如何从Code Behind展开XAML中的子菜单？

Hey there! Let's work through how to expand that submenu from your code-behind. From the menu structure shown in your image (a parent menu item with nested submenus), here are practical ways to achieve this in WPF:

1. Directly Set IsSubmenuOpen (Simplest Approach) #

If you've named your target submenu MenuItem in XAML, this is the quickest way. First, here's an example of what your XAML might look like:

<Menu x:Name="MainApplicationMenu">
    <MenuItem Header="File">
        <MenuItem x:Name="OpenSubMenu" Header="Open">
            <MenuItem Header="Recent Documents"/>
            <MenuItem Header="Browse Location"/>
        </MenuItem>
        <MenuItem Header="Save"/>
    </MenuItem>
</Menu>

Then in your code-behind, just set the IsSubmenuOpen property to true when you want to expand it (like on window load, or a button click):

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    // This will expand the "Open" submenu immediately
    OpenSubMenu.IsSubmenuOpen = true;
    // Optional: Ensure the parent "File" menu is also expanded
    (MainApplicationMenu.Items[0] as MenuItem).IsSubmenuOpen = true;
}

2. Dynamically Locate and Expand the Submenu #

If your submenu isn't named, or you need to find it dynamically (e.g., for generated menus), you can traverse the menu's item hierarchy:

private void ExpandTargetSubmenu()
{
    // First find the parent menu item (e.g., "File")
    var parentMenuItem = MainApplicationMenu.Items
        .OfType<MenuItem>()
        .FirstOrDefault(item => item.Header.ToString() == "File");

    if (parentMenuItem != null)
    {
        // Then find the target submenu (e.g., "Open")
        var targetSubmenu = parentMenuItem.Items
            .OfType<MenuItem>()
            .FirstOrDefault(subItem => subItem.Header.ToString() == "Open");

        if (targetSubmenu != null)
        {
            parentMenuItem.IsSubmenuOpen = true; // Expand parent first
            targetSubmenu.IsSubmenuOpen = true; // Expand the submenu
        }
    }
}

Call this method whenever you need to trigger the expansion (e.g., in a button click event).

3. MVVM-Friendly Approach (No Direct Code-Behind Control Access) #

If you're using the MVVM pattern, it's better to bind the IsSubmenuOpen property to a view model property instead of manipulating controls directly:

Update Your XAML: #

<MenuItem Header="Open" IsSubmenuOpen="{Binding IsOpenSubmenu}">
    <MenuItem Header="Recent Documents"/>
    <MenuItem Header="Browse Location"/>
</MenuItem>

Add Property to Your ViewModel: #

Make sure your view model implements INotifyPropertyChanged:

public class MainViewModel : INotifyPropertyChanged
{
    private bool _isOpenSubmenu;
    public bool IsOpenSubmenu
    {
        get => _isOpenSubmenu;
        set
        {
            _isOpenSubmenu = value;
            OnPropertyChanged(nameof(IsOpenSubmenu));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propertyName)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Then, whenever you want to expand the submenu, just set IsOpenSubmenu = true in your view model.

Quick Notes: #

• Always wait until the control is loaded before manipulating it (use the Loaded event instead of the window constructor, since controls aren't initialized yet in the constructor).
• For dynamically generated menus, ensure all menu items are created before you attempt to traverse and expand them.

内容的提问来源于stack exchange，提问作者user9415363