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unit Unit1; 

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, FileUtil, LResources, Forms, Controls, Graphics, Dialogs; 

type
  TMyEvent = procedure(Sender: TObject; TestParam: String) of Object;

  { TForm1 }

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    fOnMyEvent: TMyEvent;
    procedure MyEventHandler(Sender: TObject; TestParam: String);
  public
    { public declarations }
  end; 

var
  Form1: TForm1; 

implementation

{ TForm1 }

procedure TForm1.MyEventHandler(Sender: TObject; TestParam: String);
begin
  ShowMessage(Sender.ClassName + ': ' + TestParam);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  fOnMyEvent := MyEventHandler; //ier kommt der Fehler: Unit1.pas(38,33) Error: Wrong number of parameters specified for call to "MyEventHandler"

end;

initialization
  {$I Unit1.lrs}

end.