Search notes:

System.DBNull (class)

System.DBNull represents a non-existing value.
The concept of non-existing values is most probably borrowed from SQL's special null value/data type.
DBNull is a singleton.
DBNull and null are not the same thing.

Equivalence to System.Convert.DBNull

System.DBNull is equivalent to System.Convert.DBNull, as can be demonstrated with the following PowerShell statement: 
[System.Convert]::DBNull.Equals([System.DBNull]::Value)

See also

The IsDBNull method of the Microsoft.VisualBasic.Information class.
Compare with the .NET types System.Management.Automation.Language.NullString and System.Reflection.Missing, and the VBA null value.

Index

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php:78 Stack trace: #0 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(78): PDOStatement->execute(Array) #1 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(30): insert_webrequest_('/notes/Microsof...', 1758194029, '216.73.216.150', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/Microsoft/dot-net/namespaces-classes/System/DBNull(54): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78