>I have three five text boxes in my page.
>
>Part No, Description, Quantity, Unit Price and Total Cost.
>
>If the Part Number is not empty I want post back on Quanity, so that it will calculate the Total Cost.
>
>If the Part Number is empty (non-inventory part), then I don't want post back on quantity as I will calculate the price when they enter the unit price (AutoPostBack in Unit Price field).
>
>Could you please help me how to do this? Or is there a way to do this using client-side script?

I think you can put some code that will replace that value, on postbacks, on that field based on your business logic. On postback, everything is sumitted to the server. But, you can trap the postback event and act accordingly to adjust the field value after. Something like this might work:

If Page.IsPostBack Then

   ' Replace your field value here if the condition is met

End If