Computational Field by Code in Dynamics Ax

hi

some Field in Dynamic Ax are Computational and Calculation

for example net amount in Sales Order ( Accountsreceivable)

when i change SalesQty Value by code in Dynamics Ax , Computational Field not work Properly

i write  this code in a button :

SalesLine.SalesQty= 100;

I expect  dynamics ax automatically computed other field  like "net amount " , price , ...

when i enter  value in  SalesQty   Asnormal and  Without code its work Properly.

Whatinstructions or Syntax shouldI addtothis?

Thanks All