Treeview.Items.Clear() 方法在 SelectedItemChanged 事件中返回 null 异常（e.NewValue==null）

Question

I am from Iran and I cant speak English very well, sorry. I made something like OpenFileDialog in WinForms and work correctly. After, for better user interface, I tried to make it in WPF. I use TreeView and other controls for it in both platforms (Winforms and WPF) in Winforms I could do this correctly usingbelow code:

private void Folder_FileTreeView_NodeMouseDoubleClick(object sender, TreeNodeMouseClickEventArgs e)
{
    Folder_FileTreeView.Nodes.Clear();//this is necessary to clean first page node, after  get new folders
    if(e.Node.Text=="Desktop")//also this code is necessary to compare node 
    {
        //Do something
    }
}

Also in WPF I can get text of Item by below code:

private void Folder_FileTreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    if (e.NewValue!=null)
    {
        StackPanel CustomStackPanel = (StackPanel)((TreeViewItem)e.NewValue).Header;
        TextBlock textBlock = (TextBlock)CustomStackPanel.Children[1];
        nodetext = textBlock.Text;//this line return text of item for compare
    }
    Folder_FileTreeView.Items.Clear();
}

If I don't use Folder_FileTreeView.Items.Clear() the above code return folders without clearing first page, but if I do use Folder_FileTreeView.Items.Clear() e.NewValue returns null . Please help me to use together these codes: Folder_FileTreeView.Items.Clear();(or clear first page) and get text of selecteditem by user without return null

Thanks A lot

Answer 1

e.NewItem will be null if the TreeView used to have an item selected but now does not. When you clear the items, you are removing any selection, this of course changes the selection and raises the SelectedItemChanged event with null as the new selection- since there are no possible items that could be selected.

If you want to replace the items in the list with new items after the user makes a selection, the selected item will be null while that change is happening. You need to do the following:

1. Handle the SelectedItemChanged event and remember the new selected item in a variable . For example, if they click on the item for "Desktop" set a variable (eg Path ) to the path for the user's desktop (eg C:\\Users\\UserName\\Desktop).
2. Clear the list of folders in the TreeView . This will trigger SelectedItemChanged again, but you want to ignore it this time because e.NewItem == null .
3. Read all the folders in Path and make new items for each of those folders.

Answer 2

The way was found by below code

private void Folder_FileTreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    Folder_FileTreeView.SelectedItemChanged -= Folder_FileTreeView_SelectedItemChanged;
    if (e.NewValue!=null)
    {
        StackPanel CustomStackPanel = (StackPanel)((TreeViewItem)e.NewValue).Header;
        TextBlock textBlock = (TextBlock)CustomStackPanel.Children[1];
        nodetext = textBlock.Text;//this line return text of item for compare
    }
    Folder_FileTreeView.Items.Clear();
    Folder_FileTreeView.SelectedItemChanged += Folder_FileTreeView_SelectedItemChanged;
}

thank very much for every one helped me