在使用外部模型那篇水文中，有大伙伴提出：老周，你那个 Ultraman 类和 Speciality 类的的关系是不是有问题，外键不应该在 Speciality 类上吗，怎么会跑到 Ultraman 类上？因为它们是一对一关系，在配置的时候你也可以反过来，主要区别是谁引用谁的问题，由于是一对一引用，所以反过来也可以的。

今天咱们聊聊实体类构造函数的依赖注入。实体类也支持依赖注入，不过，目前版本只支持注入 EF Core 自己的服务类型，你在代码中添加的应用程序级服务类型不能注入（以后可能会支持）。总结一下，以下服务类型可以注入：

1、EF Core 框架内部注册的服务类型；

2、三个补充类型：

　　a、当前 DbContext 实例（你从 DbContext 派生的类）；

　　b、当前实体相关的 IEntityType，可以获取实体模型相关信息；

　　c、ILazyLoader 接口，延时加载时用得上。

 

现在要解决一个问题：我怎么知道 EF Core 框架内部哪些服务可以注入？这个活儿有两个方案：第一个方案是看 EF Core 源代码，在 EFCore\Infrastructure 下，注意看 EntityFrameworkServicesBuilder 类的 CoreServices 字段或 TryAddCoreServices 方法，基本齐全了。

 public static readonly IDictionary<Type, ServiceCharacteristics> CoreServices = new Dictionary<Type, ServiceCharacteristics> { { typeof(
LoggingDefinitions
), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(
IDatabaseProvider
), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(
IDbSetFinder
), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(
IDbSetInitializer
), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IDbSetSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IEntityFinderSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IStructuralTypeMaterializerSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ITypeMappingSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IModelCustomizer), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IModelCacheKeyFactory), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IModelSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IModelRuntimeInitializer), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IInternalEntrySubscriber), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IEntityEntryGraphIterator), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IValueGeneratorCache), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ISingletonOptionsInitializer), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ILoggingOptions), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ICoreSingletonOptions), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IModelValidator), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ICompiledQueryCache), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IValueConverterSelector), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IConstructorBindingFactory), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IRegisteredServices), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IPropertyParameterBindingFactory), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IParameterBindingFactories), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IMemberClassifier), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IMemoryCache), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IEvaluatableExpressionFilter), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(INavigationExpansionExtensibilityHelper), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(ILiftableConstantFactory), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IExceptionDetector), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IJsonValueReaderWriterSource), new ServiceCharacteristics(ServiceLifetime.Singleton) }, { typeof(IProviderConventionSetBuilder), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IConventionSetBuilder), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDiagnosticsLogger<>), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IInterceptors), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ILoggerFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IEntityGraphAttacher), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IKeyPropagator), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(INavigationFixer), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ILocalViewListener), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IStateManager), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IConcurrencyDetector), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IInternalEntityEntryNotifier), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IValueGenerationManager), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IChangeTrackerFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IChangeDetector), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDbContextServices), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IValueGeneratorSelector), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IExecutionStrategyFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IExecutionStrategy), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IAsyncQueryProvider), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryCompiler), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ICompiledQueryCacheKeyGenerator), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IModel), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDesignTimeModel), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IUpdateAdapterFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ICurrentDbContext), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDbContextDependencies), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDatabaseFacadeDependencies), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDbContextOptions), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDatabase), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDatabaseCreator), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDbContextTransactionManager), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryContextFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryCompilationContextFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryableMethodTranslatingExpressionVisitorFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryTranslationPreprocessorFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IQueryTranslationPostprocessorFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IShapedQueryCompilingExpressionVisitorFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IDbContextLogger), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IAdHocMapper), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ILiftableConstantProcessor), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(ILazyLoader), new ServiceCharacteristics(ServiceLifetime.Transient) }, { typeof(ILazyLoaderFactory), new ServiceCharacteristics(ServiceLifetime.Scoped) }, { typeof(IParameterBindingFactory), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(ITypeMappingSourcePlugin), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(IEvaluatableExpressionFilterPlugin), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(ISingletonOptions), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(IConventionSetPlugin), new ServiceCharacteristics(ServiceLifetime.Scoped, multipleRegistrations: true) }, { typeof(ISingletonInterceptor), new ServiceCharacteristics(ServiceLifetime.Singleton, multipleRegistrations: true) }, { typeof(IResettableService), new ServiceCharacteristics(ServiceLifetime.Scoped, multipleRegistrations: true) }, { typeof(IInterceptor), new ServiceCharacteristics(ServiceLifetime.Scoped, multipleRegistrations: true) }, { typeof(IInterceptorAggregator), new ServiceCharacteristics(ServiceLifetime.Scoped, multipleRegistrations: true) } };

