Nullable DateTime conversion

Question

Possible Duplicate    c  why cant a nullable int be assigned null as a value       Im trying to convert my reader 3  object which is datetime to be null if there is no lastPostDate for a Forum but it says Im missing a conversion  Error      Type of conditional expression cannot be determined because there is no implicit conversion between  lt null gt  and  System DateTime        public class Forums       public List lt Forum gt  GetForums                 using  SqlConnection conn   new SqlConnection ConfigurationManager ConnectionStrings  CMS   ConnectionString                         conn Open                SqlCommand cmd   new SqlCommand  sproc Forums GetForums   conn               cmd CommandType   CommandType StoredProcedure              SqlDataReader reader   cmd ExecuteReader CommandBehavior Default                List lt Forum gt  forums   new List lt Forum gt                 while  reader Read                                  var title   reader 6  ToString                    var threadCount    int reader 5                   var lastPostTitle   reader 4  ToString                       below is where im having a problem                 Nullable lt DateTime gt  lastPostDate    reader 3     DBNull Value   null   Convert ToDateTime reader 3                      var lastPostBy   reader 2  ToString                    var forumGroup   reader 1  ToString                    var description   reader 0  ToString                     Forum forum   new Forum 0     DateTime Now                      reader  Title   ToString   description                      0 false       DateTime Now true                      forumGroup   int threadCount  lastPostBy                      lastPostDate  lastPostTitle                   forums Add forum                                  return forums                                    Below is my class object for Forum with a Nullable lastPostDate      public class Forum       public Forum int forumID  string addedBy  DateTime addedDate  string title  string description  int parentID  bool moderated          string imageUrl  string updatedBy  DateTime  updatedDate  bool active  string forumGroup  int threadCount  string lastPostBy          Nullable lt DateTime gt  lastPostDate  string lastPostTitle                this ForumID   forumID          this AddedBy   addedBy          this AddedDate   addedDate          this Title   title          this Description   description          this ParentID   parentID          this Moderated   moderated          this ImageUrl   imageUrl          this UpdatedBy   updatedBy          this UpdatedDate   updatedDate          this Active   active          this ForumGroup   forumGroup          this ThreadCount   threadCount          this LastPostBy   lastPostBy          this LastPostDate   lastPostDate          this LastPostTitle   lastPostTitle             private int  forumID      public int ForumID               get   return  forumID            set    forumID   value              private string  addedBy      public string AddedBy               get   return  addedBy            set    addedBy   value              private DateTime  addedDate   DateTime Now      public DateTime AddedDate               get   return  addedDate            set    addedDate   value              private string  title           public string Title               get   return  title            set    title   value              private string  description           public string Description               get   return  description            set    description   value              private int  parentID   0      public int ParentID               get   return  parentID            set    parentID   value              private bool  moderated   false      public bool Moderated               get   return  moderated            set    moderated   value              private string  imageUrl           public string ImageUrl               get   return  imageUrl            set    imageUrl   value              private string  updatedBy           public string UpdatedBy               get   return  updatedBy            set    updatedBy   value              private DateTime   updatedDate   null      public DateTime  UpdatedDate               get   return  updatedDate            set    updatedDate   value              private bool  active   false      public bool Active               get   return  active            set    active   value              private string  forumGroup           public string ForumGroup               get   return  forumGroup            set    forumGroup   value              private int  threadCount   0      public int ThreadCount               get   return  threadCount            set    threadCount   value              private string  lastPostBy           public string LastPostBy               get   return  lastPostBy            set    lastPostBy   value              private Nullable lt DateTime gt   lastPosteDate   null      public Nullable lt DateTime gt  LastPostDate               get   return  lastPosteDate            set    lastPosteDate   value              private string  lastPostTitle           public string LastPostTitle               get   return  lastPostTitle            set    lastPostTitle   value

User · Answer

Make sure those two types are nullable DateTime  var lastPostDate   reader 3     DBNull Value                                           null                                        DateTime   Convert ToDateTime reader 3       Usage of DateTime  instead of Nullable lt DateTime gt  is a time saver    Use better indent of the   expression like I did      I have found this excellent explanations in Eric Lippert blog     The specification for the    operator states the following      The second and third operands of the    operator control the type of   the conditional expression  Let X and Y be the types of the second and   third operands  Then          If X and Y are the same type  then this is the type of the conditional   expression     Otherwise  if an implicit conversion exists from X to Y    but not from Y to X  then Y is the type of the conditional expression    Otherwise  if an implicit conversion exists from Y to X  but not from   X to Y  then X is the type of the conditional expression     Otherwise    no expression type can be determined  and a compile-time error occurs       The compiler doesn t check what is the type that can  hold  those two types    In this case     null and DateTime aren t the same type  null doesn t have an implicit conversion to DateTime   DateTime doesn t have an implicit conversion to null   So we end up with a compile-time error

User · Answer

You can try this  var lastPostDate   reader 3     DBNull Value                                   default DateTime                                     Convert ToDateTime reader 3

User · Answer

Cast the null literal   DateTime  null or  Nullable lt DateTime gt  null   You can also use default DateTime   or default Nullable lt DateTime gt    And  as other answers have noted  you can also apply the cast to the DateTime value rather than to the null literal   EDIT  adapted from my comment to Prutswonder s answer    The point is that the conditional operator does not consider the type of its assignment target  so it will only compile if there is an implicit conversion from the type of its second operand to the type of its third operand  or from the type of its third operand to the type of its second operand   For example  this won t compile   bool b   GetSomeBooleanValue    object o   b    Forty-two    42    Casting either the second or third operand to object  however  fixes the problem  because there is an implicit conversion from int to object and also from string to object   object o   b    Forty-two     object 42    or  object o   b    object  Forty-two    42

User · Answer

You might want to do it like this   DateTime  lastPostDate     DateTime   reader IsDbNull 3    null   reader 3       The problem you are having is that the ternary operator wants a viable cast between the left and right sides  And null can t be cast to DateTime   Note the above works because both sides of the ternary are object s  The object is explicitly cast to DateTime  which works  as long as reader 3  is in fact a date

[c#] Nullable DateTime conversion

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