Question

Why does FindName() return null in the following example?

XAML:

<Window x:Class="TestDynamicTextBox343.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <StackPanel>
        <Border >

            <DockPanel x:Name="FormBase" LastChildFill="True">

            </DockPanel>

        </Border>

        <Button Content="Save" Click="Button_Click"/>
    </StackPanel>
</Window>

Code Behind:

using System;
using System.Windows;
using System.Windows.Controls;

namespace TestDynamicTextBox343
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();


            StackPanel sp = new StackPanel();
            sp.Orientation = Orientation.Horizontal;

            TextBlock textBlock = new TextBlock();
            textBlock.Text = "First Name: ";

            TextBox textBox = new TextBox();
            textBox.Name = "FirstName";
            textBox.Text = "test";

            sp.Children.Add(textBlock);
            sp.Children.Add(textBox);
            FormBase.Children.Add(sp);

        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            TextBox tb = (TextBox)this.FindName("FirstName");
            Console.WriteLine(tb.Text);
        }
    }
}

Addendum to Answer:

Thanks a lot, Bruno, that worked well. In order not to add the same name twice, I wrap it with this:

void RegisterTextBox(string textBoxName, TextBox textBox)
{
    if ((TextBox)this.FindName(textBoxName) != null)
        this.UnregisterName(textBoxName);
    this.RegisterName(textBoxName, textBox);
}

Or if you will be registering anything other than TextBoxes, a generic version:

void RegisterControl<T>(string textBoxName, T textBox)
{
    if ((T)this.FindName(textBoxName) != null)
        this.UnregisterName(textBoxName);
    this.RegisterName(textBoxName, textBox);
}

Answer 1

This is related to WPF XAML Namescopes.

Because you add elements to parsed element trees, you need to call RegisterName.

        ...
        TextBox textBox = new TextBox();
        textBox.Name = "FirstName";
        textBox.Text = "test";

        this.RegisterName("FirstName", textBox);
        ...

Adding Elements to Parsed Element
Trees

Any additions to the element tree
after initial loading and processing
must call the appropriate
implementation of RegisterName for the
class that defines the XAML namescope.
Otherwise, the added object cannot be
referenced by name through methods
such as FindName. Merely setting a
Name property (or x:Name Attribute)
does not register that name into any
XAML namescope. Adding a named element
to an element tree that has a XAML
namescope also does not register the
name to the XAML namescope. Although
XAML namescopes can be nested, you
generally register names to the XAML
namescope that exists on the root
element, so that your XAML namescope
location parallels the XAML namescope
that would have been created in an
equivalent loaded XAML page. The most
common scenario for application
developers is that you will use
RegisterName to register names into
the XAML namescope on the current root
of the page. RegisterName is part of
one important scenario for finding
storyboards that will run as
animations. For more information, see
Storyboards Overview. If you call
RegisterName on an element other than
the root element in the same object
tree, the name is still registered to
the element nearest the root, as if
you had called RegisterName on the
root element.