这个字典的 Key 就是核心服务的类型。Relational 库和数据库提供者可能会补充一些服务，但大都多是替换服务，即接口类型是不变的。

第二个方案是通过一个叫 IRegisteredServices 的服务，它可以列出已注册服务的 Type 列表。

using (var c = new DemoContext()) { 
IRegisteredServices regsvcs 
= c.GetService<IRegisteredServices> (); 
 foreach (var sv in regsvcs.Services) { Console.WriteLine(sv); } …… }

这样就会列出内部已注册服务的类型。

Microsoft.EntityFrameworkCore.Diagnostics.LoggingDefinitions Microsoft.EntityFrameworkCore.Storage.IDatabaseProvider Microsoft.EntityFrameworkCore.ValueGeneration.IValueGeneratorCache Microsoft.EntityFrameworkCore.Storage.IRelationalTypeMappingSource Microsoft.EntityFrameworkCore.Storage.ISqlGenerationHelper Microsoft.EntityFrameworkCore.Metadata.IRelationalAnnotationProvider Microsoft.EntityFrameworkCore.Migrations.IMigrationsAnnotationProvider Microsoft.EntityFrameworkCore.Infrastructure.IModelValidator Microsoft.EntityFrameworkCore.Metadata.Conventions.Infrastructure.IProviderConventionSetBuilder Microsoft.EntityFrameworkCore.Update.IUpdateSqlGenerator Microsoft.EntityFrameworkCore.Query.IEvaluatableExpressionFilter Microsoft.EntityFrameworkCore.Storage.IRelationalTransactionFactory Microsoft.EntityFrameworkCore.Update.IModificationCommandBatchFactory Microsoft.EntityFrameworkCore.Update.IModificationCommandFactory Microsoft.EntityFrameworkCore.ValueGeneration.IValueGeneratorSelector Microsoft.EntityFrameworkCore.Storage.IRelationalConnection Microsoft.EntityFrameworkCore.Migrations.IMigrationsSqlGenerator Microsoft.EntityFrameworkCore.Storage.IRelationalDatabaseCreator Microsoft.EntityFrameworkCore.Migrations.IHistoryRepository Microsoft.EntityFrameworkCore.Storage.IExecutionStrategyFactory Microsoft.EntityFrameworkCore.Query.IRelationalQueryStringFactory Microsoft.EntityFrameworkCore.Query.ICompiledQueryCacheKeyGenerator Microsoft.EntityFrameworkCore.Query.IQueryCompilationContextFactory Microsoft.EntityFrameworkCore.Query.IMethodCallTranslatorProvider Microsoft.EntityFrameworkCore.Query.IAggregateMethodCallTranslatorProvider Microsoft.EntityFrameworkCore.Query.IMemberTranslatorProvider Microsoft.EntityFrameworkCore.Query.IQuerySqlGeneratorFactory Microsoft.EntityFrameworkCore.Query.IRelationalSqlTranslatingExpressionVisitorFactory Microsoft.EntityFrameworkCore.Query.ISqlExpressionFactory Microsoft.EntityFrameworkCore.Query.IQueryTranslationPostprocessorFactory Microsoft.EntityFrameworkCore.Query.IRelationalParameterBasedSqlProcessorFactory Microsoft.EntityFrameworkCore.Query.INavigationExpansionExtensibilityHelper Microsoft.EntityFrameworkCore.Query.IQueryableMethodTranslatingExpressionVisitorFactory Microsoft.EntityFrameworkCore.Storage.IExceptionDetector Microsoft.EntityFrameworkCore.Infrastructure.ISingletonOptions Microsoft.EntityFrameworkCore.SqlServer.Infrastructure.Internal.ISqlServerSingletonOptions Microsoft.EntityFrameworkCore.SqlServer.ValueGeneration.Internal.ISqlServerValueGeneratorCache Microsoft.EntityFrameworkCore.SqlServer.Update.Internal.ISqlServerUpdateSqlGenerator Microsoft.EntityFrameworkCore.SqlServer.ValueGeneration.Internal.ISqlServerSequenceValueGeneratorFactory Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.ISqlServerConnection Microsoft.EntityFrameworkCore.Storage.IParameterNameGeneratorFactory System.Collections.Generic.IComparer`1[Microsoft.EntityFrameworkCore.Update.IReadOnlyModificationCommand] Microsoft.EntityFrameworkCore.Migrations.IMigrationsIdGenerator Microsoft.EntityFrameworkCore.Update.Internal.IRowKeyValueFactoryFactory Microsoft.EntityFrameworkCore.Update.Internal.IRowForeignKeyValueFactoryFactory