>Justok,
>
>First thee are two VFP errors for which your error handler will not be called , they are insufficient memory and do level nesting too deep. The first won't call you error handler because there is no memory available to call it, the second because all of the calling stack is in used and there is no call level available to call your error handler.
>
>That said, you need separate the VFP errors from the operating system errors, the error you displayed is an OS error and not a vfp error. VFP errors are alway seen in a message box with the Cancel Ignore Suspend options. If the error is in any other form then it is not from VFP.
>
>Thirdly you error handler always returns a value, to what? Who do you think is going to receive the value returned by your error handler and what is going to be done with the value?
>
>Oh yes, the one other place where the default VFP error handler will be called is when there is an error in your error handler.


I am using Chinese Win 95 OSR-2, in case of OS error, I need to take risk on the unsaved data. I am afraid because no one know there will be an OS error if My program requesting too much!

As I previously state, Even I put that statement at VFP command prompt when just starting VFP! No one know the command request is or not over the OS nor VFP error! 2nd, as previous state:
ww(_screen) && no error
ww(NVL(_screen, .NULL.)) && no error
ww(NVL(.NULL., _screen)) && OS Error....

I think it would be the internal VFP500.DLL handling problem!
If only OS Error, why 2nd statement work fine??!!

>When you call this test function with an argument of NVL( .NULL., _SCREEN), you get a program error (usually on the
> second time you call it). If you make the same call, except with a form object (that is, o = createobject( "form")), it works fine.
> An argument of NVL( _SCREEN, .NULL.) works fine too! That's wierd.
>
> - Mark

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I agree with Mark state, it seem NVL() got problem on requesting OS to task.
For programmer view, sometime afraid if huge error occur with only very simply
command/function call because the reliability become uncertain now! >_<'