【EF Core】DbContext是如何识别出实体集合的

战机五代半 發表於 2025-6-29 22:31:00

【EF Core】DbContext是如何识别出实体集合的

在开始之前说明一下，你不要指望阅读完本文后会得到光，就算得到光你也未必能变成迪迦。本文老周仅介绍原理，可以给部分大伙伴们解惑。
咱们都知道，在派生 DbContext 类时，集体类的集合用 DbSet<TEntity> 表示，而咱们最常用的方法是在 DbContext 的派生类中公开 DbSet<TEntity> 属性。但在实例化 DbContext 后，我们并未给这些属性赋值，就能查询数据了，那么，DbContext 类（包括其子类）是如何识别出这些公共属性并填充数据的？
好，主题已经打开，接下来老周就开始表演了。有大伙伴会说了：切，这个看看源码不就知道了。是的，但有些人天生懒啊，不想看，那老周帮你看。
首先，咱们要了解，DbContext 类是如何维护实体集合的？DbContext 类中有这么个字段声明：
<div class="cnblogs_code">
<pre> private Dictionary<(Type Type, string? Name), object>? _sets;</pre>
</div>
这行代码老周严重希望你能看懂，看不懂会很麻烦的哟。这是一个字典类型，没错吧。然后，Key是啥类型，Value是啥类型？
Key：是一个二元元组，第一项为 Type 对象，第二项为字符串对象。type 指的是实体类的 Type，name 指的是你为这个实体集合分配的名字。有伙伴会问，我怎么给它命名，DbSet 实例又不是我创建的？不急，请看下文；
Value：猜得出来，这是与实体集合相关的实例，DbSet<>，实际类型是内部类 InternalDbSet<TEntity>。这个后面咱们再说。
咱们先不去关心 DbSet<TEntity> 实例是怎么创建的（因为这里面要绕绕弯子），至少咱们知道：在DbContext上声明的实体集合是缓存到一个字典中的。而把集合实例添加到字典中的是一个名为 GetOrAddSet 的方法。注意该方法是显示实现了 IDbSetCache 接口的。看看这个接口的定义：
<div class="cnblogs_code">
<pre>public interface IDbSetCache
{

object GetOrAddSet(IDbSetSource source, Type type);

object GetOrAddSet(
 IDbSetSource source,
 string entityTypeName,
 Type type);

IEnumerable<object> GetSets();
}</pre>
</div>
IDbSetSource 接口的实现者就是跟创建 DbSet 实例有关的，咱们先忽略它。把注意放那两个重载方法 GetOrAddSet 上，它的功能就是获取或者添加实体集合的引用。咱们看到，这两个重载的区别在：1、以Type为标识添加；2、以Type + name为标识添加。而 DbContext 类是显式实现了 IDbSetCache 接口的，即咱们上面提到过的，就是把 DbSet 实例存到那个名为 _sets 的字典中。
<div class="cnblogs_code">
<pre> object IDbSetCache.GetOrAddSet(
 IDbSetSource source,
 Type type)
{
 CheckDisposed();

 _sets ??= [];

 if (!_sets.TryGetValue((type, null), out var set))
 {
 set = source.Create(this, type);
 _sets[(type, null)] = set;
 _cachedResettableServices = null;
 }

 return set;
}

object IDbSetCache.GetOrAddSet(
 IDbSetSource source,
 string entityTypeName,
 Type type)
{
 CheckDisposed();

 _sets ??= [];

 if (!_sets.TryGetValue((type, entityTypeName), out var set))
 {
 set = source.Create(this, entityTypeName, type);
 _sets[(type, entityTypeName)] = set;
 _cachedResettableServices = null;
 }

 return set;
}</pre>
</div>
当添加的实体集合有名字时，字典的Key是由 type 和 entiyTypeName 组成；当集合不提供名字时，Key 就由 type 和 null 组成。
然后，DbContext 类公开一组重载方法，封装了 GetOrAddSet 方法的调用。
<div class="cnblogs_code">
<pre> public virtual DbSet<TEntity> Set< TEntity>()
 where TEntity : class
 => (DbSet<TEntity>)((IDbSetCache)this).GetOrAddSet(DbContextDependencies.SetSource, typeof(TEntity));

public virtual DbSet<TEntity> Set< TEntity>(string name)
 where TEntity : class
 => (DbSet<TEntity>)((IDbSetCache)this).GetOrAddSet(DbContextDependencies.SetSource, name, typeof(TEntity));</pre>
</div>
 
