THISQOBJECT Function inside EXTENDSWindows/Unix

The keyword THIS is used as a handy reference to the component/object name inside a 
TYPE EXTENDS QOBJECT/Q.....   END TYPE statement block.
Example: THIS.Height = (numeric-expression)

TYPE MyObj EXTENDS QFORM
   Height as INTEGER
'you can set the Height property by using this line
 SUB SetHeight()
     THIS.Height = 30
END SUB
'
END TYPE

Details:
See extending custom components