How to access private methods without helpers?

Question

In Delphi 10 Seattle I could use the following code to work around overly strict visibility restrictions.

How do I get access to private variables?

Answer 1

There is still a way to use class helpers for access of private methods in Delphi 10.1 Berlin:

type
  TBase2 = class(TObject) 
  private
    procedure UsefullButHidden;  
    procedure VirtualHidden; virtual;
    procedure PreviouslyProtected; override;
  end;

  TBase2Helper = class helper for TBase2
    procedure OpenAccess;
  end;

  procedure TBase2Helper.OpenAccess;
  var
    P : procedure of object;
  begin
    TMethod(P).Code := @TBase2.UsefullButHidden;
    TMethod(P).Data := Self;
    P; // Call UsefullButHidden;
    // etc
  end;

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Unfortunately there is no way to access strict private/private fields by class helpers with Delphi 10.1 Berlin. RTTI is an option, but can be considered slow if performance is critical.

Here is a way to define the offset to a field at startup using class helpers and RTTI:

type 
  TBase = class(TObject)
  private  // Or strict private
    FMemberVar: integer;
  end;

type
  TBaseHelper = class helper for TBase
  private
    class var MemberVarOffset: Integer;
    function GetMemberVar: Integer;
    procedure SetMemberVar(value: Integer);
  public
    class constructor Create;  // Executed at program start
    property MemberVar : Integer read GetMemberVar write SetMemberVar;
  end;

class constructor TBaseHelper.Create;
var
  ctx: TRTTIContext;
begin
  MemberVarOffset := ctx.GetType(TBase).GetField('FMemberVar').Offset;
end;

function TBaseHelper.GetMemberVar: Integer;
begin
  Result := PInteger(Pointer(NativeInt(Self) + MemberVarOffset))^;
end;

procedure TBaseHelper.SetMemberVar(value: Integer);
begin
  PInteger(Pointer(NativeInt(Self) + MemberVarOffset))^ := value;
end;

This will have the benefit that the slow RTTI part is only executed once.

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Note: Using RTTI for access of protected/private methods

The RTL/VCL/FMX have not declared visibility for access of protected/private methods with RTTI. It must be set with the local directive {$RTTI}.

Using RTTI for access of private/protected methods in other code requires for example setting :

{$RTTI EXPLICIT METHODS([vcPublic, vcProtected, vcPrivate])}