Microsoft.EntityFrameworkCore.Update.Internal.IRowIndexValueFactoryFactory Microsoft.EntityFrameworkCore.Update.Internal.IRowIdentityMapFactory Microsoft.EntityFrameworkCore.Infrastructure.IModelCustomizer Microsoft.EntityFrameworkCore.Infrastructure.IModelRuntimeInitializer Microsoft.EntityFrameworkCore.Migrations.IMigrator Microsoft.EntityFrameworkCore.Migrations.IMigrationCommandExecutor Microsoft.EntityFrameworkCore.Migrations.IMigrationsAssembly Microsoft.EntityFrameworkCore.Storage.IDatabase Microsoft.EntityFrameworkCore.Update.IBatchExecutor Microsoft.EntityFrameworkCore.Storage.IRelationalCommandBuilderFactory Microsoft.EntityFrameworkCore.Storage.IRawSqlCommandBuilder Microsoft.EntityFrameworkCore.Update.ICommandBatchPreparer Microsoft.EntityFrameworkCore.Migrations.IMigrationsModelDiffer Microsoft.EntityFrameworkCore.Storage.ITypeMappingSource Microsoft.EntityFrameworkCore.Storage.IDatabaseCreator Microsoft.EntityFrameworkCore.Storage.IDbContextTransactionManager Microsoft.EntityFrameworkCore.Query.IQueryContextFactory Microsoft.EntityFrameworkCore.Storage.Internal.INamedConnectionStringResolver Microsoft.EntityFrameworkCore.Storage.IDatabaseFacadeDependencies Microsoft.EntityFrameworkCore.Storage.IRelationalDatabaseFacadeDependencies Microsoft.EntityFrameworkCore.Diagnostics.IRelationalConnectionDiagnosticsLogger Microsoft.EntityFrameworkCore.Diagnostics.IDiagnosticsLogger`1[Microsoft.EntityFrameworkCore.DbLoggerCategory+Database+Connection] Microsoft.EntityFrameworkCore.Diagnostics.IRelationalCommandDiagnosticsLogger Microsoft.EntityFrameworkCore.Diagnostics.IDiagnosticsLogger`1[Microsoft.EntityFrameworkCore.DbLoggerCategory+Database+Command] Microsoft.EntityFrameworkCore.Diagnostics.IInterceptorAggregator Microsoft.EntityFrameworkCore.Query.IShapedQueryCompilingExpressionVisitorFactory Microsoft.EntityFrameworkCore.Query.IQueryTranslationPreprocessorFactory Microsoft.EntityFrameworkCore.Metadata.IAdHocMapper Microsoft.EntityFrameworkCore.Query.ISqlAliasManagerFactory Microsoft.EntityFrameworkCore.Query.ILiftableConstantFactory Microsoft.EntityFrameworkCore.Query.IRelationalLiftableConstantFactory Microsoft.EntityFrameworkCore.Query.ILiftableConstantProcessor Microsoft.EntityFrameworkCore.Storage.RelationalSqlGenerationHelperDependencies Microsoft.EntityFrameworkCore.Storage.RelationalTypeMappingSourceDependencies Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelValidatorDependencies Microsoft.EntityFrameworkCore.Update.UpdateSqlGeneratorDependencies Microsoft.EntityFrameworkCore.Metadata.RelationalAnnotationProviderDependencies Microsoft.EntityFrameworkCore.Migrations.MigrationsAnnotationProviderDependencies Microsoft.EntityFrameworkCore.Storage.ParameterNameGeneratorDependencies Microsoft.EntityFrameworkCore.Storage.RelationalTransactionFactoryDependencies Microsoft.EntityFrameworkCore.Storage.RelationalCommandBuilderDependencies Microsoft.EntityFrameworkCore.Query.QuerySqlGeneratorDependencies Microsoft.EntityFrameworkCore.Query.RelationalEvaluatableExpressionFilterDependencies Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelDependencies Microsoft.EntityFrameworkCore.Infrastructure.RelationalModelRuntimeInitializerDependencies Microsoft.EntityFrameworkCore.Query.RelationalLiftableConstantExpressionDependencies Microsoft.EntityFrameworkCore.Migrations.MigrationsSqlGeneratorDependencies Microsoft.EntityFrameworkCore.Metadata.Conventions.Infrastructure.RelationalConventionSetBuilderDependencies