Ghost1372

everything can be handy

BindingProxy 

1
public class BindingProxy : Freezable

BindingProxy Located in the HandyControl.Tools namespace.

1
Using HandyControl.Tools;

The primary purpose of this class is to define objects that have a modifiable and a read-only state, but the interesting feature in our case is that Freezable objects can inherit the DataContext even when they’re not in the visual or logical tree.

Let’s illustrate with a simple example: we want to display a list of products in a DataGrid. In the grid, we want to be able to show or hide the Price column, based on the value of a ShowPrice property exposed by the ViewModel. The obvious approach is to bind the Visibility of the column to the ShowPrice property:

1
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False" Visibility="{Binding ShowPrice, Converter={StaticResource visibilityConverter}}"/>

Unfortunately, changing the value of ShowPrice has no effect, and the column is always visible, If we look at the Output window in Visual Studio, we notice the following line:

1
System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=ShowPrice; DataItem=null; target element is ‘DataGridTextColumn’ (HashCode=32685253); target property is ‘Visibility’ (type ‘Visibility’)

The solution to our problem is actually quite simple, and takes advantage of the Freezable class.

Example

We can declare an instance of this class in the resources of the DataGrid, and bind the Value property to the current DataContext:

1
2
3
<DataGrid.Resources>
    <hc:BindingProxy x:Key="proxy" Value="{Binding}" />
</DataGrid.Resources>

The last step is to specify this BindingProxy object (easily accessible with StaticResource) as the Source for the binding:

1
<DataGridTextColumn Header="Price" Binding="{Binding Price}" IsReadOnly="False" Visibility="{Binding Value.ShowPrice, Converter={StaticResource visibilityConverter}, Source={StaticResource proxy}}"/>

Binding path has been prefixed with Value, since the path is now relative to the BindingProxy object.

The binding now works correctly, and the column is properly shown or hidden based on the ShowPrice property.

0%