FinanceManager.ViewModel.Validation.ValidationErrors.this C# (CSharp) Method

        public string this[string fieldName]
        {
            get
            {
                return validationErrors.ContainsKey(fieldName) ?
                    validationErrors[fieldName] : string.Empty;
            }
            set
            {
                if (validationErrors.ContainsKey(fieldName))
                {
                    if (string.IsNullOrWhiteSpace(value))
                    {
                        validationErrors.Remove(fieldName);
                    }
                    else
                    {
                        validationErrors[fieldName] = value;
                    }
                }
                else
                {
                    if (!string.IsNullOrWhiteSpace(value))
                    {
                        validationErrors.Add(fieldName, value);
                    }
                }
                OnPropertyChanged("IsValid");
                OnPropertyChanged();
            }
        }