c# - Exception: The null value cannot be assigned to a member with type System.Int32 which is a non-nullable value type -

Can anyone explain why this exception is coming in the following LINQ query:

 
 Code> Back to DC (Classification of C. Categorization where CID == Select the Classification ID Classification () {Description = C. Description, PainterID = Convert. ToInt16 (c.ParentId),}). Single & lt; Classification & gt; ();   
 is DC detacentext, classification is a class in which the intribute is ParentId. In the case of parent field empty in ParentId column SQL Server database is a nullable int, the statement returns invalid operation exceptions.  
 In other words, why the above query fails and 'int y = convert.OINT16 (empty);' Works?   
 
 
 if  can be c.PartentId   null , Then  Convert.ToInt16 (null)  throws an exception.  
 Since you indicate that the  classification ParentId is an integer, is there any reason that you are using  Convert.ToInt16  instead of a little? Do not you want ToInt32 instead? For that matter, why convert all to? Just:    ParentId = c.ParentId ?? 0   
 ... and just to knit-off, technically you do not need to specify your type at the end of the link expression:  
 . Single & lt; Classification & gt; ()   
 You can forget this, because it is determined by the compiler, and just do it:  
 . Single ()   
 
  Update:   
 Oh, I see, I'm sorry I got your original question wrong I read from Why the question really does:  
  int y = convert toInt16 (empty);   
 While working, the same thing in Linq2Sql expression throws an exception.  
 I do not have a very good answer, except for other things, expression is the same in the  view  code, t is actually handled by 2 differnet Linq implementation (an interface There may be different backing develations as well).  
 In this case:  
  int y = Convert.ToInt16 (empty);   
 You are making a direct call to convert. ToInt16 This is a narrow  default & lt; T & gt; , where t is the desired type (hence it returns 0 in this case).  
 However, when a Linq2Sql expression is used in the expression and its launch is assigned to the Linq2Entities or Linq2Sql process. There may be a bug somewhere in that stuff, used as the original Linq2Objects (or whatever you want to call it), it actually works fine:  
  [ TestMethod] // Exam Public Zero TestLinqToObjects () {var stuff = new list & lt; Int? & Gt; () {tap}; Var y = (Select from Convert to x to ToInt32 (x)). first (); Decide. First (0, Y); }   
 The above "selection" works, however Linq2Sql or EntityFramework will cause a different implementation of the Linq processor to handle the expression by putting the same Linq expression with the collection; ND is somewhat different to try to assess the expression, or to change some of it to a SQL statement, or there may be other bugs that others do not apply.  
 I know that does not really solve your problem, but can it help explain it?