validation.cs

public class ValidatedObservable : ObservableBase<string> { }
    public abstract class Validation
    {
        private readonly ValidatedObservable _validatedObservable;
        private readonly bool _validated;
        private readonly string _id;
        private readonly ISet<string> _mustBeValidated;

        protected Validation(ValidatedObservable validatedObservable, ISet<string> mustBeValidated, bool validated, string id)
        {
            _validatedObservable = validatedObservable;
            _mustBeValidated = mustBeValidated;
            _validated = validated;
            _id = id;
        }

        public void Execute()
        {
            if (_mustBeValidated.Count != 0)
            {
                return;
            }

            if (Validate())
            {
                _validated = true;
                _validatedObservable.OnNext(_id);
            }
        }

        public abstract bool Validate();

        public void OnValidated(string id)
        {
            _mustBeValidated.Remove(id);
            if (!_validated) {
                Execute();
            }
        }
    }