Delphi GetClass

Question

My question is how to access a class which is in an other unit? For an example:

program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils,
  Classes,
  System,
  StrUtils,
  Math,
  TypInfo,
  Data in 'Data.pas';

var
  Str, name, value                      : string;
  List, tmpList                         : TStringList;
  i                                     : Integer;
  Obj                                   : TObject;
  CRef                                  : TPersistentClass;
  d                                     : TData;
begin
  d := TData(GetClass('Data.TData').Create);
  Writeln(Format('%s', [d.Name]));
  Readln;
  Readln;
end.

And the Data unit :

unit Data;

interface
 uses
  SysUtils,
  Classes;
type
  TData = class(TObject)
    FName : string;
  published
    property Name : string read FName write FName;
  end;
type
  TIn = class(TObject)
    FName : string;
  published
    property Name : string read FName write FName;
  end;
implementation

end.

The problem is that the method GetClass return me always nil. I know that there is a questions like this one but they doesn't helped me.

Thanks in advance!

Answer 1

If you are using one of the later Delphi versions you can use the RTTI unit.

uses RTTI;
..
  var
    R : TRttiContext;
  begin
    R.FindType('Data.TData')
...

In your example TIn is not a inner class, but inner classes can also be accessed like this:

R.FindType('Data.TData.TIn')

Answer 2

Given that d is defined as TData, can't you simply use d := TData.Create()?

Anyway if you want to create class from its name, you have to (look this link) call RegisterClass before using GetClass or that class won't be recognized and you get a nil.

Answer 3

You didn't register the class. You need to call RegisterClass.

How can I register class if I have only the name in string?

Typically you would place a call to RegisterClass in the initialization section unit that declares the class.

When you come to call RegisterClass you will discover that the class needs to derived from TPersistent:

procedure RegisterClass(AClass: TPersistentClass);

Answer 4

Please read the GetClass documentation. It only works with registered persistent classes. So The first thing you must change in order to use it is descend from TPersistent, ie

type
  TData = class(TPersistent)
    FName : string;
  published
    property Name : string read FName write FName;
  end;

and then you have to make sure that the class is registred, ie you have to call RegisterClasses, perhaps in the initialization section of the unit

initialization
  RegisterClasses([TData]);