When you declare a property, you usually need to declare a private field to store the data for the property, then specify methods for reading and writing the property value. Often, you don't need to use a method to read the value, but can just point to the stored data instead. 

For the shape control, you will declare a field that holds the current shape, then declare a property that reads that field and writes to it through a method call. 

Add the following declarations to TSampleShape:

type
  TSampleShape = class(TGraphicControl)
  private
    FShape: TSampleShapeType;  { field to hold property value }
    procedure SetShape(Value: TSampleShapeType);
  published
    property Shape: TSampleShapeType read FShape write SetShape;
  end;

class PACKAGE TSampleShape : public TGraphicControl
{
private:
    TSampleShapeType FShape;
    void __fastcall SetShape(TSampleShapeType Value);
__published:
    __property TSampleShapeType Shape = {read=FShape, write=SetShape, nodefault};
};

Now all that remains is to add the implementation of SetShape.