Update DataGrid Background from Binding in ViewModel

Question

Today I found something strange (for me). If I want to update a DataGrid through a property in my ViewModel the Binding wont get notified. The special think here is (I think) the Binding is bound to another object (part of an Collection) not directly the property i change.

I have prepared some sample code for you. But first a little (Depper) explanation.

Of course it is just a sample but here I have a ViewModel with two public Properties (Items and CurrentItem). Items is a ObservableCollection and serves as the ItemsSource of my DataGrid. CurrentItem is a String which serves as indicator for a converter to set the background colour (for the grid).

I add two instances o String to my Collection and after the program is started the behaviour is as expected. The first line is green the second is white (set through the converter and the properties).

But if I change the value of CurrentItem after the program was loaded (lets say through the button) the colours wont update on my Datagrid.

If I create a breakpoint at the beginning of the converter I can see (after the loading process) the converter wont execute again so it has to be a Problem with the Binding. I think the problem is my property which is not part of the items in my Collection. The OnPropertyChange method seems to not trigger the update for the RowStyle properly.

In real life the model class of the collections is not a string and the model class implements INotifyPropertyChange (but I don´t think this is the problem cause i just don´t update anything in the model).

I need this kind of behaviour to visible highlight more rows based on a dynamic indicator (similar to the example). If no one knows a better way I think I will implement some kind of Property in my models and update the property with a method from the ViewModel.

ViewModel:

public class MainWindowViewModel : INotifyPropertyChanged
{
    public static MainWindowViewModel instance = null;

    private string _CurrentItem;

    public event PropertyChangedEventHandler PropertyChanged;

    protected void OnPropertyChanged(string Property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(Property));
        }
    }

    public string CurrentItem
    {
        get
        {
            return _CurrentItem;
        }
        set
        {
            if (value != _CurrentItem)
            {
                _CurrentItem = value;
                OnPropertyChanged("CurrentItem");
                OnPropertyChanged("Items");
            }
        }
    }
    public ObservableCollection<String> Items { get; set; }

    public MainWindowViewModel()
    {
        instance = this;
        Items = new ObservableCollection<string>();

        CurrentItem = "First";

        Items.Add("First");
        Items.Add("Second");
        Items.Add("First");

    }

View XAML

<Window.DataContext>
    <local:MainWindowViewModel />
</Window.DataContext>
<Window.Resources>
    <local:StringToColorConverter x:Key="StringToColorConverter" />
</Window.Resources>
<DockPanel Margin="30">
    <Button DockPanel.Dock="bottom" Content="From First to Second" Click="Button_Click" />
    <DataGrid IsReadOnly="True" ItemsSource="{Binding Items}" ColumnWidth="*" AutoGenerateColumns="False">
        <DataGrid.RowStyle>
            <Style TargetType="DataGridRow">
                <Setter Property="Background" 
                        Value="{Binding Converter={StaticResource StringToColorConverter}}" />
            </Style>
        </DataGrid.RowStyle>
        <DataGrid.Columns>
            <DataGridTextColumn Header="Text" Binding="{Binding}" />
        </DataGrid.Columns>
    </DataGrid>
</DockPanel>

Converter

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    var item = value as string;

    if (item == MainWindowViewModel.instance?.CurrentItem)
        return "Green";
    return "White";
}

So sorry for the long post I hope you can comprehend my problem and of course maybe help me :)

Answer 1

You can involve CurrentItem using an IMultiValueConverter

<DataGrid.RowStyle>
    <Style TargetType="DataGridRow">
        <Setter Property="Background">
            <Setter.Value>
                <MultiBinding Converter="{StaticResource MultiStringToColorConverter}">
                    <Binding />
                    <Binding Path="DataContext.CurrentItem" 
                             RelativeSource="{RelativeSource FindAncestor, 
                                              AncestorType={x:Type Window}}"/>
                </MultiBinding>
            </Setter.Value>
        </Setter>
    </Style>
</DataGrid.RowStyle>

The Converter

public class MultiStringToColorConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter,
                  System.Globalization.CultureInfo culture)
    {
        var item = values[0] as string;
        var current = values[1] as string;

        if (item == current)
            return new SolidColorBrush(Colors.Green);
        return new SolidColorBrush(Colors.White);
    }

    public object[] ConvertBack(object values, Type[] targetType, object parameter,
                    System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Answer 2

Everything is normal !

It is by design. Your background is bound to the n-th object of the DataGrid. It is not bound to CurrentItem, so there is no reason the binding updates the n-th line background.

Because you have an ObservableCollection, you could put a IsSelected property in MyItem class And you should make MyItem ràise a PropertyChanged event on IsSelected property.

Of course MyItem would implement INotifyPropertyChanged

Last, you should change the binding :

<DataGrid.RowStyle>
  <Style TargetType="DataGridRow">
    <Setter Property="Background" 
                    Value="{Binding Path=IsSelected,
                                    Converter={StaticResource BooleanToColorConverter}}" />
  </Style>
</DataGrid.RowStyle>

Of course changing the StringToColorConverter into BooleanToColorConvert wouldbe trivial.

Regards