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Leonie ChristianiLeonie Christiani 

how to allow users to edit a particular field only when it is null

Looking to make a field editable by users only when it is null.  Once the field has a value i would like it to become read only.

Thank you!

VinayVinay (Salesforce Developers) 
You can use validation for that.  Try below.

Hope this helps..

Leonie ChristianiLeonie Christiani
It works when i try to change the text in the account field however when i try to create a new account i get the same error when the field is either left blank or filled in.
Thank you
VinayVinay (Salesforce Developers) 
Check this.