delphi - 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