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New Message on MumbaiUserGroup
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From: Rohit_BD
Message 2 in Discussion
A null basically means that a variable hasn't been initialised to a valid
address. Note that in .Net, variables of user-types or non-integral types are
internally treated as pointers.
Example:
int i; //Integral type - i represents "normal" data
MyClass o; //Non-integral type - o represents "address" data
Here "i" represents the name given to the block of memory allocated by the
run-time, and this block of memory is normal integer data. But "o" is the name
of a block of memory which contains, as it's data the *address* of some other
block of memory - "o" is a pointer variable, although Java, C# & VB.NET hide
this from us. The address (or data in "o") is null by default (just as "bool"
is initialised to false or "int" is initialised to 0), and is valid when the
"new" operator is invoked with the class name. DBNull represents a "value" of
null in an object - i.e., while the object (pointer) itself may not be null,
what it points to contains null as data. To make it clear, suppose we declare a
class MyClass as -
class MyClass
{
/**/ public String s; //s is the data held by the object.
/**/ MyClass() //Constructor
/**/ {
/**/ /**/ s = null; /* Initialise to null */
/**/ }
}
Now, difference in null & DBNull
MyClass myObj; /* myObj is going to be null and accessing it will result in
null-reference exception. */
myObj.s = "abcd"; /* Statement fails, null-ref. exception on myObj. */
myObj = new MyClass(); /* Here new operator returns an address which is now
stored in myObj. */
myObj.s.ToString() /* Fails, since s is null even though mObj is not (in this
example I have used a non-integral type). */
A more familiar example is that of a dataview where although the dataview is
not null, and has rows & columns in it, a particular column may contain a null
in it.
DataView dv = GetDataView(); /* Fire SQL query */
Assume that a column (on 0th index of 0th row in the dataview) contains NULL
since the underlying database table had that column to have NULL, then,
dv[0][0] will not be null, but dv[0][0].ToString() will fail, since dv[0][0]
contains a null value. In this particular example, dv[0][0].ToString() operates
not on dv[0][0], but on the value it contains - it might appear to contradict
the MyClass example that I gave above myObj.s.ToString() is equivalent to
dv[0][0].ToString().
A simple example in C notation:
int *p; //p contains random data since C does not initialise.
int i; //i contains random data since C does not initialise.
i = 0; //valid
*p = 0; /* Invalid, will result in program crash, since p will contain some
random data, and due to the "*" system iterprets it as an address - trying to
access random memory location causes invalid memory access fault. */
p = /* valid, assign address of i using the " operator. */
p = new int; /* valid, assign address of memory block using "new" operator. */
*p = 0; /* Now it's valid, since p already has valid data (address). */
Hope this helps.
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