Async Validation

1. Determine the type of the validation error (e.g. string).

2. Derive your view model from the corresponding AsyncValidatableBindableBase:

   using Ditto.AsyncMvvm;
   
   class UniversalAnswerViewModel : AsyncValidatableBindableBase<string>
   {
       private int _answer;
       public int Answer
       {
           get { return _answer; }
           set { SetProperty(ref _answer, value); }
       }
   }

3. Override GetErrorsAsync():

       protected override async Task<IEnumerable<string>> GetErrorsAsync(string propertyName)
       {
           if (propertyName == "Answer")
           {
               await Task.Delay(TimeSpan.FromDays(7500000 * 365.25));
               if (Answer != 42)
                   yield return "Answer is wrong.";
           }
       }

Done! However, a small optimization for WPF exists.