How do I read custom keyboard shortcut from user in WPF?

Question

In my application, I want to let users customize keyboard shortcuts, just like it's done in Visual Studio's keyboard options. The user can focus a blank text box and then type any shortcut he wants to assign to a command.

The closest I've come to make it work is by subscribing to the TextBox.PreviewKeyDown event, setting it as handled to prevent actual text input in the text box. I then ignore the KeyDown events associated with modifier keys (is there a cleaner way to determine if a Key is a modifier key?).

// Code-behind
private void ShortcutTextBox_PreviewKeyDown(object sender, KeyEventArgs e)
{
    // The text box grabs all input
    e.Handled = true;

    if (e.Key == Key.LeftCtrl || 
        e.Key == Key.RightCtrl || 
        e.Key == Key.LeftAlt ||
        e.Key == Key.RightAlt || 
        e.Key == Key.LeftShift ||
        e.Key == Key.RightShift)
        return;

    string shortcutText = "";
    if ((Keyboard.Modifiers & ModifierKeys.Control) == ModifierKeys.Control)
        shortcutText += "Ctrl+";
    if ((Keyboard.Modifiers & ModifierKeys.Shift) == ModifierKeys.Shift)
        shortcutText += "Shift+";
    if ((Keyboard.Modifiers & ModifierKeys.Alt) == ModifierKeys.Alt)
        shortcutText += "Alt+";
    _ShortcutTextBox.Text = shortcutText + e.Key.ToString();

}

The above works for any shortcut starting with Ctrl and Ctrl+Shift, but fails for any Alt shortcuts. The e.Key is always set to Key.System when I press a shortcut containing Alt.

How can I record Alt shortcuts from the user? Is there a better, more robust way to record shortcuts form the user?