help with invalad cast exception
 

hello, i get a invalid cast exception for the following line : costList = (List<mycost>)abf.Deserialize(df);

exact error is : Unable to cast object of type 'System.Collections.Generic.List`1[test.Form1+comoditys]' to type 'System.Collections.Generic.List`1[test.Form2+mycost]'.

i have looked this up online and cannot find a good answer to this issue can anyone help me out?