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Difference between null and System.DBNull.Value

      S.No     null                                DBNull


      1        Conceptual Difference:              Conceptual Difference:
               The keyword null represents         The class System.DbNull
               an invalid reference.               represents a nonexistent value in a
                                                   database field.

      2        What is the purpose of null         What is the purpose of DBNull
               keyword ?                           class ?
               NULL is used to check if a          DBNULL is used to check if a
               whole record is not found in        certain field in a database is null.
               the database.

      3        Is it a Value / Reference /         Is it a Value / Reference /
               Nullable type ?                     Nullable type ?
               null is the default value of        DBNull does not match the type of
               reference-type variables.           a nullable value type, a reference
               Ordinary value types cannot be      type, or a value type, and therefore
               null.                               is not directly assignable.

                                                   Please look at: Handle null, then
                                                   DBNull to avoid
                                                   InvalidCastException

                                                   Reason:
                                                   Of DBNull class only one instance
                                                   can exist as it a singleton class.
                                                   Sole instance is DBNull.Value.

Handle null, then DBNull to avoid InvalidCastException:

For instance, sometimes in code you will see items in a data row being used without
checking for null first. When the underlying field is not null, this is OK, but if a null is
available, the following code will not work:

Example-1

Decimal amount = (Decimal)row["AmountValue"]; //returned value is DBNull

Because DBNull is not the same type, an InvalidCastException occurs. Rather, a null
check has to be performed. The safe alternative to this is the following:

Example-2

Decimal amount = 0;
If (!row.IsNull("AmountValue"))
  amount = (Decimal)row["AmountValue"];

If you want to set a row's value, you can do something like below. The row takes an
instance of type object, and therefore can be assigned DBNull:

Example-3

If (amountValue > 0)
  row["AmountValue"] = amountValue;
Else
  row["AmountValue"] = DBNull.Value;

The value property returns the actual instance of DBNull (a static property). Sometimes, a
helper routine would be good to handle this conversion for you, reducing the amount to a
total of one line instead of four. That may not seem like much, but a table with thirty
columns will make this much coding a chore.

Checking null and DBNull values

How to check for null value ?

   1. Create the variable. A variable must be declared before using it. The code below
allocates memory and declares the variable for use.
   string myNullVar = null;
   The code uses the "null" value assignment for the new variable.
   2. Check if the variable is null. The code below checks if the variable is null. If it is null,
the code assigns it a color value.
   if(myNullVar == null) {
   myNullVar = "purple";
   }

   3.Check if the variable is not null. In some cases, you may wan to reassign the variable
back to null. The following code checks if the variable is not null, and assigns it a null
value.
  if(myNullVar != null) {
  myNullVar = null;
  }

How to check for DBNull values ?

   4. Declare a variable. Just like section 1, to check for DBNull, a variable needs to be
declared. However, the variable is assigned the DBNull value.
   string myNullVar = System.DBNull;
   5. Use the "Equals()" method from the string class to check for the DBNull value. The
code below checks for this value and assigns myNullVar a color.
   if (myNullVar.Equals(System.DBNull.Value)) {
   myNullVar = "purple";
   }
   6.Evaluate if the variable is not null. Similar to section one, you may want to check if the
variable is not null to reassign it back to DBNull. The "!" character evaluates to "not" in the
following example.
   if(!myNullValue.Equals(System.DBNull.Value)) {
   myNullValue = System.DBNull;
   }
Note:
 However note that neither checking null of nor DBNull.value will work if the index is out of
range. If we check for an index or a column that doesn't exist, it will throw an index out of
range exception.