根据这个逻辑，那么，咱们在继承 DbContext 类时，这样写也可以（假设实体类为 Student）：
<div class="cnblogs_code">
<pre>public class MyDbContext : DbContext
{
 public DbSet<Student> Students => Set<Student>();
 // 或者
 public DbSet<Student> Students => Set<Student>("stu");
}</pre>
</div>
不过，咱们通常的写法是实体集合作为公共属性：
<div class="cnblogs_code">
<pre>public class MyDbContext : DbContext
{
 public DbSet<Student> Students { get; set; }
}</pre>
</div>
那 DbContext 类是怎么识别并调用 GetOrAddSet 方法的？
这就要用到另一个辅助—— IDbSetInitializer，其实现类为 DbSetInitializer。
<div class="cnblogs_code">
<pre>public class DbSetInitializer : IDbSetInitializer
{
private readonly IDbSetFinder _setFinder;
private readonly IDbSetSource _setSource;

public DbSetInitializer(
 IDbSetFinder setFinder,
 IDbSetSource setSource)
{
 _setFinder = setFinder;
 _setSource = setSource;
}

public virtual void InitializeSets(DbContext context)
{
 foreach (var setInfo in _setFinder.FindSets(context.GetType()).Where(p => p.Setter != null))
 {
 setInfo.Setter!.SetClrValueUsingContainingEntity(
 context,
 ((IDbSetCache)context).GetOrAddSet(_setSource, setInfo.Type));
 }
}
}</pre>
</div>
这个 InitializeSets 方法就是在 DbContext 类的构造函数中调用的。
<div class="cnblogs_code">
<pre> public DbContext(DbContextOptions options)
{
 ……

 ServiceProviderCache.Instance.GetOrAdd(options, providerRequired: false)
 .GetRequiredService<IDbSetInitializer>()
 .InitializeSets(this);

 EntityFrameworkMetricsData.ReportDbContextInitializing();
}</pre>
</div>
由于各种辅助类型间有依赖关系，因此，EF Core 内部其实也使用了服务容器技术来自动实例化。咱们回到上面 InitializeSets 方法的实现代码上。从源代码中我们看到，其实完成从 DbContext 的公共属性识别 DbSet<> 这一功能的是名为 IDbSetFinder 的组件，它的内部实现类为 DbSetFinder。
<div class="cnblogs_code">
<pre>public class DbSetFinder : IDbSetFinder
{
private readonly ConcurrentDictionary<Type, IReadOnlyList<DbSetProperty>> _cache = new();

public virtual IReadOnlyList<DbSetProperty> FindSets(Type contextType)
 => _cache.GetOrAdd(contextType, FindSetsNonCached);

private static DbSetProperty[] FindSetsNonCached(Type contextType)
{
 var factory = ClrPropertySetterFactory.Instance;

 return contextType.GetRuntimeProperties()
 .Where(
 p => !&& !&& p.DeclaringType != typeof&&&& p.PropertyType.GetGenericTypeDefinition() == typeof(DbSet<>))
 .OrderBy(p => p.Name)
 .Select(
 p => new DbSetProperty(
 p.Name,
 p.PropertyType.GenericTypeArguments.Single(),
 p.SetMethod == null ? null : factory.Create(p)))
 .ToArray();
}
}</pre>
</div>
总结一下，就是在 DbContext 的派生类中查找符合以下条件的属性：
1、非静态属性；
2、不能是索引器；
3、属性是 DbSet<> 类型，并且有泛型参数（即实体类型）；
4、外加一条，属性具有 set 访问器（这个条件是在 InitializeSets 方法的代码中，Where 方法筛选出来）。
 