Microsoft.EntityFrameworkCore.Update.ModificationCommandBatchFactoryDependencies Microsoft.EntityFrameworkCore.Update.Internal.CommandBatchPreparerDependencies Microsoft.EntityFrameworkCore.Storage.RelationalDatabaseCreatorDependencies Microsoft.EntityFrameworkCore.Migrations.HistoryRepositoryDependencies Microsoft.EntityFrameworkCore.Query.RelationalCompiledQueryCacheKeyGeneratorDependencies Microsoft.EntityFrameworkCore.Query.RelationalMethodCallTranslatorProviderDependencies Microsoft.EntityFrameworkCore.Query.RelationalAggregateMethodCallTranslatorProviderDependencies Microsoft.EntityFrameworkCore.Query.RelationalMemberTranslatorProviderDependencies Microsoft.EntityFrameworkCore.Query.SqlExpressionFactoryDependencies Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitorDependencies Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitorDependencies Microsoft.EntityFrameworkCore.Query.RelationalShapedQueryCompilingExpressionVisitorDependencies Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPreprocessorDependencies Microsoft.EntityFrameworkCore.Query.RelationalQueryTranslationPostprocessorDependencies Microsoft.EntityFrameworkCore.Query.RelationalParameterBasedSqlProcessorDependencies Microsoft.EntityFrameworkCore.Storage.RelationalConnectionDependencies Microsoft.EntityFrameworkCore.Storage.RelationalDatabaseDependencies Microsoft.EntityFrameworkCore.Query.RelationalQueryContextDependencies Microsoft.EntityFrameworkCore.Query.RelationalQueryCompilationContextDependencies Microsoft.EntityFrameworkCore.Metadata.RelationalAdHocMapperDependencies Microsoft.EntityFrameworkCore.Infrastructure.IDbSetFinder Microsoft.EntityFrameworkCore.Internal.IDbSetInitializer Microsoft.EntityFrameworkCore.Internal.IDbSetSource Microsoft.EntityFrameworkCore.Internal.IEntityFinderSource Microsoft.EntityFrameworkCore.Query.IEntityMaterializerSource Microsoft.EntityFrameworkCore.Metadata.Conventions.Infrastructure.IConventionSetBuilder Microsoft.EntityFrameworkCore.Infrastructure.IModelCacheKeyFactory Microsoft.Extensions.Logging.ILoggerFactory Microsoft.EntityFrameworkCore.Infrastructure.IModelSource Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IInternalEntityEntrySubscriber Microsoft.EntityFrameworkCore.ChangeTracking.IEntityEntryGraphIterator Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IEntityGraphAttacher Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IKeyPropagator Microsoft.EntityFrameworkCore.ChangeTracking.Internal.INavigationFixer Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ILocalViewListener Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IStateManager Microsoft.EntityFrameworkCore.Infrastructure.IConcurrencyDetector Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IInternalEntityEntryNotifier Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IValueGenerationManager Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IChangeTrackerFactory Microsoft.EntityFrameworkCore.ChangeTracking.Internal.IChangeDetector Microsoft.EntityFrameworkCore.Internal.IDbContextServices Microsoft.EntityFrameworkCore.Internal.IDbContextDependencies Microsoft.EntityFrameworkCore.Storage.IExecutionStrategy Microsoft.EntityFrameworkCore.Query.Internal.ICompiledQueryCache Microsoft.EntityFrameworkCore.Query.IAsyncQueryProvider Microsoft.EntityFrameworkCore.Query.Internal.IQueryCompiler Microsoft.EntityFrameworkCore.Internal.ISingletonOptionsInitializer Microsoft.EntityFrameworkCore.Diagnostics.IDiagnosticsLogger`1[TLoggerCategory] Microsoft.EntityFrameworkCore.Diagnostics.IInterceptors