刚开始接触C#，，没看C#这门语言的书，手头上有一本.NET编程实例。
响应一个按扭时做出在代码段中添加了这两行，为什么提示错误？

  private: System::Void button3_Click(System::Object^  sender, System::EventArgs^  e) {
                 this->FormBorderStyle=FormBorderStyle::Sizable;
                 this->WindowState=FormWindowState::Maximized;
             }

为什么说:
'Sizable' : is not a member of 'System::Windows::Forms::Form::FormBorderStyle'
See declaration of 'System::Windows::Forms::Form::FormBorderStyle'
error C2065: 'Sizable' : undeclared identifier