【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 表示，而咱们最常用的方法是在 DbContext 的派生类中公开 DbSet 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：
private Dictionary<(Type Type, string? Name), object>? _sets;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。这个后面咱们再说。
咱们先不去关心 DbSet 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：
public interface IDbSetCache
{
object GetOrAddSet(IDbSetSource source, Type type);
object GetOrAddSet(
IDbSetSource source,
string entityTypeName,
Type type);
IEnumerable<object> GetSets();
}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。
object IDbSetCache.GetOrAddSet(
IDbSetSource source,
Type type)
{
CheckDisposed();
_sets ??= [];
if (!_sets.TryGetValue((type, null), out var set))
{
set = source.Create(this, type);
<em><strong>_sets[(type, </strong></em><em><strong>null)] = set</strong></em><em><strong>;</strong></em>
_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);
<em><strong>_sets[(type, entityTypeName)] </strong></em><em><strong>= set</strong></em><em><strong>;</strong></em>
_cachedResettableServices = null;
}
return set;
}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。
public virtual DbSet<TEntity> Set< TEntity>()
where TEntity : class
=> (DbSet<TEntity>)((IDbSetCache)this).<strong>GetOrAddSet(DbContextDependencies.SetSource, typeof</strong><strong>(TEntity))</strong>;
public virtual DbSet<TEntity> Set< TEntity>(string name)
where TEntity : class
=> (DbSet<TEntity>)((IDbSetCache)this).<strong>GetOrAddSet(DbContextDependencies.SetSource, name, typeof(TEntity))</strong>;
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）：
public class MyDbContext : DbContext
{
public DbSet<Student> Students => Set<Student>();
// 或者
public DbSet<Student> Students => Set<Student>("stu");
}不过，咱们通常的写法是实体集合作为公共属性：
public class MyDbContext : DbContext
{
public DbSet<Student> Students { get; set; }
}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。
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 <strong>setInfo in _setFinder.FindSets(context.GetType()).Where(p => p.Setter != null</strong><strong>)</strong>)
{
setInfo.Setter!.SetClrValueUsingContainingEntity(
context,
((IDbSetCache)context).<strong>GetOrAddSet(_setSource, setInfo.Type))</strong>;
}
}
}这个 InitializeSets 方法就是在 DbContext 类的构造函数中调用的。
public DbContext(DbContextOptions options)
{
……
ServiceProviderCache.Instance.GetOrAdd(options, providerRequired: false)
<strong>.GetRequiredService</strong><strong><IDbSetInitializer></strong><strong>()
.InitializeSets(</strong><strong>this</strong><strong>)</strong>;
EntityFrameworkMetricsData.ReportDbContextInitializing();
}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。
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()
<strong>.Where(
</strong> <strong>p </strong><strong>=> !</strong><strong>&& !</strong><strong>&& p.DeclaringType != typeof</strong><strong>&&</strong><strong>&& p.PropertyType.GetGenericTypeDefinition() == typeof(DbSet<></strong><strong>))</strong>
.OrderBy(p => p.Name)
.Select(
p => new DbSetProperty(
p.Name,
p.PropertyType.GenericTypeArguments.Single(),
p.SetMethod == null ? null : factory.Create(p)))
.ToArray();
}
}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。
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
=> <strong><em>(c, name) => new InternalDbSet<TEntity></em></strong><strong><em>(c, name)</em></strong>;
}所以，默认创建的 DbSet 实例其实是 InternalDbSet 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。
public virtual EntityFrameworkServicesBuilder TryAddCoreServices()
{
TryAdd<<strong>IDbSetFinder, DbSetFinder</strong>>();
TryAdd<<strong>IDbSetInitializer, DbSetInitializer</strong>>();
TryAdd<<strong>IDbSetSource, DbSetSource</strong>>();
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();
ServiceCollectionMap.TryAddSingleton<IRegisteredServices>(
new RegisteredServices(ServiceCollectionMap.ServiceCollection.Select(s => s.ServiceType)));
return this;
}
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。
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