MultiBindingConvertor not working on grid in WPF MVVM application

I've got a column in a WPF Telerik grid I need to restrict based on two thing开发者_高级运维s.

An IsEditable property on ViewModel and an IsManualChange property which is Property of the List the grid is bound on ...

I wrote a MultiBoolConvertor to handle this and implemented in the WPF as follows:

<telerik:GridViewComboBoxColumn 
    Header="Selection" 
    DataMemberBinding="{Binding HandHeldDifference.GRSSelection}" 
    ItemsSource="{Binding Path=SelectionOptions}">
    <telerik:GridViewComboBoxColumn.IsReadOnly>
        <MultiBinding Converter="{StaticResource MultiBoolConv}" 
            ConverterParameter="True">
            <Binding 
                RelativeSource="{RelativeSource FindAncestor, 
                    AncestorType={x:Type StackPanel}}" 
                Path="DataContext.IsEditable" />
            <Binding Path="IsManualChange" />
        </MultiBinding>
    </telerik:GridViewComboBoxColumn.IsReadOnly>
</telerik:GridViewComboBoxColumn>

However the values that come into the Convertor are a bool (from the ViewModel) and a DependencyProperty.UnsetValue from the IsManualChange!

public object Convert(object[] values, 
                        Type targetType, 
                        object parameter, 
                        CultureInfo culture)
{
    var defaultReturn = false;
    if (parameter != null)
    {
        bool.TryParse(parameter.ToString(), out defaultReturn);
    }

    if (values == null) return defaultReturn;
    if (values.Length < 2) return defaultReturn;
    if (values[0] is bool && values[1] is bool)
    {
        return ((bool) values[0]) && ((bool) values[1]);
    }
    return defaultReturn;
}

The second value obviously fails the "values[1] is bool" comparison

A clue maybe that the converter is only being called once, rather than per line as I'd expect.

Does anyone know how I can get this working please?

Before the DataContext is pass down, you will get DependencyProperty.UnsetValue and need to handle that case. So you can do something like:

bool isManualChange= (value[1] is bool) ? (bool)value[1] : false; // or true depending on which should be the default

From the link above:

UnsetValue is a sentinel value that is used for scenarios where the WPF property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than null, because null could be a valid property value, as well as a valid (and frequently used) DefaultValue.

So basically the DataContext is currently null, so it can't find a property named IsManualChange so you get UnsetValue.

Once the context is setup, your converter should get called again.

Turns out the Telerik grid binds the IsReadOnly to the ViewModel and you have to use IsReadOnlyBinding when you want to bind to an item in the ItemsSource!