References:
http://stackoverflow.com/questions/4958379/what-is-the-difference-between-null-and-
system-dbnull-value

http://www.dotnetspider.com/forum/315649-Difference-between-NULL-DBNULL.aspx

http://aspalliance.com/1460_More_InDepth_About_Nulls_And_DBNull.4

http://msdn.microsoft.com/en-us/library/system.dbnull.value.aspx

http://msdn.microsoft.com/en-us/library/edakx9da.aspx

http://www.ehow.com/how_5158769_check-null-value-c.html

http://dotnetanalysis.blogspot.in/2012/07/c-difference-between-dbnullvalue-and.html

And, further updates on difference between questions and answers, please visit my
blog @ http://onlydifferencefaqs.blogspot.in/

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Difference between null and system.db null.value

  • 1. Difference between null and System.DBNull.Value S.No null DBNull 1 Conceptual Difference: Conceptual Difference: The keyword null represents The class System.DbNull an invalid reference. represents a nonexistent value in a database field. 2 What is the purpose of null What is the purpose of DBNull keyword ? class ? NULL is used to check if a DBNULL is used to check if a whole record is not found in certain field in a database is null. the database. 3 Is it a Value / Reference / Is it a Value / Reference / Nullable type ? Nullable type ? null is the default value of DBNull does not match the type of reference-type variables. a nullable value type, a reference Ordinary value types cannot be type, or a value type, and therefore null. is not directly assignable. Please look at: Handle null, then DBNull to avoid InvalidCastException Reason: Of DBNull class only one instance can exist as it a singleton class. Sole instance is DBNull.Value. Handle null, then DBNull to avoid InvalidCastException: For instance, sometimes in code you will see items in a data row being used without checking for null first. When the underlying field is not null, this is OK, but if a null is available, the following code will not work: Example-1 Decimal amount = (Decimal)row["AmountValue"]; //returned value is DBNull Because DBNull is not the same type, an InvalidCastException occurs. Rather, a null check has to be performed. The safe alternative to this is the following: Example-2 Decimal amount = 0; If (!row.IsNull("AmountValue")) amount = (Decimal)row["AmountValue"]; If you want to set a row's value, you can do something like below. The row takes an
  • 2. instance of type object, and therefore can be assigned DBNull: Example-3 If (amountValue > 0) row["AmountValue"] = amountValue; Else row["AmountValue"] = DBNull.Value; The value property returns the actual instance of DBNull (a static property). Sometimes, a helper routine would be good to handle this conversion for you, reducing the amount to a total of one line instead of four. That may not seem like much, but a table with thirty columns will make this much coding a chore. Checking null and DBNull values How to check for null value ? 1. Create the variable. A variable must be declared before using it. The code below allocates memory and declares the variable for use. string myNullVar = null; The code uses the "null" value assignment for the new variable. 2. Check if the variable is null. The code below checks if the variable is null. If it is null, the code assigns it a color value. if(myNullVar == null) { myNullVar = "purple"; } 3.Check if the variable is not null. In some cases, you may wan to reassign the variable back to null. The following code checks if the variable is not null, and assigns it a null value. if(myNullVar != null) { myNullVar = null; } How to check for DBNull values ? 4. Declare a variable. Just like section 1, to check for DBNull, a variable needs to be declared. However, the variable is assigned the DBNull value. string myNullVar = System.DBNull; 5. Use the "Equals()" method from the string class to check for the DBNull value. The code below checks for this value and assigns myNullVar a color. if (myNullVar.Equals(System.DBNull.Value)) { myNullVar = "purple"; } 6.Evaluate if the variable is not null. Similar to section one, you may want to check if the variable is not null to reassign it back to DBNull. The "!" character evaluates to "not" in the following example. if(!myNullValue.Equals(System.DBNull.Value)) { myNullValue = System.DBNull; }
  • 3. Note: However note that neither checking null of nor DBNull.value will work if the index is out of range. If we check for an index or a column that doesn't exist, it will throw an index out of range exception. References: http://stackoverflow.com/questions/4958379/what-is-the-difference-between-null-and- system-dbnull-value http://www.dotnetspider.com/forum/315649-Difference-between-NULL-DBNULL.aspx http://aspalliance.com/1460_More_InDepth_About_Nulls_And_DBNull.4 http://msdn.microsoft.com/en-us/library/system.dbnull.value.aspx http://msdn.microsoft.com/en-us/library/edakx9da.aspx http://www.ehow.com/how_5158769_check-null-value-c.html http://dotnetanalysis.blogspot.in/2012/07/c-difference-between-dbnullvalue-and.html And, further updates on difference between questions and answers, please visit my blog @ http://onlydifferencefaqs.blogspot.in/