em có đoạn code như này mong a chị giúp đỡ.

DataTable dt = new DataTable();
 dt = (DataTable)Session["GioHang"];

khi chạy báo lỗi 

An exception of type 'System.InvalidCastException' occurred in App_Web_0v0jypz2.dll but was not handled in user code

Additional information: Unable to cast object of type 'System.String' to type 'System.Data.DataTable'.