Thanks all of you, I understand this issue more now.

>>In a form, I have the formula to check a field X of a table --- if X = "" ...
>>It works fine when I run the form in VFP. But it has wrong operant error after I complie it to an exe file.
>>Finally I change it to if empty(X) ... then it works fine in both VFP and in its exe file. Does it mean we should use the empty() instead of = "" if we want to check empty field?
>
>See the command SET EXACT.
>Depending on SET EXACT the results they are different.
>
>Example:
>
>

>lc = "    "
>SET EXACT OFF
>? "" = lc
>  >>> .F.
>? EMPTY(lc)
>  >>> .T.
>SET EXACT ON
>? "" = lc
>  >>> .T.
>? EMPTY(lc)
>  >>> .T.
>

>
>I advise to you to use EMPTY()