这当然很有趣，因为我假定在每个页面中设置SearchHandler会解决这个问题，但是我将各个页面中的Shell.SearchHandler XAML移到了C#中（如下），而且它仍然只利用了使用的第一个。我很困惑。我是否需要每次都从头开始重建整个 shell 或其他什么？我并不反对在渲染之前将所有这些都移到代码vs XAML中，但这似乎是不必要的，只是感觉我错过了什么。
Users.xaml.cs

public UsersPage()
    {
        InitializeComponent();

        Shell.SetSearchHandler(this, new UserSearchHandler
        {
            Placeholder = "Enter first or last name",
            ShowsResults = true,
            SelectedItemNavigationTarget = typeof(UserDetailPage),
            Users = UserData.Users,
            ItemTemplate = new DataTemplate(() =>
            {
                Grid grid = new Grid { Padding = 10 };
                grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(0.15, GridUnitType.Star) });
                grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(0.85, GridUnitType.Star) });

                Image image = new Image { HeightRequest = 40, WidthRequest = 40 };
                image.SetBinding(Image.SourceProperty, "Images[1].ImageUri");
                Label nameLabel = new Label { FontAttributes = FontAttributes.Bold, VerticalOptions = LayoutOptions.Center };
                nameLabel.SetBinding(Label.TextProperty, "Name");

                grid.Children.Add(image);
                grid.Children.Add(nameLabel);

                return grid;
            })
        });
    }

ContactsPage.xaml.cs

public ContactsPage()
    {
        InitializeComponent();

        Shell.SetSearchHandler(this, new ExternalContactSearchHandler
        {
            Placeholder = "Enter search term",
            ShowsResults = true,
            SelectedItemNavigationTarget = typeof(UserDetailPage),
            ExternalContacts = ExternalContactData.ExternalContacts,
            ItemTemplate = new DataTemplate(() =>
            {
                Grid grid = new Grid { Padding = 10 };
                grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });
                grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });
                grid.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });

                Label firstNameLabel = new Label { FontAttributes = FontAttributes.Bold, HorizontalOptions = LayoutOptions.Start, VerticalOptions = LayoutOptions.Center };
                firstNameLabel.SetBinding(Image.SourceProperty, "FirstName");
                Label lastNameLabel = new Label { FontAttributes = FontAttributes.Bold, HorizontalOptions = LayoutOptions.Start, VerticalOptions = LayoutOptions.Center };
                lastNameLabel.SetBinding(Label.TextProperty, "LastName");
                Label titleLabel = new Label { FontAttributes = FontAttributes.Bold, HorizontalOptions = LayoutOptions.End, VerticalOptions = LayoutOptions.Center };
                titleLabel.SetBinding(Label.TextProperty, "Title");

                grid.Children.Add(firstNameLabel);
                grid.Children.Add(lastNameLabel);
                grid.Children.Add(titleLabel);

                return grid;
            })
        });
    }

Shell.SetSearchHandler会影响整个Shell。它是全域的。当您呼叫它时，所有的页面都会受到影响。
将它放在构造函数中是无效的，因为在加载Shell时调用了该构造函数：所有的页面都被“构造”了，即使是那些还看不见的页面。

未来：修复“MAUI错误-未对选项卡页调用OnAppearing”时

将以Shell.SetSearchHandler...开头的代码移到每个页面的OnAppearing方法中。该方法在该页面即将可见时运行：

protected override void OnAppearing(...)
{
    base.OnAppearing();

    Shell.SetSearchHandler...
    ...
}

替代解决方案

Shell有一个Navigating事件，在转到新页面时调用该事件。使用此事件可根据导航到的页面设置不同的搜索处理程序：

protected override void OnNavigating(ShellNavigatingEventArgs args)
{
    base.OnNavigating(args);

    // Test which page is next.
    if (args.Target == /*...uri of your page...*/)
    {
        // ... your code here to set search handler ...
    }
}