Invoice create and handle response

repository.cs

var newInvoice = repository.Create<Invoice>(invs).FirstOrDefault();
 
if (newInvoice != null)
{
       if (newInvoice.ValidationStatus == ValidationStatus.ERROR)
       {
              for (int i = 0; i < newInvoice.ValidationErrors.Count; i++)
              {
                     returnString = returnString + "\n" + newInvoice.ValidationErrors[i].Message;
              }
       }
       else
       {
              return newInvoice.ValidationStatus.ToString();
       }
}
 
return returnString;