在 .NET Core 控制台应用程序中针对 EF Core DbContext 服务错误建立了依赖项注入

Question

我创建了一个 .net Core 控制台应用程序，并添加了以下依赖项注入包：

Microsoft.Extensions.DependencyInjection

要注入 EF Core DbContext 服务，以下是该项目的代码片段：

static void Main(string[] args)
        {
            // Create service collection and configure our services
            var services = ConfigureServices();

            // Generate a provider
            var serviceProvider = services.BuildServiceProvider();

            // Kick off our actual code
            serviceProvider.GetService<Startup>().Run();
        }

        public static IConfiguration LoadConfiguration()
        {
            var builder = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
                .AddJsonFile($"appsettings.{Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") ?? "Production"}.json", optional: true)
                .AddEnvironmentVariables();

            return builder.Build();
        }


private static IServiceCollection ConfigureServices()
        {
            IServiceCollection services = new ServiceCollection();

            // Set up the objects we need to get to configuration settings
            var configuration = LoadConfiguration();


            // IMPORTANT! Register our cvonfig file, db connection string, and application entry point(startup)
            services
                .AddSingleton(configuration)
                .AddConnection(configuration)
                .AddStartup();
            
            return services;
        }

服务池类别：

  public static class ServicesPool
    {
        public static IServiceCollection AddStartup(this IServiceCollection services)
        {
            services.AddTransient<Startup>();

            return services;
        }

        public static IServiceCollection AddConfiggguration(this IServiceCollection services, IConfiguration configuration)
        {
            services.AddSingleton(configuration);

            return services;
        }

        public static IServiceCollection AddConnection(this IServiceCollection services, IConfiguration configuration)
        {
            var connection = configuration.GetConnectionString("DEV_CS");
            services.AddDbContext<MigrationDbContext>(options =>
            options.UseSqlServer(connection, b => b.MigrationsAssembly("migration.presentence")));

            return services;
        }

    }

MigrationDbContext 服务从 EntityFramwork Core 实现 DbContext：

public class MigrationDbContext : DbContext
    {
        public MigrationDbContext(DbContextOptions dbContextOptions) : base(dbContextOptions)
        {
        }

        public DbSet<RootItemMigrationEntity> RootItems { get; set; }

    }

该应用程序运行没有任何问题，但是当我尝试使用以下命令创建初始迁移时：

添加迁移“初始”

出现错误并显示以下消息：

无法创建“MigrationDbContext”类型的对象。有关设计时支持的不同模式，请参阅 https://go.microsoft.com/fwlink/?linkid=851728

我一直在调查这个问题，并尝试了多种解决方案，但仍无法解决。因此，我将分享错误的堆栈跟踪，希望能得到您的帮助！

堆栈跟踪：

PM> Add-Migration 'Initial' -verbos 使用项目 'migration.presentence'。使用启动项目“e-commerce.migration”。构建开始...构建成功。C:\Program Files\dotnet\dotnet.exe exec --depsfile C:\dev\backlogheros\e-commerce.migration\e-commerce.migration\bin\Debug\netcoreapp5.0\e-commerce.migration.deps。 json --additionalprobingpath C:\Users\yousi.nuget\packages --runtimeconfig C:\dev\backlogheros\e-commerce.migration\e-commerce.migration\bin\Debug\netcoreapp5.0\e-commerce.migration。 runtimeconfig.json C:\Users\yousi.nuget\packages\microsoft.entityframeworkcore.tools\5.0.1\tools\netcoreapp2.0\any\ef.dll 迁移添加初始 --json --verbose --no-color - -前缀输出--程序集C：\ dev \ backlogheros \ e-commerce.migration \ e-commerce.migration \ bin \ Debug \ netcoreapp5.0 \ migration.presentence.dll --启动程序集C：\ dev \ backlogheros \e-commerce.migration\e-commerce.migration\bin\Debug\netcoreapp5.0\e-commerce.migration.dll --project-dir C:\dev\backlogheros\e-commerce.migration\migration.presentence
- -language C# --working-dir C:\dev\backlogheros\e-commerce.migration --root-namespace migration.presentence 使用程序集“migration.presentence”。使用启动程序集“e-commerce.migration”。使用应用程序库“C:\dev\backlogheros\e-commerce.migration\e-commerce.migration\bin\Debug\netcoreapp5.0”。使用工作目录“C:\dev\backlogheros\e-commerce.migration\e-commerce.migration”。使用根命名空间“migration.presentence”。使用项目目录“C:\dev\backlogheros\e-commerce.migration\migration.presentence”。剩余参数： . 查找 DbContext 类...查找 IDesignTimeDbContextFactory 实现...在程序集“e-commerce.migration”中查找应用程序服务提供程序...查找 Microsoft.Extensions.Hosting 服务提供程序...没有静态方法“CreateHostBuilder(string[])”在“程序”类中找到。未找到应用服务提供商。正在项目中查找 DbContext 类...找到 DbContext 'MigrationDbContext'。Microsoft.EntityFrameworkCore.Design.OperationException：无法创建“MigrationDbContext”类型的对象。有关设计时支持的不同模式，请参阅 https://go.microsoft.com/fwlink/?linkid=851728 ---> System.InvalidOperationException: 尝试时无法解析类型“Microsoft.EntityFrameworkCore.DbContextOptions”的服务激活“migration.presentence.MigrationDbContext”。

