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If a window is not defined, GetProperty("WindowName", "HANDLE") causes run-time error.
Maybe in this case the function should return 0?

Now in event handler I am using:
IF IsWindowDefined("WindowName") .and. (nHWnd == GetProperty("WindowName", "HANDLE"))
...
ENDIF

It could be shortened:
IF nHWnd == GetProperty("WindowName", "HANDLE")
...
ENDIF

It is the responsibility of the programmer to check if the window/control is defined. All properties produces runtime error if a control or window is not defined.

The default return values when a programming error occurs at runtime generate black boxes in the code that are difficult to debug. A well written code can never produce an execution error, this last one more that a complication is a help and insurance for the programmer.

Claudio, thank you very much for your explanation.