descendant type, Delphi raises runtime error 10 (EInvalidCast).

• You should use the as operator instead of a type cast when typecasting an object reference. The only exception is when you know the type from an earlier use of the is operator.

• If the desired type is an interface, Delphi calls the QueryInterface method, passing the interface type’s GUID as the first argument. If the object does not implement the interface, Delphi raises runtime error 23 (EIntfCastError).

Example

// When any check box is checked, enable the OK button.
// This event handler can be used for multiple check boxes.
procedure TForm1.CheckBox1Click(Sender: TObject);
begin
  if (Sender as TCheckBox).Checked then
    OkButton.Enabled := True;
end;

See Also

Interface Keyword, Is Keyword, TObject Type