Microsoft.EntityFrameworkCore.Diagnostics.ILoggingOptions Microsoft.EntityFrameworkCore.Infrastructure.ICoreSingletonOptions Microsoft.EntityFrameworkCore.Metadata.IModel Microsoft.EntityFrameworkCore.Metadata.IDesignTimeModel Microsoft.EntityFrameworkCore.Infrastructure.ICurrentDbContext Microsoft.EntityFrameworkCore.Infrastructure.IDbContextOptions Microsoft.EntityFrameworkCore.Infrastructure.IResettableService Microsoft.EntityFrameworkCore.Storage.ValueConversion.IValueConverterSelector Microsoft.EntityFrameworkCore.Metadata.IConstructorBindingFactory Microsoft.EntityFrameworkCore.Infrastructure.Internal.ILazyLoaderFactory Microsoft.EntityFrameworkCore.Infrastructure.ILazyLoader Microsoft.EntityFrameworkCore.Metadata.IParameterBindingFactories Microsoft.EntityFrameworkCore.Metadata.Internal.IMemberClassifier Microsoft.EntityFrameworkCore.Metadata.IPropertyParameterBindingFactory Microsoft.EntityFrameworkCore.Metadata.IParameterBindingFactory Microsoft.Extensions.Caching.Memory.IMemoryCache Microsoft.EntityFrameworkCore.Update.IUpdateAdapterFactory Microsoft.EntityFrameworkCore.Storage.Json.IJsonValueReaderWriterSource Microsoft.EntityFrameworkCore.Diagnostics.IDbContextLogger System.Diagnostics.DiagnosticSource Microsoft.EntityFrameworkCore.Metadata.LazyLoaderParameterBindingFactoryDependencies Microsoft.EntityFrameworkCore.Storage.DatabaseProviderDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelSourceDependencies Microsoft.EntityFrameworkCore.ValueGeneration.ValueGeneratorCacheDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelValidatorDependencies Microsoft.EntityFrameworkCore.Storage.TypeMappingSourceDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelCustomizerDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelCacheKeyFactoryDependencies Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverterSelectorDependencies Microsoft.EntityFrameworkCore.Query.Internal.EntityMaterializerSourceDependencies Microsoft.EntityFrameworkCore.Query.EvaluatableExpressionFilterDependencies Microsoft.EntityFrameworkCore.Infrastructure.RuntimeModelDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelRuntimeInitializerDependencies Microsoft.EntityFrameworkCore.Query.NavigationExpansionExtensibilityHelperDependencies Microsoft.EntityFrameworkCore.Storage.Json.JsonValueReaderWriterSourceDependencies Microsoft.EntityFrameworkCore.Query.LiftableConstantExpressionDependencies Microsoft.EntityFrameworkCore.Metadata.Conventions.Infrastructure.ProviderConventionSetBuilderDependencies Microsoft.EntityFrameworkCore.Query.QueryCompilationContextDependencies Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManagerDependencies Microsoft.EntityFrameworkCore.Storage.ExecutionStrategyDependencies Microsoft.EntityFrameworkCore.Query.CompiledQueryCacheKeyGeneratorDependencies Microsoft.EntityFrameworkCore.Query.QueryContextDependencies Microsoft.EntityFrameworkCore.Query.QueryableMethodTranslatingExpressionVisitorDependencies Microsoft.EntityFrameworkCore.Query.QueryTranslationPreprocessorDependencies Microsoft.EntityFrameworkCore.Query.QueryTranslationPostprocessorDependencies Microsoft.EntityFrameworkCore.Query.ShapedQueryCompilingExpressionVisitorDependencies Microsoft.EntityFrameworkCore.ValueGeneration.ValueGeneratorSelectorDependencies Microsoft.EntityFrameworkCore.Storage.DatabaseDependencies Microsoft.EntityFrameworkCore.Infrastructure.ModelDependencies Microsoft.EntityFrameworkCore.ModelCreationDependencies Microsoft.EntityFrameworkCore.Metadata.AdHocMapperDependencies

