I have a error Object cannot be cast from DBNull to other types?

Question

My code is : I am retrieving the data fron database when i am doing the breakpoint it show the data in list,but it also give me a error

    public static List<StudentScore> GetAllScore()
   {
       SqlConnection conn = MyDB.GetConnection();
       string selectStm = "SELECT en.CourseID,en.Score,s.StudentID FROM EnrollmentTable en,Student s WHERE en.StudentID = s.StudentID";
       SqlCommand command = new SqlCommand(selectStm, conn);
       List<StudentScore> aStudentScore = new List<StudentScore>();
       try
       {
           conn.Open();
           SqlDataReader reader = command.ExecuteReader();          
           Console.WriteLine(reader.HasRows.ToString());
           while (reader.Read())
           {
               StudentTable st = new StudentTable();
               CourseTable cr = new CourseTable();
               Enrollment enr = new Enrollment();
               StudentScore score = new StudentScore();
               enr.CourseData = cr;
               enr.StudentData = st;                                    
                   //score.EnrollmentData.StudentData.StudentID = reader["StudentID"].ToString();
                   //score.EnrollmentData.CourseData.CourseID = reader["CourseID"].ToString();                  
                   st.StudentID = reader["StudentID"].ToString();
               cr.CourseID = reader["CourseID"].ToString();
               score.Score = Convert.ToInt32(reader["Score"]);
               score.EnrollmentData = enr; 
               aStudentScore.Add(score);
           }
           reader.Close();
           return aStudentScore;
       }
       catch (SqlException ex)
       {
           throw ex;
       }
       finally
       {
           conn.Close();
       }

   }


}

}

It takes a data from database but show mw a this error.....Object cannot be cast from DBNull to other types so what it mean please tell me how to fix it?

Answer 1

It means you have a NULL value in the database. You have to check for it in the code, or change your column schemas to NOT NULL .

st.StudentID = reader["StudentID"] == DBNull.Value ? null : reader["StudentID"].ToString();
cr.CourseID = reader["CourseID"] == DBNull.Value ? null : reader["CourseID"].ToString();
score.Score = reader["Score"] == DBNull.Value ? 0 : Convert.ToInt32(reader["Score"]);

You have to deal with the null values in the C# objects now.

Answer 2

you need to check if the reader is of type DBNULL

Call IsDBNull() on the reader to check the column before attempting to convert it:

using (reader = server.ExecuteReader(CommandType.Text, TopIDQuery, paramet))
{
   while (reader.Read())
   {
       var column = reader.GetOrdinal("TopID");

       if (!reader.IsDBNull(column))
          topID = Convert.ToInt32(reader[column]);
       }
   }
}

Or, compare against DBNull.Value:

var value = reader["TopID"];

if (value != DBNull.Value)
{
    topID = Convert.ToInt32(value);
}

Answer 3

DBNull is used to represent a null value in a DataBase.

You should check if the value isn't DBNull before casting ir.

object score = reader["Score"];

score.Score = score == DBNull.Value ? 0 : Convert.ToInt32(score);