Why isn't a property in my ViewModel updated when DataGrid changes?

Question

I\'m trying to create a UserControl, that will let me edit a Dictionary of type Dictionary in a grid (just editing entries so far, not adding o

Answer 1

There's actually two issues here: your DictionaryEntry class should implement INotifyPropertyChanged to work correctly with the binding engine, and secondarily it should implement IEditableObject because you want to edit items in a data grid and avoid 'random results'. So your class should look something like this...

public class DictionaryEntry : INotifyPropertyChanged, IEditableObject
{
    private string _k;
    [Description("The key")]
    public string K
    {
        [DebuggerStepThrough]
        get { return _k; }
        [DebuggerStepThrough]
        set
        {
            if (value != _k)
            {
                _k = value;
                OnPropertyChanged("K");
            }
        }
    }
    private string _v;
    [Description("The value")]
    public string V
    {
        [DebuggerStepThrough]
        get { return _v; }
        [DebuggerStepThrough]
        set
        {
            if (value != _v)
            {
                _v = value;
                OnPropertyChanged("V");
            }
        }
    }
    #region INotifyPropertyChanged Implementation
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string name)
    {
        var handler = System.Threading.Interlocked.CompareExchange(ref PropertyChanged, null, null);
        if (handler != null)
        {
            handler(this, new PropertyChangedEventArgs(name));
        }
    }
    #endregion
    #region IEditableObject
    public void BeginEdit()
    {
        // implementation goes here
    }
    public void CancelEdit()
    {
        // implementation goes here
    }
    public void EndEdit()
    {
        // implementation goes here
    }
    #endregion
}

In your ViewModel (or code behind) you would instantiate it like this...

    public ObservableCollection<DictionaryEntry> MyItems { get; set; } 
    public ViewModel()
    {
        MyItems = new ObservableCollection<DictionaryEntry>();
        MyItems.Add(new DictionaryEntry{K="string1", V="value1"});
        MyItems.Add(new DictionaryEntry { K = "color", V = "red" });
    }

...which is pretty close to what you have. And the Xaml would look like this...

    <DataGrid ItemsSource="{Binding MyItems}" AutoGenerateColumns="True">
    </DataGrid>

Those things will bring about the behaviour you are after. I.e., edits will be sticky.

On the IEditableObject interface vis-à-vis DataGrids, it's a known 'gotcha' and there's a description of it here... http://blogs.msdn.com/b/vinsibal/archive/2009/04/07/5-random-gotchas-with-the-wpf-datagrid.aspx

which says...

If you are not familiar with IEditableObject, see this MSDN article which has a good explanation and code sample. The DataGrid has baked in functionality for transactional editing via the IEditableObject interface. When you begin editing a cell, the DataGrid gets into cell editing mode as well as row editing mode. What this means is that you can cancel/commit cells as well as cancel/commit rows. For example, I edit cell 0 and press tab to the next cell. Cell 0 is committed when pressing tab. I start typing in cell 1 and realize I want to cancel the operation. I press ‘Esc’ which reverts cell 1. I now realize I want to cancel the whole operation so I press ‘Esc’ again and now cell 0 is reverted back to its original value.