Supports

This week I had to maintain some legacy code that uses many interfaces and supports in order to find out if a specific interface is supported:

type
  IMyInterface = interface(IInterface)
    ['{F500EEDD-032E-4D15-A8C0-EFF673C2AF01}']
    procedure DoSomething;
  end;

procedure MyProcedure(const AIntf: IInterface);

implementation

uses
  System.SysUtils;

procedure MyProcedure(const AIntf: IInterface);
var
  lMyIntf: IMyInterface;
begin
  if Supports(AIntf, IInterface, lMyIntf) then
    lMyIntf.DoSomething;
end;

One of the tasks I had to do is to replace the interfaces with pure Delphi objects. That’s why I wondered if there is a chance to write something similar for objects.

I played a bit and found the following:

type
  TObjectHelper = class helper for TObject
  public
    function Supports<T: class>(out ADest: T): Boolean;
  end;

...

{ TObjectHelper }

function TObjectHelper.Supports<T>(out ADest: T): Boolean;
begin
  if Self.InheritsFrom(T) then
  begin
    ADest := T(Self);
    Result := True;
  end
  else
  begin
    ADest := nil;
    Result := False;
  end;
end;

...

var
  lObject: TObject;
  lMyObject: TMyObject;
begin
  ...
  if lObject.Supports<TMyObject>(lMyObject) then
    lMyObject.DoSomething;