Since this is a Calculated Field, it should not be stored in a Table, but simply re-calculated , as needed. Since you're not storing it, you can use the Control Source of the third Textbox to do the math.
In Form Design View
- Right-Click on the 'results' Textbox
- Click on Properties
- Click on the Data Tab
- In the Control Source Property enter this: = Nz([Textbox1],0) + Nz([Textbox2],0)
Of course you'll have to replace Textbox1 and Textbox2 with your actual names of your Textboxes.
The above assumes that when you said 'add two fields' you actually meant 'add,' in the mathematical sense. If you meant 'add two fields' as in adding ABC to DEF and getting ABCDEF, the Control Source would be slightly different:
=[Textbox1] & [Textbox2]
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007