到了这里，本文的主题就有了答案了：
DbContext 构造函数 --> IDbSetInitializer --> IDbSetFinder
还差一步，前面咱们说过，DbSet<> 实例是由 IDbSetSource 负责创建的，其内部实现类是 DbSetSource。
<div class="cnblogs_code">
<pre>public class DbSetSource : IDbSetSource
{
private static readonly MethodInfo GenericCreateSet
 = typeof(DbSetSource).GetTypeInfo().GetDeclaredMethod(nameof(CreateSetFactory))!;

private readonly ConcurrentDictionary<(Type Type, string? Name), Func<DbContext, string?, object>> _cache = new();

public virtual object Create(DbContext context, Type type)
 => CreateCore(context, type, null, GenericCreateSet);

public virtual object Create(DbContext context, string name, Type type)
 => CreateCore(context, type, name, GenericCreateSet);

private object CreateCore(DbContext context, Type type, string? name, MethodInfo createMethod)
 => _cache.GetOrAdd(
 (type, name),
 static (t, createMethod) => (Func<DbContext, string?, object>)createMethod
 .MakeGenericMethod(t.Type)
 .Invoke(null, null)!,
 createMethod)(context, name);

private static Func<DbContext, string?, object> CreateSetFactory<TEntity>()
 where TEntity : class
 => (c, name) => new InternalDbSet<TEntity>(c, name);
}</pre>
</div>
所以，默认创建的 DbSet<> 实例其实是 InternalDbSet<TEntity> 类型。
所有的组件都是通过 EntityFrameworkServicesBuilder 类的相关方法来添加到服务容器中的。
<div class="cnblogs_code">
<pre> public virtual EntityFrameworkServicesBuilder TryAddCoreServices()
{
 TryAdd<IDbSetFinder, DbSetFinder>();
 TryAdd<IDbSetInitializer, DbSetInitializer>();
 TryAdd<IDbSetSource, DbSetSource>();
 TryAdd<IEntityFinderSource, EntityFinderSource>();
 TryAdd<IEntityMaterializerSource, EntityMaterializerSource>();
 TryAdd<IProviderConventionSetBuilder, ProviderConventionSetBuilder>();
 TryAdd<IConventionSetBuilder, RuntimeConventionSetBuilder>();
 TryAdd<IModelCustomizer, ModelCustomizer>();
 TryAdd<IModelCacheKeyFactory, ModelCacheKeyFactory>();
 TryAdd<ILoggerFactory>(p => ScopedLoggerFactory.Create(p, null));
 TryAdd<IModelSource, ModelSource>();
 TryAdd<IModelRuntimeInitializer, ModelRuntimeInitializer>();
 TryAdd<IInternalEntityEntrySubscriber, InternalEntityEntrySubscriber>();
 TryAdd<IEntityEntryGraphIterator, EntityEntryGraphIterator>();
 TryAdd<IEntityGraphAttacher, EntityGraphAttacher>();
 TryAdd<IValueGeneratorCache, ValueGeneratorCache>();
 TryAdd<IKeyPropagator, KeyPropagator>();
 TryAdd<INavigationFixer, NavigationFixer>();
 TryAdd<ILocalViewListener, LocalViewListener>();
 TryAdd<IStateManager, StateManager>();
 TryAdd<IConcurrencyDetector, ConcurrencyDetector>();
 TryAdd<IInternalEntityEntryNotifier, InternalEntityEntryNotifier>();
 TryAdd<IValueGenerationManager, ValueGenerationManager>();
 TryAdd<IChangeTrackerFactory, ChangeTrackerFactory>();
 TryAdd<IChangeDetector, ChangeDetector>();
 TryAdd<IDbContextServices, DbContextServices>();
 TryAdd<IDbContextDependencies, DbContextDependencies>();
 TryAdd<IDatabaseFacadeDependencies, DatabaseFacadeDependencies>();
 TryAdd<IValueGeneratorSelector, ValueGeneratorSelector>();
 TryAdd<IModelValidator, ModelValidator>();
 TryAdd<IExecutionStrategyFactory, ExecutionStrategyFactory>();
