Question

I have following code. Here listClear is an object, which is filled by ViewModel. I am using properties of this object to fill a Grid. In below code what property should I use to make Button disable in DataTrigger. I want Button to be disabled when Grid is empty, otherwise it should be enabled.

<Button Grid.Column="3" Margin="2" Command="{Binding Path=ClearCommand}" Content="Clear">
    <Button.Style>
        <Style TargetType="Button">
            <Setter Property="IsEnabled" Value="True" />

                <Style.Triggers>
                    <DataTrigger Binding="{Binding Path=listClear}" Value="">
                        <Setter Property="IsEnabled" Value="False"/>
                    </DataTrigger>
                </Style.Triggers>
        </Style>
    </Button.Style>
</Button>

Answer 1

You can use the List Count property for this to indicate empty

Example:

public partial class MainWindow : Window 
{
    private ObservableCollection<string> myVar = new ObservableCollection<string>();

    public MainWindow()
    {
        InitializeComponent();
        MyList.Add("test");
    }

    public ObservableCollection<string> MyList
    {
        get { return myVar; }
        set { myVar = value; }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        MyList.Clear();
    }

}

Xaml:

<Window x:Class="WpfApplication11.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication11"
        Title="MainWindow" Height="136.3" Width="208" x:Name="UI">
    <Grid DataContext="{Binding ElementName=UI}">
        <Button Content="Clear" Click="Button_Click">
            <Button.Style>
                <Style TargetType="Button">
                    <Setter Property="IsEnabled" Value="True" />
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding Path=MyList.Count}" Value="0">
                            <Setter Property="IsEnabled" Value="False"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
        </Button>
    </Grid>
</Window>

If your only using IEnumerable<T> its a bit more difficult because IEnumerable has no public properties to bint to, you would have to make a converter.

Something like this

public class IsEmptyConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value is IEnumerable)
        {
            var enumerable = (IEnumerable)value;
            foreach (var item in enumerable)
            {
                return false;
            }
        }
        return true;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Xaml:

<Window x:Class="WpfApplication11.MainWindow"
        xmlns:local="clr-namespace:Namespace for converter"
....
....
 <Window.Resources>
        <local:IsEmptyConverter x:Key="IsEmptyConverter" />
 </Window.Resources>
....
....
   <DataTrigger Binding="{Binding Path=MyList, Converter={StaticResource IsEmptyConverter}}" Value="true">