c# - Incorrect reading data from .bin file -

i have 2 programs: 1 test students take , second teachers(teachers create variants of test , add questions it); teacher program creates .bin files tests , student's open , take tests. data structure(class question) similiar in both programs; teacher's program class code:

[serializable]     public class question     {         public question(string q_text, dictionary<string, bool> ans, image img)         {             text = q_text;             answers = ans;             image = img;             isanswered = false;         }         public string text { get; set; }         public dictionary<string, bool> answers { get; set; }         public image image { get; set; }         public bool isanswered;          public static void persistobject(dictionary<int, question> q, stream stream)         {             binaryformatter formatter = new binaryformatter();             formatter.serialize(stream, q);             stream.close();         }          public static dictionary<int, question> loadobject(stream stream)         {             try             {                 binaryformatter formatter = new binaryformatter();                  dictionary<int, question> deserializedobject = (dictionary<int, question>)formatter.deserialize(stream);                 stream.close();                 return deserializedobject;             }             catch             {                 return null;             }         }      } } 

and student's program class code:

[serializable]     class question     {         public question(string text, image img, dictionary<string, bool> ans)         {             question_text = text;             image = img;             answers = ans;             isanswered = false;         }         public string question_text { get; set; }         public dictionary<string, bool> answers { get; set; }         public image image { get; set; }         public bool isanswered;          public static dictionary<int, question> loadobject(stream stream)         {             try             {                 binaryformatter formatter = new binaryformatter();                  dictionary<int, question> deserializedobject = (dictionary<int, question>)formatter.deserialize(stream);                 stream.close();                 return deserializedobject;             }             catch             {                 return null;             }         }     } 

but when try read test in student program:

string file =  path + test_number + ".bin"; stream mystream = file.open(file, filemode.open); questions = question.loadobject(mystream); 

it sets questions null. when read files in teachers program it's ok.(perhaps, it's ok, because create files in teachers mode too). type of questions dictionary<int,question> .what's problem?