Change from Window to UserControl

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I am having some styles defined for list view. The style appear will depends on the data whether it is first level or second level (using expander). In order to define it, a class of GroupItemStyleSelector has been created. Unfortunately it throw an exception of null reference of Window.

Since my xaml file is created with <UserControl>, how can I change the code of GroupItemStyleSelector to avoid error? I have tried to use this and UserControl. But it couldn't locate the Style.

The codes are refer from online materials.

xaml

    <local:GroupItemStyleSelector x:Key="groupItemStyleSelector" />
                <!-- Style for the first level GroupItem -->
                <Style x:Key="FirstLevel" TargetType="{x:Type GroupItem}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type GroupItem}">
                                <Expander IsExpanded="True">
                                    <Expander.Header>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Text="{Binding Name}" FontWeight="Bold" VerticalAlignment="Bottom" />
                                            <TextBlock Text="{Binding ItemCount}" Foreground="Silver" FontStyle="Italic" Margin="10,0,0,0" VerticalAlignment="Bottom" />
                                            <TextBlock Text=" item(s)" Foreground="Silver" FontStyle="Italic" VerticalAlignment="Bottom" />
                                        </StackPanel>
                                    </Expander.Header>
                                    <ItemsPresenter />
                                </Expander>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
                <!-- Style for the second level GroupItem -->
                <Style x:Key="SecondLevel" TargetType="{x:Type GroupItem}">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type GroupItem}">
                                <Expander IsExpanded="True" Margin="15,0,0,0">
                                    <Expander.Header>
                                        <StackPanel Orientation="Horizontal">
                                            <TextBlock Text="{Binding Name}" FontWeight="Bold" VerticalAlignment="Bottom" />
                                            <TextBlock Text="{Binding ItemCount}" Foreground="Silver" FontStyle="Italic" Margin="10,0,0,0" VerticalAlignment="Bottom" />
                                            <TextBlock Text=" item(s)" Foreground="Silver" FontStyle="Italic" VerticalAlignment="Bottom" />
                                        </StackPanel>
                                    </Expander.Header>
                                    <ItemsPresenter />
                                </Expander>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>

cs

public class GroupItemStyleSelector : StyleSelector
    {
        public override Style SelectStyle(object item, DependencyObject container)
        {
            Style s;

            CollectionViewGroup group = item as CollectionViewGroup;
            Window window = Application.Current.MainWindow;

            if (!group.IsBottomLevel)
            {
                s = window.FindResource("FirstLevel") as Style; 
            }
            else
            {
                s = window.FindResource("SecondLevel") as Style;
            }

            return s;
        }
    }

asked 17 secs ago

user1850936

54

Change from Window to UserControl