WPF: How can you add a new menuitem to a menu at runtime?

I have successfully added menu items to a pre-defined menu item. In the following code, the LanguageMenu is defined in design view in th xaml, and then added the sub items in C#.

XAML:

<MenuItem Name="LanguageMenu" Header="_Language">
  <MenuItem Header="English" IsCheckable="True" Click="File_Language_Click"/>
</MenuItem>

C#:

// Clear the existing item(s) (this will actually remove the "English" element defined in XAML)
LanguageMenu.Items.Clear(); 

// Dynamically get flag images from a specified folder to use for definingthe menu items 
string[] files = Directory.GetFiles(Settings.LanguagePath, "*.png");
foreach (string imagePath in files)
{
  // Create the new menu item
  MenuItem item = new MenuItem();

  // Set the text of the menu item to the name of the file (removing the path and extention)
  item.Header = imagePath.Replace(Settings.LanguagePath, "").Replace(".png", "").Trim("\\".ToCharArray());
  if (File.Exists(imagePath))
  {
    // Create image element to set as icon on the menu element
    Image icon = new Image();
    BitmapImage bmImage = new BitmapImage();
    bmImage.BeginInit();
    bmImage.UriSource = new Uri(imagePath, UriKind.Absolute);
    bmImage.EndInit();
    icon.Source = bmImage;
    icon.MaxWidth = 25;
    item.Icon = icon;
  }

  // Hook up the event handler (in this case the method File_Language_Click handles all these menu items)
  item.Click += new RoutedEventHandler(File_Language_Click); 

  // Add menu item as child to pre-defined menu item
  LanguageMenu.Items.Add(item); // Add menu item as child to pre-defined menu item
}

I ran into the same problem. In my case the problem was that in the XAML the <menuitem> was directly contained in a <toolbar>. Once I put the <menuitem> inside a <menu> it started working. So:

<toolbar>
   <menuitem>
   </menuitem>
</toolbar>   

is bad

<toolbar>
    <menu>
        <menuitem>
        </menuitem>
    </menu>
</toolbar>   

is good

//Add to main menu
MenuItem newMenuItem1 = new MenuItem();
newMenuItem1.Header = "Test 123";
this.MainMenu.Items.Add(newMenuItem1);

//Add to a sub item
MenuItem newMenuItem2 = new MenuItem();
MenuItem newExistMenuItem = (MenuItem)this.MainMenu.Items[0];
newMenuItem2.Header = "Test 456";
newExistMenuItem.Items.Add(newMenuItem2);