Can I use ObservableDictionary to show errors in MvvmCross

Understanding ObservableDictionary

An `ObservableDictionary` is a custom implementation that combines the functionality of a dictionary with the notification capabilities of `INotifyCollectionChanged` and `INotifyPropertyChanged`. This allows it to notify observers when items are added, removed, or modified, which is essential for updating the UI in real-time.

Implementing ObservableDictionary

To create an `ObservableDictionary`, you need to implement several interfaces, including `IDictionary`, `ICollection`, `IEnumerable`, `INotifyCollectionChanged`, and `INotifyPropertyChanged`. This ensures that your dictionary can be used interchangeably with the standard `Dictionary` class while providing the necessary notifications for data binding.

Here's a simplified example of how you might define an `ObservableDictionary`:

csharp
public class ObservableDictionary : 
    IDictionary, 
    ICollection>, 
    IEnumerable>, 
    INotifyCollectionChanged, 
    INotifyPropertyChanged
{
    private Dictionary _dictionary = new Dictionary();

    public event NotifyCollectionChangedEventHandler CollectionChanged;
    public event PropertyChangedEventHandler PropertyChanged;

    public void Add(TKey key, TValue value)
    {
        _dictionary.Add(key, value);
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, new KeyValuePair(key, value)));
    }

    public bool Remove(TKey key)
    {
        if (_dictionary.TryGetValue(key, out TValue value))
        {
            _dictionary.Remove(key);
            OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, new KeyValuePair(key, value)));
            return true;
        }
        return false;
    }

    // Implement other IDictionary methods...

    protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        CollectionChanged?.Invoke(this, e);
    }

    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Using ObservableDictionary for Error Messages

In your MvvmCross application, you can use an `ObservableDictionary` to store error messages. Each key in the dictionary could represent a field or property in your view model, and the corresponding value would be the error message associated with that field.

Here's how you might use it in a view model:

csharp
public class MyViewModel : MvxViewModel
{
    private ObservableDictionary _errors = new ObservableDictionary();

    public ObservableDictionary Errors
    {
        get { return _errors; }
        set { SetProperty(ref _errors, value); }
    }

    // Example method to add an error
    public void AddError(string fieldName, string errorMessage)
    {
        Errors.Add(fieldName, errorMessage);
    }

    // Example method to remove an error
    public void RemoveError(string fieldName)
    {
        Errors.Remove(fieldName);
    }
}

Binding Errors in the View

To display these errors in your view, you can bind to the `Errors` dictionary. However, MvvmCross does not natively support binding to dictionaries like `IDataErrorInfo` does in WPF. You might need to create custom bindings or properties in your view model to expose error messages for specific fields.

For example, you could create a property for each field's error message:

csharp
public string NameError
{
    get { return Errors.ContainsKey("Name") ? Errors["Name"] : null; }
}

Then, bind this property in your view:

xml

Conclusion

While using an `ObservableDictionary` for error messages in MvvmCross is possible, it requires additional work to implement the dictionary and handle data binding. You might also consider using third-party libraries like `MvvmValidation` to simplify the validation process[1].