What is the difference between null and System DBNull Value

Question

Is there any difference between null and System DBNull Value  If yes  what is it   I noticed this behavior now -   while  rdr Read          if  rdr  Id      null    if  rdr  Id      System DBNull Value                  int x   Convert ToInt32 rdr  Id               While I retrieve data from the database using a sql datareader  though there is no value returned if rdr  Id      null  returned true and eventually threw an exception for casting a null as integer    But  this if I use if  rdr  Id      System DBNull Value  returns false   What s the difference between null and System DBNull Value

User · Accepted Answer

Well  null is not an instance of any type  Rather  it is an invalid reference   However  System DbNull Value  is a valid reference to an instance of System DbNull  System DbNull is a singleton and System DbNull Value gives you a reference to the single instance of that class  that represents nonexistent  values in the database    We would normally say null  but I don t want to confound the issue   So  there s a big conceptual difference between the two  The keyword null represents an invalid reference  The class System DbNull represents a nonexistent value in a database field  In general  we should try avoid using the same thing  in this case null  to represent two very different concepts  in this case an invalid reference versus a nonexistent value in a database field    Keep in mind  this is why a lot of people advocate using the null object pattern in general  which is exactly what System DbNull is an example of

User · Answer

Null is similar to zero pointer in C    So it is a reference which not pointing to any value   DBNull Value is completely different and is a constant which is returned when a field value contains NULL

User · Answer

DBNull Value is what the  NET Database providers return to represent a null entry in the database   DBNull Value is not null and comparissons to null for column values retrieved from a database row will not work  you should always compare to DBNull Value   http   msdn microsoft com en-us library system dbnull value aspx

User · Answer

DataRow has a method that is called IsNull   that you can use to test the column if it has a null value - regarding to the null as it s seen by the database   DataRow  col    null will allways be false   use  DataRow r  if  r IsNull  col          instead

User · Answer

From the documentation of the DBNull class      Do not confuse the notion of null in an object-oriented programming language with a DBNull object  In an object-oriented programming language  null means the absence of a reference to an object  DBNull represents an uninitialized variant or nonexistent database column

User · Answer

DBNull Value is annoying to have to deal with   I use static methods that check if it s DBNull and then return the value   SqlDataReader r        String firstName   getString r COL Firstname     private static String getString Object o       if  o    DBNull Value  return null     return  String  o      Also  when inserting values into a DataRow  you can t use  null   you have to use DBNull Value   Have two representations of  null  is a bad design for no apparent benefit

[c#] What is the difference between null and System.DBNull.Value?

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