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

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?

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.

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.

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.

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

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.