Question

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <StackPanel>
            <TextBox Name="TextBox1" HorizontalAlignment="Left" PreviewKeyUp="TextBox_PreviewKeyUp"></TextBox>
        </StackPanel>
    </Grid>
</Window>  

---

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void TextBox_PreviewKeyUp(object sender, KeyEventArgs e)
        {
            TextBox1.Text = "Some text inside Textbox";
            MessageBox.Show("TextBox1 Width = " + TextBox1.Width + " & TextBox1.ActualWidth = " + TextBox1.ActualWidth); // NaN & 10
        }
    }
}

How do I dynamically get the width of TextBox? Width and ActualWidth give me only the default compile time values, but I need the Width property after it has been compiled.

Answer 1

Width and ActualWidth do not give you the default compile time values.

Width is the requested size.

ActualWidth is the rendered size.
From MSDN

ActualWidth is a calculated value, and there can be multiple or incremental reported changes to the value because of operations by the layout system. If you get the value while layout is still iterating, the layout system might still be calculating the required measure of space for child objects, constraints by the parent object, and so on.

In your case, if you want the actual size of the item, use ActualWidth.

Edit: Based on your comment, I’m showing you how to make your specific method work, by using a BeginInvoke,to give the ActualWidth a chance to be calculated:

private void TextBox1_PreviewKeyUp_1(object sender, KeyEventArgs e)
{
        TextBox1.Text = "Some other text";
        Dispatcher.BeginInvoke((Action)new Action(() =>
            MessageBox.Show("TextBox1 Width = " + TextBox1.Width + " & TextBox1.ActualWidth = " + TextBox1.ActualWidth)),
          System.Windows.Threading.DispatcherPriority.Input);
}