前言

我们都知道.NET Core提供了对依赖项注入的内置支持。我们通常在ASP.NET Core中使用它（从Startup.cs文件中的ConfigureServices方法开始），但是该功能不限于此框架，我们可以在WPF和Windows Forms应用程序中使用它。

实践

新建项目
将所需的NuGet包添加到项目中。
- Microsoft.Extensions.DependencyInjection
- Microsoft.Extensions.Options.ConfigurationExtensions
- Microsoft.Extensions.Configuration.Json
然后，将一个名为appsettings.json的文件添加到项目的根文件夹。将其“ 构建操作”属性设置为“ 内容”，将“复制到输出目录”设置为“ 复制”（如果较新）：
```
{
"AppSettings": {
    "AppName": "SampleNetCore3WpfDependencyInjection"
    }
}
```
创建一个AppSettings.cs文件来保存配置设置。该文件将映射我们在appsettings.json中编写的设置：
```
public class AppSettings
    {
        public string AppName { get; set; } 
    }
```

创建一个示例服务：

public interface ISampleService
    {
        Task<string> GetCurrentDate();
    }

public class SampleService : ISampleService
    {
        public async Task<string> GetCurrentDate() => 
        await Task.FromResult(DateTime.Now.ToLongDateString());
    }

然后像往常一样在IOC容器中注册服务：

services.AddScoped<ISampleService, SampleService>();

打开App.xaml文件并删除Application类的StartupUri属性。然后，我们需要重写App.xaml.cs中的OnStartup方法：

public partial class App : Application
    {
        public IServiceProvider ServiceProvider { get; private set; }
        public IConfiguration Configuration { get; private set; }

        protected override void OnStartup(StartupEventArgs e)
        {
            var builder = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);

            Configuration = builder.Build();

            var serviceCollection = new ServiceCollection();
            ConfigureServices(serviceCollection);

            ServiceProvider = serviceCollection.BuildServiceProvider();

            var mainWindow = ServiceProvider.GetRequiredService<MainWindow>();
            mainWindow.Show();
        }

        private void ConfigureServices(IServiceCollection services)
        {
            services.Configure<AppSettings>(Configuration.GetSection(nameof(AppSettings)));
            services.AddScoped<ISampleService, SampleService>();
            services.AddTransient(typeof(MainWindow));
        }
    }

MainWindow简单布局及代码改造

如上所述，MainWindow位于IOC容器中。因此，当我们从服务提供商处获得服务时，它将自动注入所有必需的服务（如果有）。：

public partial class MainWindow : Window
    {
        private readonly ISampleService sampleService;
        private readonly AppSettings settings;

        public MainWindow(ISampleService sampleService,
            IOptions<AppSettings> settings)
        {
            InitializeComponent();

            this.sampleService = sampleService;
            this.settings = settings.Value;
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            var serviceData =await sampleService.GetCurrentDate();
            var settingsData = settings;
            TextBox1.Text = $"serviceData:{serviceData}{Environment.NewLine}settingsData:{settings.AppName}";
        }
    }