<TabControl
    assist:SelectorDragDropAttach.IsItemsDragDropEnabled="True"
    AlternationCount="{Binding ClassInfos.Count}"
    ContentTemplate="{StaticResource contentTemplate}"
    ItemContainerStyle="{StaticResource TabItemStyle}"
    ItemsSource="{Binding ClassInfos}"
    SelectedIndex="0" />

public static class SelectorDragDropAttach
{
    public static bool GetIsItemsDragDropEnabled(Selector scrollViewer)
    {
        return (bool)scrollViewer.GetValue(IsItemsDragDropEnabledProperty);
    }

    public static void SetIsItemsDragDropEnabled(Selector scrollViewer, bool value)
    {
        scrollViewer.SetValue(IsItemsDragDropEnabledProperty, value);
    }

    public static readonly DependencyProperty IsItemsDragDropEnabledProperty =
        DependencyProperty.RegisterAttached("IsItemsDragDropEnabled", typeof(bool), typeof(SelectorDragDropAttach), new PropertyMetadata(false, OnIsItemsDragDropEnabledChanged));

    private static readonly DependencyProperty SelectorDragDropProperty =
        DependencyProperty.RegisterAttached("SelectorDragDrop", typeof(SelectorDragDrop), typeof(SelectorDragDropAttach), new PropertyMetadata(null));

    private static void OnIsItemsDragDropEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        bool b = (bool)e.NewValue;
        Selector selector = d as Selector;
        var selectorDragDrop = selector?.GetValue(SelectorDragDropProperty) as SelectorDragDrop;
        if (selectorDragDrop != null)
            selectorDragDrop.Selector = null;
        if (b == false)
        {
            selector?.SetValue(SelectorDragDropProperty, null);
            return;
        }
        selector?.SetValue(SelectorDragDropProperty, new SelectorDragDrop(selector));

    }

}

void selector_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    if (this.IsMouseOverScrollbar)
    {
        //Set the flag to false when cursor is over scrollbar.
        this.canInitiateDrag = false;
        return;
    }

    int index = this.IndexUnderDragCursor;
    this.canInitiateDrag = index > -1;

    if (this.canInitiateDrag)
    {
        // Remember the location and index of the SelectorItem the user clicked on for later.
        this.ptMouseDown = GetMousePosition(this.selector);
        this.indexToSelect = index;
    }
    else
    {
        this.ptMouseDown = new Point(-10000, -10000);
        this.indexToSelect = -1;
    }
}

void selector_PreviewMouseMove(object sender, MouseEventArgs e)
{
    if (!this.CanStartDragOperation)
        return;

    // Select the item the user clicked on.
    if (this.selector.SelectedIndex != this.indexToSelect)
        this.selector.SelectedIndex = this.indexToSelect;

    // If the item at the selected index is null, there's nothing
    // we can do, so just return;
    if (this.selector.SelectedItem == null)
        return;

    UIElement itemToDrag = this.GetSelectorItem(this.selector.SelectedIndex);
    if (itemToDrag == null)
        return;

    AdornerLayer adornerLayer = this.ShowDragAdornerResolved ? this.InitializeAdornerLayer(itemToDrag) : null;

    this.InitializeDragOperation(itemToDrag);
    this.PerformDragOperation();
    this.FinishDragOperation(itemToDrag, adornerLayer);
}

void selector_Drop(object sender, DragEventArgs e)
{
    if (this.ItemUnderDragCursor != null)
        this.ItemUnderDragCursor = null;

    e.Effects = DragDropEffects.None;

    var itemsSource = this.selector.ItemsSource;
    if (itemsSource == null) return;

    int itemsCount = 0;
    Type type = null;
    foreach (object obj in itemsSource)
    {
        type = obj.GetType();
        itemsCount++;
    }

    if (itemsCount < 1) return;
    if (!e.Data.GetDataPresent(type))
        return;

    object data = e.Data.GetData(type);
    if (data == null)
        return;

    int oldIndex = -1;
    int index = 0;
    foreach (object obj in itemsSource)
    {
        if (obj == data)
        {
            oldIndex = index;
            break;
        }
        index++;
    }
    int newIndex = this.IndexUnderDragCursor;

    if (newIndex < 0)
    {
        if (itemsCount == 0)
            newIndex = 0;
        else if (oldIndex < 0)
            newIndex = itemsCount;
        else
            return;
    }
    if (oldIndex == newIndex)
        return;

    if (this.ProcessDrop != null)
    {
        // Let the client code process the drop.
        ProcessDropEventArgs args = new ProcessDropEventArgs(itemsSource, data, oldIndex, newIndex, e.AllowedEffects);
        this.ProcessDrop(this, args);
        e.Effects = args.Effects;
    }
    else
    {
        dynamic dItemsSource = itemsSource;
        if (oldIndex > -1)
            dItemsSource.Move(oldIndex, newIndex);
        else
            dItemsSource.Insert(newIndex, data);
        e.Effects = DragDropEffects.Move;
    }
}