.NET 9 MAUI自定义控件如何调用非默认构造函数？

由于XAML解析控件时必须要有默认无参构造函数，我们可以通过以下几种方式绕开限制，同时保留依赖注入的带参构造逻辑：

1. 默认构造+显式初始化方法 #

这是兼容性最好的方案，既满足XAML解析要求，又能传入所需依赖：

树形控件后台代码 #

public partial class CompanyTreeControl : ContentView
{
    // 供XAML解析用的默认构造函数
    public CompanyTreeControl()
    {
        InitializeComponent();
    }

    // 自定义初始化方法，传入依赖服务
    public void Setup(DataService service, CompanyTreeViewBuilder treeBuilder)
    {
        // 执行原本在带参构造中的逻辑，比如初始化树数据、绑定构建器
        DataService = service;
        TreeBuilder = treeBuilder;
        LoadTreeData();
    }

    private DataService DataService { get; set; }
    private CompanyTreeViewBuilder TreeBuilder { get; set; }

    private void LoadTreeData()
    {
        // 调用服务获取数据，用构建器生成树形结构的逻辑
        var companyData = DataService.GetCompanyHierarchy();
        TreeBuilder.BuildTree(companyData, this.TreeView);
    }
}

父页面XAML #

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="clr-namespace:YourSharedControls;assembly=SharedTreeControls"
             x:Class="YourParentApp.Pages.CompanyListPage">
    <!-- 引用控件并命名，方便后台调用初始化方法 -->
    <controls:CompanyTreeControl x:Name="companyTreeView" />
</ContentPage>

父页面后台代码 #

public partial class CompanyListPage : ContentPage
{
    private readonly DataService _dataService;
    private readonly CompanyTreeViewBuilder _treeBuilder;

    // 父页面通过依赖注入获取所需服务
    public CompanyListPage(DataService dataService, CompanyTreeViewBuilder treeBuilder)
    {
        InitializeComponent();
        _dataService = dataService;
        _treeBuilder = treeBuilder;
    }

    protected override void OnAppearing()
    {
        base.OnAppearing();
        // 页面显示时初始化树形控件
        companyTreeView.Setup(_dataService, _treeBuilder);
    }
}

2. 纯代码后台实例化控件 #

放弃在XAML中直接引用，转而在父页面后台用带参构造创建控件，适合不需要设计器预览的场景：

父页面XAML #

<ContentPage ...>
    <!-- 留一个容器用于添加控件 -->
    <StackLayout x:Name="treeHostContainer" />
</ContentPage>

父页面后台代码 #

public partial class CompanyListPage : ContentPage
{
    public CompanyListPage(DataService dataService, CompanyTreeViewBuilder treeBuilder)
    {
        InitializeComponent();
        // 直接用带参构造创建控件并添加到容器
        var treeControl = new CompanyTreeControl(dataService, treeBuilder);
        treeHostContainer.Children.Add(treeControl);
    }
}

3. MAUI专属：使用x:FactoryMethod从DI容器解析（仅MAUI可用） #

如果是MAUI项目，可以利用x:FactoryMethod指定一个从依赖注入容器获取服务的创建方法，完美兼顾XAML设计器和依赖注入：

创建控件工厂类 #

namespace YourSharedControls.Factories;

public static class CompanyTreeControlFactory
{
    public static CompanyTreeControl Create()
    {
        // 从MAUI的全局服务容器中获取依赖
        var dataService = MauiProgram.Current.Services.GetRequiredService<DataService>();
        var treeBuilder = MauiProgram.Current.Services.GetRequiredService<CompanyTreeViewBuilder>();
        
        return new CompanyTreeControl(dataService, treeBuilder);
    }
}

父页面XAML引用 #

<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:controls="clr-namespace:YourSharedControls;assembly=SharedTreeControls"
             xmlns:factories="clr-namespace:YourSharedControls.Factories"
             x:Class="YourParentApp.Pages.CompanyListPage">
    <!-- 通过工厂方法创建控件，无需默认构造（但控件仍可保留默认构造用于设计器） -->
    <controls:CompanyTreeControl x:FactoryMethod="factories:CompanyTreeControlFactory.Create" />
</ContentPage>

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