I've been reading through some of the other questions tagged with validation and similar. We have a Windows desktop application (WPF) that has many textboxes contained in a dialog that need to be validated, each one differently (needs to be a number, needs to be between 2 numbers, needs to be greater than zero, etc.).
We can't use the same UI that most website forms use when you try to Submit or move to another page, where some text is displayed (in red, for example) near the invalid fields, indicating both which fields are invalid and what is the expected input/what is wrong with the current input. In such cases, the height of the webpage is just extended to accommodate the additional text, if needed.
In our application's case, real estate is limited, so the controls and window cannot be easily resized. The question is, what are some rules/no-no's on validating the input during user interaction?
Do we change the values from incorrect ones to a default correct one? If so, when — on field exit, on dialog OK, or…? Tooltips seems to be a viable communication method.