Question

I’m binding TreeView. My reason is treeview level 0 is unselectable. When i click level 0 treeviewitem, current item must be collapse and first child item must be selected.

├ Item 1   //<- level 0. this item must be unselectable
├─ Child Item 11 //<- level 1
├─ Child Item 12
├ Item 2   //<- level 0. When i click this item, that is automatically collapse
├─ Child Item 21
├─ Child Item 22

How to do this using style?

Answer 1

I’d do it in my view model. The view model for level 0 items would have:

public bool IsSelected
{
    get { return false; }
    set
    {
        // error checking is omitted
        Children[0].IsSelected = value;

        // let WPF know that IsSelected may have changed from what it's expecting
        this.Dispatcher.BeginInvoke((ThreadStart)delegate
        {
            this.OnPropertyChanged(() => this.IsSelected);
        });
    }
}

Your XAML would look like:

<TreeView>
    <TreeView.ItemContainerStyle>
        <Style TargetType="TreeViewItem">
            <Setter Property="IsSelected" Value="{Binding IsSelected}"/>
        </Style>
    </TreeView.ItemContainerStyle>
</TreeView>

Now whenever the user clicks on a level-one item, the VM will refuse to be selected and instead select its first child item.

You can use exactly the same technique to handle your requirements around collapsing levels.