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TreeViewItem item = (TreeViewItem)DieTreeView.ItemContainerGenerator.ContainerFromItem(fileItem);					

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public class Item :ICloneable
{
    public string Name { get; set; }
    public string Path { get; set; }              
   
    public Item() { }

    public Object Clone()
    {
        return new Item { Name = this.Name, Path = this.Path };
    }
}					

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public class DirectoryItem : Item, ICloneable
{
    public ObservableCollection<Item> Items { get; set; }

    public bool IsExpanded { get; set; }

    public DirectoryItem() { Items = new ObservableCollection<Item>(); }

    public new object Clone()
    {
        var newList = new ObservableCollection<Item>(GetAllItems(this.Items));

        return new DirectoryItem { Name = this.Name, Path = this.Path, Items = newList, IsExpanded = this.IsExpanded };
    }


    private static ObservableCollection<Item> GetAllItems(ObservableCollection<Item> items)
    {
        var list = new ObservableCollection<Item>();

        foreach (Item it in items)
        {
            if (it.GetType() == typeof(DirectoryItem))
            {
                DirectoryItem dirIt = it as DirectoryItem;
                if (dirIt.Items.Count > 0)
                {
                    DirectoryItem newDirItem = (DirectoryItem)dirIt.Clone();
                    newDirItem.Items = GetAllItems(dirIt.Items);//Rekursion
                    list.Add(newDirItem);
                }
            }
            else
            {
                list.Add(new FileItem() { Name = it.Name, Path = it.Path });
            }
        }
        return list;
    }
}					

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public class FileItem : Item , ICloneable
{
    public List<string> KeywordList { get; set; }

    public FileItem() { KeywordList = new List<string>(); }

    public new FileItem Clone()
    {
        return new FileItem { Name = this.Name, Path = this.Path , KeywordList = this.KeywordList};
    }
}					

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<HierarchicalDataTemplate DataType="{x:Type Model:DirectoryItem}" ItemsSource="{Binding Items}">
    <StackPanel Orientation="Horizontal">
        <Image Width="28" Height="28" Source="Bild1.png"></Image>
        <TextBlock VerticalAlignment="Center" FontSize="16" Foreground="#FFC5C9CE" Margin="8,0,0,0" Text="{Binding Path=Name}" ToolTip="{Binding Path=Path}" />
    </StackPanel>
</HierarchicalDataTemplate>

<DataTemplate DataType="{x:Type Model:FileItem}">
    <StackPanel Orientation="Horizontal">
        <Image Width="20" Height="20" Source="Bild2.png"></Image>
        <TextBlock  FontSize="14" Foreground="#FFC5C9CE" Margin="8,0,0,0" Text="{Binding Path=Name}" ToolTip="{Binding Path=Path}" />
    </StackPanel>
</DataTemplate>					

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//hier startet das Prozedere....(Ein Filteraudruck in einem Kentextmenue wurde geändert)
void myContextItem_KeyWordSelectionChanged(object sender, List<string> keywords)
{
    foreach (Item it in DieTreeView.Items)
    {
        if (it.GetType() == typeof(DirectoryItem))
        {
            GetFileItems((DirectoryItem)it);
        }
    }
}

private void GetFileItems(DirectoryItem dirItem)
{
    foreach (Item it in dirItem.Items)
    {
        if (it.GetType() == typeof(DirectoryItem))
        {
            GetFileItems((DirectoryItem)it);//Rekursion da Unterordner vorhanden sind.
        }
        else if (it.GetType() == typeof(FileItem))
            CompareFileItemKeywordList((FileItem)it);
    }
}

private void CompareFileItemKeywordList(FileItem fileItem)
{
    TreeViewItem item = (TreeViewItem)SignalTreeView2.ThisTreeView.ItemContainerGenerator.ContainerFromItem(fileItem);
    item.Visibility = System.Windows.Visibility.Hidden;
    
    //in dieser Methode ist Item immer null.
}					

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foreach (Item it in DieTreeView.Items)
{
    if (it.GetType() == typeof(FileItem))
    {
        TreeViewItem item = (TreeViewItem)SignalTreeView2.ThisTreeView.ItemContainerGenerator.ContainerFromItem((FileItem)it);
        item.Visibility = System.Windows.Visibility.Hidden;
      
    }
}