public IEnumerable <ValidationResult> Validate(ValidationContext validationContext)
{
if (Have.Count() != Want.Count())
{
yield break;
}
if (Have.All(i => Want.Any(j => j.Id == i.Id)) &&
Want.All(i => Have.Any(j => j.Id == i.Id)))
{
yield return(new ValidationResult(
"It is not possible to create a trade for the same items.", new[] { "Have", "Want" }));
}
}