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unit Unit1;

interface

uses ...

type
  TBla = class
  public
    x : integer;
  end;

...

var
  b : TBla;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
     b.x := 0; {speicherzugriffsfehler hier, warum?)
end;

end.					

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procedure TForm1.Button1Click(Sender: TObject);
begin
  b := TBla.Create;
  try
    b.x := 10;
  finally
    b.Free;
  end;
end;