public void Exception_Validation_Sends_ValidationUpdate() { var data = new ExceptionTest { MustBePositive = 5 }; var observer = new ExpressionObserver(data, nameof(data.MustBePositive), false); var validationMessageFound = false; observer.Where(o => o is IValidationStatus).Subscribe(_ => validationMessageFound = true); observer.SetValue(-5); Assert.True(validationMessageFound); }