How to use VisualTreeHelper to get all TextBoxes

Question

On the Page, I add UserControl into GridView dynamically. So, each UserControl can contain different kind of controls ( TextBox, CheckBox, Radio Button)

How to get a collection of control of textbox  using VisualTreeHelper and check which textBox is empty.

I found a code similiar to this problem and modified it but not working.

I dont know what this means in the code and if it is required?

            list.AddRange(AllTextBoxes(child))

            Should I use MyList.Select() or MyList.Where() ?

For example , the name of UserControl is : UserForm. This will be added into the GridView on the current page

Thanks

void FindTextBoxes()
{

   List <TextBox> MyList = AllTextBoxes(UserForm);

   var count = MyList.Where(x= > if(string.IsEmptyOrNull(x.Text));

}

List <TextBox> AllTextBoxes(DependencyObject parent)
{
    var list = new List <TextBox>();

    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
    {
        var child = VisualTreeHelper.GetChild(parent, i);

        if (child is TextBox)

            list.Add(child as TextBox);

        list.AddRange(AllTextBoxes(child));
    }
    return list;
}