Auto highlight text in a textbox control

In Windows Forms and WPF:

textbox.SelectionStart = 0;
textbox.SelectionLength = textbox.Text.Length;

If you want to do it for your whole WPF application you can do the following: - In the file App.xaml.cs

    protected override void OnStartup(StartupEventArgs e)
    {
        //works for tab into textbox
        EventManager.RegisterClassHandler(typeof(TextBox),
            TextBox.GotFocusEvent,
            new RoutedEventHandler(TextBox_GotFocus));
        //works for click textbox
        EventManager.RegisterClassHandler(typeof(Window),
            Window.GotMouseCaptureEvent,
            new RoutedEventHandler(Window_MouseCapture));

        base.OnStartup(e);
    }
    private void TextBox_GotFocus(object sender, RoutedEventArgs e)
    {
        (sender as TextBox).SelectAll();
    }

    private void Window_MouseCapture(object sender, RoutedEventArgs e)
    {
        var textBox = e.OriginalSource as TextBox;
        if (textBox != null)
             textBox.SelectAll(); 
    }

In ASP.NET:

textbox.Attributes.Add("onfocus","this.select();");

If you need to do this for a large number of textboxes (in Silverlight or WPF), then you can use the technique used in the blog post: http://dnchannel.blogspot.com/2010/01/silverlight-3-auto-select-text-in.html. It uses Attached Properties and Routed Events.

If you wanted to only select all the text when the user first clicks in the box, and then let them click in the middle of the text if they want, this is the code I ended up using.

Just handling the FocusEnter event doesn't work, because the Click event comes afterwards, and overrides the selection if you SelectAll() in the Focus event.

private bool isFirstTimeEntering;
private void textBox_Enter(object sender, EventArgs e)
{
    isFirstTimeEntering = true;
}

private void textBox_Click(object sender, EventArgs e)
{
    switch (isFirstTimeEntering)
    {
        case true:
            isFirstTimeEntering = false;
            break;
        case false:
            return;
    }

    textBox.SelectAll();
    textBox.SelectionStart = 0;
    textBox.SelectionLength = textBox.Text.Length;
}

if you want to select all on "On_Enter Event" this won't Help you achieving your goal. Try using "On_Click Event"

    private void textBox_Click(object sender, EventArgs e)
    {
        textBox.Focus();
        textBox.SelectAll();
    }

In window form c#. If you use Enter event it will not work. try to use MouseUp event

    bool FlagEntered;
    private void textBox1_MouseUp(object sender, MouseEventArgs e)
    {
        if ((sender as TextBox).SelectedText == "" && !FlagEntered)
        {
            (sender as TextBox).SelectAll();
            FlagEntered = true;
        }
    }

    private void textBox1_Leave(object sender, EventArgs e)
    {
        FlagEntered = false;
    }