严重注意：

实体构造函数不能注入 IRegisteredServices  类型的服务，因为 EF Core 内部实现时，本身就是从 IRegisteredServices.Services 集合中查找可以注入的服务的，而 IRegisteredServices.Services 是不包含它自己的。说人话就是：用 IRegisteredServices 类注入是找不到该服务的。

 

然后解决第二个问题：在实体类的构造函数中注入服务有啥用？大多数时候没啥用，但当你要使用某些服务来获取特殊信息，或执行特殊操作（如在实体类中执行 SQL 语句）时就用得上。

咱们举个例子，就用官方文档最喜欢的 Blog 和 Post 实体来演示。其中，Blog 类的构造函数将注入 IEntityType 服务。然后可以利用这个服务来获取 Blog 实体有几个属性，有几个键，有几个导航属性。

public class Blog { // 私有字段 private 
IEntityType? _thisEntity; 
 // 私有构造器 private Blog(IEntityType t) => _thisEntity = t; // 公共构造器 public Blog() { } public int BlogID { get; set; } public string DisplayName { get; set; } = string.Empty; public string Url { get; set; } = string.Empty; public List<Post> PostList { get; set; } = new(); // 以下属性通过注入的 IEntityType 获取相关值 public int PropertyCount => _thisEntity?.GetProperties().Count() ?? 0; public int NavigationCount => _thisEntity?.GetNavigations().Count() ?? 0; public int KeyCount => _thisEntity?.GetKeys().Count() ?? 0; } public class Post { public int PostID { get; set; } public string Header { get; set; } = string.Empty; public Blog? TheBlog { get; set; } }

注意，你自己 new 一个 Blog 实例是没有注入功能的，所以要定义一个无参数的公共构造函数，以供外部初始化用。另一个私有构造函数有一个 IEntityType 类型的参数，可以接收注入。因为这构造函数只由 EF Core 内部调用，代码中调用无法注入，故声明为私有即可。EF Core 在查找构造函数时，不管你是公有的还是私有的，只要不是静态的，一律能发现。

在访问 Blog 实体时就可以获取键、属性、导航属性的数量。

var q = c.Blogs.Include(b => b.PostList).ToArray(); var first = q.FirstOrDefault(); if (first != null) { Console.WriteLine($"实体{first.GetType().Name}有{first. PropertyCount}个属性，{first.KeyCount}个键，{first. NavigationCount}个导航属性"); }

得到的输出如下：

实体Blog有3个属性，1个键，1个导航属性

 

除了构造函数，也可以从属性注入。例如

[PrimaryKey(nameof(Screen.ScrID))] public class Screen { [Column("scr_id")] public int ScrID { get; set; } [Column("width")] public int Width { get; set; } [Column("height")] public int Height { get; set; } //----------- 注意以下属性 -------------- public 
MyDBContext? ThisContext
 { get; set; } } public class MyDBContext : DbContext { public MyDBContext(DbContextOptions<MyDBContext> options) : base(options) { } public DbSet<Screen> Screens { get; set; } }

上面示例中，MyDBContext 实例会注入到每个 Screen 实例的 ThisContext 属性。即每个实体实例的 ThisContext 属性都引用同一个 MyDBContext 对象。

var q = dc.Screens.FirstOrDefault(); if (q != null) { if (q.ThisContext != null) { Console.WriteLine("ThisContext属性：{0}", q.ThisContext); } } // 输出： // ThisContext属性：MyDBContext

好了，今天就水到这里了。因为这个主题确实太简单了，老周也不必举太多例子。