Using the NULL indicator

For parameters that have text values, use NULL to indicate that the event handler should execute only if the value is not specified or NULL. NULL must be specified in all capital letters:

<PARAMETER name="overriddenBy">NULL</PARAMETER>

This example runs the event handler only if overriddenBy is not specified and has no value.

Note: A zero-length string value is not NULL.

The user can combine the not and the NULL indicators to specify that an event handler should run only when a parameter has a value. For example:

<PARAMETER name="overriddenBy">!NULL</PARAMETER>