WPF how do I create a textbox dynamically and find the textbox on a button click?

If you want to do a comprehensive search through the visual tree of controls, you can use the VisualTreeHelper class.

Use the following code to iterate through all of the visual children of a control:

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

    if (child is TextBox)
        // Do something
}

If you want to search down into the tree, you will want to perform this loop recursively, like so:

public delegate void TextBoxOperation(TextBox box);

public bool SearchChildren(DependencyObject parent, TextBoxOperation op)
{
    for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
    {
        DependencyObject child = VisualTreeHelper.GetChild(parent, i);

        TextBox box = child as TextBox;

        if (box != null)
        {
            op.Invoke(box);
            return true;
        }

        bool found = SearchChildren(child, op);

        if (found)
            return true;
    }
}

You can get your original click handler to work by registering the name of the text box:

someStackPanel.RegisterName(txtNumber.Name, txtNumber);

This will then allow you to call FindName on the StackPanel and find the TextBox.

Josh G had the clue that fixed this code: use RegisterName().

Three benefits here:

1. Doesn't use a member variable to save the reference to the dynamically created TextBox.
2. Compiles.

3. Complete code.

   using System;
   using System.Windows;
   using System.Windows.Controls;
   
   namespace AddControlsDynamically
   {
       public partial class Window1 : Window
       {
           public void Window_Loaded(object sender, RoutedEventArgs e)
           {
               GenerateControls();
           }
           public void GenerateControls()
           {
               Button btnClickMe = new Button();
               btnClickMe.Content = "Click Me";
               btnClickMe.Name = "btnClickMe";
               btnClickMe.Click += new RoutedEventHandler(this.CallMeClick);
               someStackPanel.Children.Add(btnClickMe);
               TextBox txtNumber = new TextBox();
               txtNumber.Name = "txtNumber";
               txtNumber.Text = "1776";
               someStackPanel.Children.Add(txtNumber);
               someStackPanel.RegisterName(txtNumber.Name, txtNumber);
           }
           protected void CallMeClick(object sender, RoutedEventArgs e)
           {
               TextBox txtNumber = (TextBox) this.someStackPanel.FindName("txtNumber");
               string message = string.Format("The number is {0}", txtNumber.Text);
               MessageBox.Show(message);
           }
       }
   }
   

Another method is to set the associated TextBox as Button Tag when instanciating them.

btnClickMe.Tag = txtNumber;

This way you can retrieve it back in event handler.

protected void ClickMeClick(object sender, RoutedEventArgs e)
{
    Button btnClickMe = sender as Button;
    if (btnClickMe != null)
    {
        TextBox txtNumber = btnClickMe.Tag as TextBox;
        // ...
    }
}