 TryAdd(p => p.GetRequiredService<IExecutionStrategyFactory>().Create());
 TryAdd<ICompiledQueryCache, CompiledQueryCache>();
 TryAdd<IAsyncQueryProvider, EntityQueryProvider>();
 TryAdd<IQueryCompiler, QueryCompiler>();
 TryAdd<ICompiledQueryCacheKeyGenerator, CompiledQueryCacheKeyGenerator>();
 TryAdd<ISingletonOptionsInitializer, SingletonOptionsInitializer>();
 TryAdd(typeof(IDiagnosticsLogger<>), typeof(DiagnosticsLogger<>));
 TryAdd<IInterceptors, Interceptors>();
 TryAdd<IInterceptorAggregator, SaveChangesInterceptorAggregator>();
 TryAdd<IInterceptorAggregator, IdentityResolutionInterceptorAggregator>();
 TryAdd<IInterceptorAggregator, QueryExpressionInterceptorAggregator>();
 TryAdd<ILoggingOptions, LoggingOptions>();
 TryAdd<ICoreSingletonOptions, CoreSingletonOptions>();
 TryAdd<ISingletonOptions, ILoggingOptions>(p => p.GetRequiredService<ILoggingOptions>());
 TryAdd<ISingletonOptions, ICoreSingletonOptions>(p => p.GetRequiredService<ICoreSingletonOptions>());
 TryAdd(p => GetContextServices(p).Model);
 TryAdd<IDesignTimeModel>(p => new DesignTimeModel(GetContextServices(p)));
 TryAdd(p => GetContextServices(p).CurrentContext);
 TryAdd<IDbContextOptions>(p => GetContextServices(p).ContextOptions);
 TryAdd<IResettableService, ILazyLoaderFactory>(p => p.GetRequiredService<ILazyLoaderFactory>());
 TryAdd<IResettableService, IStateManager>(p => p.GetRequiredService<IStateManager>());
 TryAdd<IResettableService, IDbContextTransactionManager>(p => p.GetRequiredService<IDbContextTransactionManager>());
 TryAdd<IEvaluatableExpressionFilter, EvaluatableExpressionFilter>();
 TryAdd<IValueConverterSelector, ValueConverterSelector>();
 TryAdd<IConstructorBindingFactory, ConstructorBindingFactory>();
 TryAdd<ILazyLoaderFactory, LazyLoaderFactory>();
 TryAdd<ILazyLoader>(p => p.GetRequiredService<ILazyLoaderFactory>().Create());
 TryAdd<IParameterBindingFactories, ParameterBindingFactories>();
 TryAdd<IMemberClassifier, MemberClassifier>();
 TryAdd<IPropertyParameterBindingFactory, PropertyParameterBindingFactory>();
 TryAdd<IParameterBindingFactory, LazyLoaderParameterBindingFactory>();
 TryAdd<IParameterBindingFactory, ContextParameterBindingFactory>();
 TryAdd<IParameterBindingFactory, EntityTypeParameterBindingFactory>();
 TryAdd<IMemoryCache>(_ => new MemoryCache(new MemoryCacheOptions { SizeLimit = 10240 }));
 TryAdd<IUpdateAdapterFactory, UpdateAdapterFactory>();
 TryAdd<IQueryCompilationContextFactory, QueryCompilationContextFactory>();
 TryAdd<IQueryTranslationPreprocessorFactory, QueryTranslationPreprocessorFactory>();
 TryAdd<IQueryTranslationPostprocessorFactory, QueryTranslationPostprocessorFactory>();
 TryAdd<INavigationExpansionExtensibilityHelper, NavigationExpansionExtensibilityHelper>();
 TryAdd<IExceptionDetector, ExceptionDetector>();
 TryAdd<IAdHocMapper, AdHocMapper>();
 TryAdd<IJsonValueReaderWriterSource, JsonValueReaderWriterSource>();
 TryAdd<ILiftableConstantFactory, LiftableConstantFactory>();
 TryAdd<ILiftableConstantProcessor, LiftableConstantProcessor>();

