>I have a table with code an description
>In the form I use a textbox to get these values. For the code I use an input mask of 99999.
>I check for the code so that it does not repeat when adding a new item. Also I check that both code and description are added.
>The problem is when I add an item, the default value for the code is zero, although no value is displayed. If I check for this value for not null it returns false (because it has a zero). If I add a description and save it, it saves the description and .null. for the code. If in the list I have another item with a zero code, the routine will tell that the code already exists.

Have you tried testing for EMPTY() rather than null? A var with a value of zero won't evaluate to null but will evaluate to empty. For example:

lnVar = 0
? ISNULL(lnVar)
? EMPTY(lnVar)

will yield:
.F.
.T.

lnVar is not null but is empty in this case. HTH, Bill