WPF DataGridTemplateColumn Visibility Binding under MVVM

Question

I have a DataGrid bound to an ICollectionView in my ViewModel. The DataGrid is inside a UserControl which is used in a few different data scenarios, some of which require ce

Answer 1

One hundred thanks to SliverNinja and this article DataGridContextHelper. Links to source code already not working and was not able download, so i wrote my own Attached Proeprty to make it work for all possible cases (DataContext changed, Attached Property value changed, Column added)

My application use DataGrid with AutoGenerateColumns=False and use DataGridTemplateColumn, so DataContext was set before columns added to grid.

Here is Attached Property class:

public sealed class DataGridColumnDataContextForwardBehavior
{
    private DataGrid dataGrid = null;

    public DataGridColumnDataContextForwardBehavior(DataGrid dataGrid)
    {
        this.dataGrid = dataGrid;

        dataGrid.Columns.CollectionChanged += DataGridColumns_CollectionChanged;
    }

    private void DataGridColumns_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
    {
        var IsDataContextForwardingEnabled = GetIsDataContextForwardingEnabled(dataGrid);

        if (IsDataContextForwardingEnabled && dataGrid.DataContext != null)
        {
            if (e.Action == NotifyCollectionChangedAction.Add)
            {
                foreach (DataGridColumn column in e.NewItems)
                {
                    column.SetValue(FrameworkElement.DataContextProperty, dataGrid.DataContext);
                }
            }
        }
    }

    static DataGridColumnDataContextForwardBehavior()
    {
        FrameworkElement.DataContextProperty.AddOwner(typeof(DataGridColumn));
        FrameworkElement.DataContextProperty.OverrideMetadata(typeof(DataGrid), 
            new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.Inherits, new PropertyChangedCallback(OnDataContextChanged)));
    }

    public static readonly DependencyProperty IsDataContextForwardingEnabledProperty = 
        DependencyProperty.RegisterAttached("IsDataContextForwardingEnabled", typeof(bool), typeof(DataGridColumnDataContextForwardBehavior), 
            new FrameworkPropertyMetadata(false, OnIsDataContextForwardingEnabledChanged));

    public static void OnDataContextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        DataGrid dataGrid = obj as DataGrid;
        if (dataGrid == null) return;

        var IsDataContextForwardingEnabled = GetIsDataContextForwardingEnabled(dataGrid);
        if (IsDataContextForwardingEnabled)
        {
            foreach (DataGridColumn col in dataGrid.Columns)
            {
                col.SetValue(FrameworkElement.DataContextProperty, e.NewValue);
            }
        }
    }

    static void OnIsDataContextForwardingEnabledChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        var dataGrid = obj as DataGrid;
        if (dataGrid == null) return;

        new DataGridColumnDataContextForwardBehavior(dataGrid);

        if (!(e.NewValue is bool)) return;

        if ((bool)e.NewValue && dataGrid.DataContext != null)
            OnDataContextChanged(obj, new DependencyPropertyChangedEventArgs(FrameworkElement.DataContextProperty, dataGrid.DataContext, dataGrid.DataContext));
    }

    public static bool GetIsDataContextForwardingEnabled(DependencyObject dataGrid)
    {
        return (bool)dataGrid.GetValue(IsDataContextForwardingEnabledProperty);
    }

    public static void SetIsDataContextForwardingEnabled(DependencyObject dataGrid, bool value)
    {
        dataGrid.SetValue(IsDataContextForwardingEnabledProperty, value);
    }
}

Another non obvious things is how to properly use binding for DataGridTemplateColumn:

It is important to use Path=DataContext.MyProp and RelativeSource Self

Only thing i don't like in current implementation - to handle DataGrid.Columns.CollectionChanged event i create instance of my class and do not keep reference for it. So in theory GC may kill it within the time, not sure how to handle it correctly at present moment, will think on it and update my post later. Any ideas and critique are welcome.