 TryAdd(
 p => p.GetService<IDbContextOptions>()?.FindExtension<CoreOptionsExtension>()?.DbContextLogger
 ?? new NullDbContextLogger());

 // This has to be lazy to avoid creating instances that are not disposed
 ServiceCollectionMap
 .TryAddSingleton<DiagnosticSource>(_ => new DiagnosticListener(DbLoggerCategory.Name));

 ServiceCollectionMap.GetInfrastructure()
 .AddDependencySingleton<LazyLoaderParameterBindingFactoryDependencies>()
 .AddDependencySingleton<DatabaseProviderDependencies>()
 .AddDependencySingleton<ModelSourceDependencies>()
 .AddDependencySingleton<ValueGeneratorCacheDependencies>()
 .AddDependencySingleton<ModelValidatorDependencies>()
 .AddDependencySingleton<TypeMappingSourceDependencies>()
 .AddDependencySingleton<ModelCustomizerDependencies>()
 .AddDependencySingleton<ModelCacheKeyFactoryDependencies>()
 .AddDependencySingleton<ValueConverterSelectorDependencies>()
 .AddDependencySingleton<EntityMaterializerSourceDependencies>()
 .AddDependencySingleton<EvaluatableExpressionFilterDependencies>()
 .AddDependencySingleton<RuntimeModelDependencies>()
 .AddDependencySingleton<ModelRuntimeInitializerDependencies>()
 .AddDependencySingleton<NavigationExpansionExtensibilityHelperDependencies>()
 .AddDependencySingleton<JsonValueReaderWriterSourceDependencies>()
 .AddDependencySingleton<LiftableConstantExpressionDependencies>()
 .AddDependencyScoped<ProviderConventionSetBuilderDependencies>()
 .AddDependencyScoped<QueryCompilationContextDependencies>()
 .AddDependencyScoped<StateManagerDependencies>()
 .AddDependencyScoped<ExecutionStrategyDependencies>()
 .AddDependencyScoped<CompiledQueryCacheKeyGeneratorDependencies>()
 .AddDependencyScoped<QueryContextDependencies>()
 .AddDependencyScoped<QueryableMethodTranslatingExpressionVisitorDependencies>()
 .AddDependencyScoped<QueryTranslationPreprocessorDependencies>()
 .AddDependencyScoped<QueryTranslationPostprocessorDependencies>()
 .AddDependencyScoped<ShapedQueryCompilingExpressionVisitorDependencies>()
 .AddDependencyScoped<ValueGeneratorSelectorDependencies>()
 .AddDependencyScoped<DatabaseDependencies>()
 .AddDependencyScoped<ModelDependencies>()
 .AddDependencyScoped<ModelCreationDependencies>()
 .AddDependencyScoped<AdHocMapperDependencies>();

 ServiceCollectionMap.TryAddSingleton<IRegisteredServices>(
 new RegisteredServices(ServiceCollectionMap.ServiceCollection.Select(s => s.ServiceType)));

 return this;
}</pre>
</div>
 
DbContext 对象在初始化时只是查找实体集合，此时还没有任何查询被执行。当咱们要访问实体数据时，DbSet<> 会把查询任务交给 IAsyncQueryProvider 接口的实现类去处理，它的内部实现类是 EntityQueryProvider。
EntityQueryProvider 内部基于 LINQ 生成表达式树，表达式树传递给 IQueryCompiler 去编译并运行。IQueryCompiler 接口有个内部实现类叫 QueryCompiler。
后面就一路往下传递到数据库层，执行生成的SQL。当然这里头还包含很多复杂的组件，此处咱们就不继续挖，否则要挖到明天早上。
 
本文老周只讲述了和 DbContext 类添加实体集合相关的组件，其他组件等后面说到相关内容再介绍。咱们总不能一口气把整个框架都说一遍的，太复杂了。
  
来源：https://www.cnblogs.com/tcjiaan/p/18956032

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【EF Core】DbContext是如何识别出实体集合的