foreach (TreeViewItem treeitem in thetreeView.Items)
{
   treeitem.IsExpanded = true;
}

<TreeView.ItemContainerStyle>
    <Style TargetType="TreeViewItem">
        <Setter Property="IsExpanded" Value="True" />
    </Style>
</TreeView.ItemContainerStyle>

private void btnCollapseAll_Click(object sender, RoutedEventArgs e)
    {
        foreach (var item in treeView.Items)
        {
            DependencyObject dObject = treeView.ItemContainerGenerator.ContainerFromItem(item);
            CollapseTreeviewItems(((TreeViewItem)dObject));
        }
    }

    private void btnExpandAll_Click(object sender, RoutedEventArgs e)
    {
        foreach (var item in treeView.Items)
        {
            DependencyObject dObject = treeView.ItemContainerGenerator.ContainerFromItem(item);
            ((TreeViewItem)dObject).ExpandSubtree();
        }
    }

    private void CollapseTreeviewItems(TreeViewItem Item)
    {
        Item.IsExpanded = false;

        foreach (var item in Item.Items)
        {
            DependencyObject dObject = treeView.ItemContainerGenerator.ContainerFromItem(item);

            if (dObject != null)
            {
                ((TreeViewItem)dObject).IsExpanded = false;

                if (((TreeViewItem)dObject).HasItems)
                {
                    CollapseTreeviewItems(((TreeViewItem)dObject));
                }
            }
        }
    }