一个方法

 public static T GetVisualChild<T>(Visual parent) where T : Visual
        {
            T childContent = default(T);
            int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
            for (int i = 0; i < numVisuals; i++)
            {
                Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
                childContent = v as T;
                if (childContent == null)
                {
                    childContent = GetVisualChild<T>(v);
                }
                if (childContent != null)
                {
                    break;
                }
            }
            return childContent;
        }