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What is ObservableCollection and why use it for lists? How do you bind to nested properties and handle null paths safely? What is RelativeSource binding and when do you need it? How do you efficiently update large observable collections? How do you implement grouping in a CollectionView?
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How do you implement input validation with data binding?

Intermediate

Answer

Expose validation state on the view model (per-field error properties or INotifyDataErrorInfo), bind UI to show errors (DataTrigger or an error label), and disable submit via a command's CanExecute. The CommunityToolkit and behaviors package also offer validation behaviors for common rules.
public bool HasEmailError => string.IsNullOrEmpty(Email) ||
    !Email.Contains('@');
// Submit command CanExecute returns !HasEmailError && ...
Real-world example A signup form highlights invalid fields and keeps the Create button disabled until all validations pass.

Common follow-ups: What is INotifyDataErrorInfo? | How do you disable submit on errors?

Validation Data Binding & MVVM Advanced Data Binding & Collections
All 6 Advanced Data Binding & Collections questions