How to attach eventhandler to DataGrid Combobox Selection Changed

Using the DataGrid in the Community ToolKit. How can one bind an eventhandler to listen for a change in the DataGridComboBoxColumn?

Unfortunately, you can't. One way to achieve this is to use the DataGridTemplateColumn with a CheckBox in it.

For example:

*.xaml

<toolkit:DataGrid.Columns>
    <toolkit:DataGridTemplateColumn Header="IsEnabled">
        <toolkit:DataGridTemplateColumn.CellTemplate>
            <DataTemplate>
                <ComboBox
                    Loaded="ComboBox_Loaded"
                    SelectionChanged="ComboBox_SelectionChanged" />
            </DataTemplate>
        </toolkit:DataGridTemplateColumn.CellTemplate>
    </toolkit:DataGridTemplateColumn>
</toolkit:DataGrid.Columns>

then in code-behind you'll have:

*.xaml.cs

private void ComboBox_Loaded(object sender, Microsoft.UI.Xaml.RoutedEventArgs e)
{
    if (sender is not ComboBox comboBox)
    {
        return;
    }

    comboBox.ItemsSource = ComboBoxItems;
    comboBox.SelectedIndex = 0;
}

private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    System.Diagnostics.Debug.WriteLine($"Selected item: {e.AddedItems.ToString()}");
}

UPDATE

You can also create a custom DataGridComboBoxColumn:

public class DataGridComboBoxColumnEx : DataGridComboBoxColumn
{
    public event SelectionChangedEventHandler? SelectionChanged;

    protected override FrameworkElement GenerateEditingElement(DataGridCell cell, object dataItem)
    {
        var comboBox = (ComboBox)base.GenerateEditingElement(cell, dataItem);
        
        comboBox.SelectionChanged -= ComboBox_SelectionChanged;
        comboBox.SelectionChanged += ComboBox_SelectionChanged;
        return comboBox;
    }

    private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        SelectionChanged?.Invoke(sender, e);
    }
}

Then use it like a plain DataGridComboBoxColumn:

<local:DataGridComboBoxColumnEx
    SelectionChanged="ComboBox_SelectionChanged"
    ItemsSource="{x:Bind ComboBoxItems}"
    Binding="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
    Header="Enabled" />