In place editing definition: A label fields that when I click turns into an input field and allows me to type modify the text. Then either by clicking out or pressing tab it will automatically save the data.
Now imagine I have a currency field that only accepts numbers. The users types in "L" but the input field automatically filters out the "L". How can I tell the user that it is invalid?
Does it make sense to display error messages for in-place editing?
Yes, it does.
How? It depends on your concept and your layout. Consider one of following:
As an addition to the previous answers, which were already good:
As a design rule, whenever an “error message” request, idea or discussion pops up, I always ask myself the question:
How could I have improved the UI to prevent the user from doing something “wrong” in the first place.
A worst-practice example, that still I run into too often, is a create-password dialog, where no hint is given about requirements (eg at least 1 symbol), and then an error message shows up telling me that the password is unacceptable. Better to display hints upfront about requirements, possibly with live green checkboxes appearing once the password is OK.
Now to your question on numbers in fields: Probably best to make clear to the user upfront that only numbers are allowed. Maybe have a “numbers only please” message appear always when they switch to edit mode. Or display the warning message only when they enter an invalid symbol (which is automatically removed).
Yes, it definitely does.
In-place editable fields are like any other ones, and a user should either be notified about invalid input that cannot be saved, or restricted from it (e.g. being not allowed to enter a letter into a digits-only field).