at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider provider)   
at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider provider, Type instanceType, Object[] parameters)    
at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetServiceOrCreateInstance(IServiceProvider provider, Type type)    
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_4.<FindContextTypes>b__13()
--- End of inner exception stack trace ---    
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_4.<FindContextTypes>b__13()
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)    
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)    
at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace)    
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)    
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

Answer 1

迁移工具必须实例化您的 DbContext 才能创建迁移。它有 3 种方法可以做到这一点：

1. 由服务商解决
2. 新的（）
3. IDesignTimeDbContextFactory

由服务提供商解决

如果您希望迁移工具使用应用程序配置并解析来自服务提供商的 DbContext，则需要满足多个要求，如页面https://go.microsoft.com/fwlink/?linkid=851728中所述。

首先，您需要有一个DbContextOptions在 DbContext 上接受 a 的构造函数。DbContextOptions<TContext>该部分没问题，仅当程序集中有多个 DbContext 时才需要指定。

public static CreateHostBuilder(string[] args)其次，您必须使用主机构建器（ASP.NET、HTTP 或 Generic），该工具将在类中查找方法Program来访问它。这会对您的基础设施类产生很多副作用，因为它将IHostBuilder为您完成大部分ServiceCollection、 和ConfigurationBuilder内容。

此外，您的 DbContext 需要注册为单例，这可能是一个问题。

程序.cs

internal class Program
{
    public static void LoadConfiguration(HostBuilderContext host, IConfigurationBuilder builder)
    {
        builder
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
            .AddJsonFile($"appsettings.{Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") ?? "Production"}.json", optional: true)
            .AddEnvironmentVariables();
    }

    private static void ConfigureServices(HostBuilderContext host, IServiceCollection services)
    {
        services
            .AddDbContext<MigrationDbContext>(options =>
            {
                options.UseSqlServer(
                    host.Configuration.GetConnectionString("DEV_CS"), builder =>
                        builder.MigrationsAssembly("migration.presentence"));
            }, ServiceLifetime.Singleton)
            .AddHostedService<Startup>();
    }

    private static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration(LoadConfiguration)
            .ConfigureServices(ConfigureServices);

    private static async Task Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }
}

为了使您的主机构建器运行您的自定义代码，您还需要将该Startup类转换为IHostedService诸如此类的BackgroundService实例。

启动.cs

internal class Startup : BackgroundService
{
    private readonly MigrationDbContext context;

    public Startup(MigrationDbContext context)
    {
        this.context = context ?? throw new ArgumentNullException(nameof(context));
    }

    protected override Task ExecuteAsync(CancellationToken stoppingToken)
    {
        throw new NotImplementedException();
    }
}

新的（）

另一种选择是在 DbContext 上使用默认构造。但是，在这种情况下，您不能DbContextOptions同时拥有默认构造函数和构造函数。伤心。

IDesignTimeDbContextFactory

根据您的情况，最简单的解决方案可能是实施IDesignTimeDbContextFactory<TContext>. 如果工具找不到该类IHostBuilder并且 DbContext 使用DbContextOptions构造函数，则该类将由工具使用。实现非常简单：

internal class MigrationDbContextFactory : IDesignTimeDbContextFactory<MigrationDbContext>
{
    public MigrationDbContext CreateDbContext(string[] args)
    {
        var optionsBuilder = new DbContextOptionsBuilder<MigrationDbContext>();
        optionsBuilder.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=Test",
            b => b.MigrationsAssembly("migration.presentence"));
        return new MigrationDbContext(optionsBuilder.Options);
    }
}