【EF Core】實體類的依賴注入
在使用外部模型那篇水文中，有大伙伴提出：老周，你那個 Ultraman 類和 Speciality 類的的關系是不是有問題，外鍵不應該在 Speciality 類上嗎，怎么會跑到 Ultraman 類上？因為它們是一對一關系，在配置的時候你也可以反過來，主要區別是誰引用誰的問題，由于是一對一引用，所以反過來也可以的。
今天咱們聊聊實體類構造函數的依賴注入。實體類也支持依賴注入，不過，目前版本只支持注入 EF Core 自己的服務類型，你在代碼中添加的應用程序級服務類型不能注入（以后可能會支持）。總結一下，以下服務類型可以注入：
1、EF Core 框架內部注冊的服務類型；
2、三個補充類型：
a、當前 DbContext 實例（你從 DbContext 派生的類）；
b、當前實體相關的 IEntityType，可以獲取實體模型相關信息；
c、ILazyLoader 接口，延時加載時用得上。
現在要解決一個問題：我怎么知道 EF Core 框架內部哪些服務可以注入？這個活兒有兩個方案：第一個方案是看 EF Core 源代碼，在 EFCore\Infrastructure 下，注意看 EntityFrameworkServicesBuilder 類的 CoreServices 字段或 TryAddCoreServices 方法，基本齊全了。
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) } };
這個字典的 Key 就是核心服務的類型。Relational 庫和數據庫提供者可能會補充一些服務，但大都多是替換服務，即接口類型是不變的。
第二個方案是通過一個叫 IRegisteredServices 的服務，它可以列出已注冊服務的 Type 列表。
using (var c = new DemoContext()) { IRegisteredServices regsvcs = c.GetService<IRegisteredServices> (); foreach (var sv in regsvcs.Services) { Console.WriteLine(sv); } …… }
這樣就會列出內部已注冊服務的類型。
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
嚴重注意：實體構造函數不能注入 IRegisteredServices 類型的服務，因為 EF Core 內部實現時，本身就是從 IRegisteredServices.Services 集合中查找可以注入的服務的，而 IRegisteredServices.Services 是不包含它自己的。說人話就是：用 IRegisteredServices 類注入是找不到該服務的。
然后解決第二個問題：在實體類的構造函數中注入服務有啥用？大多數時候沒啥用，但當你要使用某些服務來獲取特殊信息，或執行特殊操作（如在實體類中執行 SQL 語句）時就用得上。
咱們舉個例子，就用官方文檔最喜歡的 Blog 和 Post 實體來演示。其中，Blog 類的構造函數將注入 IEntityType 服務。然后可以利用這個服務來獲取 Blog 實體有幾個屬性，有幾個鍵，有幾個導航屬性。
public class Blog { // 私有字段 private IEntityType? _thisEntity; // 私有構造器 private Blog(IEntityType t) => _thisEntity = t; // 公共構造器 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(); // 以下屬性通過注入的 IEntityType 獲取相關值 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 一個 Blog 實例是沒有注入功能的，所以要定義一個無參數的公共構造函數，以供外部初始化用。另一個私有構造函數有一個 IEntityType 類型的參數，可以接收注入。因為這構造函數只由 EF Core 內部調用，代碼中調用無法注入，故聲明為私有即可。EF Core 在查找構造函數時，不管你是公有的還是私有的，只要不是靜態的，一律能發現。
在訪問 Blog 實體時就可以獲取鍵、屬性、導航屬性的數量。
var q = c.Blogs.Include(b => b.PostList).ToArray(); var first = q.FirstOrDefault(); if (first != null) { Console.WriteLine($"實體{first.GetType().Name}有{first. PropertyCount}個屬性，{first.KeyCount}個鍵，{first. NavigationCount}個導航屬性"); }
得到的輸出如下：
實體Blog有3個屬性，1個鍵，1個導航屬性
除了構造函數，也可以從屬性注入。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; } //----------- 注意以下屬性 -------------- 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 實例會注入到每個 Screen 實例的 ThisContext 屬性。即每個實體實例的 ThisContext 屬性都引用同一個 MyDBContext 對象。
var q = dc.Screens.FirstOrDefault(); if (q != null) { if (q.ThisContext != null) { Console.WriteLine("ThisContext屬性：{0}", q.ThisContext); } } // 輸出： // ThisContext屬性：MyDBContext
好了，今天就水到這里了。因為這個主題確實太簡單了，老周也不必舉太多例子。
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