Difference between null and System.DBNull.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
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