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?

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

And how do I get access to plain or virtual private methods?

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

Previously I would use a class helper to break open the base class.

type
  TBaseHelper = class helper for TBase
    function GetMemberVar: integer;

In Delphi 10.1 Berlin, class helpers no longer have access to private members of the subject class or record.

Is there an alternative way to access private members?

Answer 1

If there is extended RTTI info generated for the class private members - fields and/or methods you can use it to gain access to them.

Of course, accessing through RTTI is way slower than it was through class helpers.

Accessing methods:

var
  Base: TBase2;
  Method: TRttiMethod;

  Method := TRttiContext.Create.GetType(TBase2).GetMethod('UsefullButHidden');
  Method.Invoke(Base, []);

Accessing variables:

var
  Base: TBase;
  v: TValue;

  v := TRttiContext.Create.GetType(TBase).GetField('FMemberVar').GetValue(Base);

---

Default RTTI information generated for RTL/VCL/FMX classes is following

• Fields - private, protected, public, published
• Methods - public, published
• Properties - public, published

Unfortunately, that means accessing private methods via RTTI for core Delphi libraries is not available. @LU RD's answer covers hack that allows private method access for classes without extended RTTI.

Working with RTTI

Answer 2

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;

---

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.

---

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])}

Answer 3

If you want a clean way that does not impact performance, you still can access private fields from a record helper using the with statement.

function TValueHelper.GetAsInteger: Integer;
begin
  with Self do begin
    Result := FData.FAsSLong;
  end;
end;

I hope they keep this method open, because we have code with high performance demands.

Answer 4

Assuming that extended RTTI is not available, then without resorting to what would be considered hacking, you cannot access private members from code in a different unit. Of course, if RTTI is available it can be used.

It is my understanding that the ability to crack private members using helpers was an unintentional accident. The intention is that private members only be visible from code in the same unit, and strict private members only be visible from code in the same class. This change corrects the accident.

Without the ability to have the compiler crack the class for you, you would need to resort to other ways to do so. For instance, you could re-declare enough of the TBase class to be able to trick the compiler into telling you where a member lived.

type
  THackBase = class(TObject)
  private
    FMemberVar: integer;
  end;

Now you can write

var
  obj: TBase;
....
MemberVar := THackBase(obj).FMemberVar;  

But this is horrendously brittle and will break as soon as the layout of TBase is changed.

That will work for data members, but for non-virtual methods, you'd probably need to use runtime disassembly techniques to find the location of the code. For virtual members this technique can be used to find the VMT offset.

Further reading:

• http://hallvards.blogspot.nl/2004/06/hack-5-access-to-private-fields.html
• https://bitbucket.org/NickHodges/delphi-unit-tests/wiki/Accessing%20Private%20Members

Answer 5

Just use 'with' statement to access private fields !

See the sample code below, taken from this article I noticed today. (Thanks, Mr.DEKO as always !)

This hack is originally reported on QualityPortal in August 2019 as described on above aritcle. (login required)

---

before rewrite (using "asm" method)

function TPropertyEditorHelper.GetPropList: PInstPropList;
{$IF CompilerVersion < 31.0}
begin
  Result := Self.FPropList;
end;
{$ELSE}
// http://d.hatena.ne.jp/tales/20160420/1461081751
asm
  MOV EAX, Self.FPropList;
end;
{$IFEND}

rewrite using 'with'

function TPropertyEditorHelper.GetPropList: PInstPropList;
begin
  with Self do
    Result := FPropList;
end;

I was amazed it's so simple :-)