.NET 通过源码深究依赖注入原理

public IWebHost Build()
{ 
    var hostingServices = BuildCommonServices(out var hostingStartupErrors);// 构建WebHost通用服务

    var hostingServiceProvider = GetProviderFromFactory(hostingServices);
    
    // 获取ServiceProvider
    IServiceProvider GetProviderFromFactory(IServiceCollection collection)
    {
        // 构建IServiceProvider对象
        var provider = collection.BuildServiceProvider(); 
        // 获取服务
        var factory = provider.GetService<IServiceProviderFactory<IServiceCollection>>();

        // 是否使用默认的DefaultServiceProviderFactory类
        if (factory != null && !(factory is DefaultServiceProviderFactory))
        {
            using (provider)
            {
                return factory.CreateServiceProvider(factory.CreateBuilder(collection));
            }
        }
        
        return provider;
    }
}

public static ServiceProvider BuildServiceProvider(this IServiceCollection services, ServiceProviderOptions options)
{
    // 生成ServiceProvider对象
    return new ServiceProvider(services, options);
}

internal ServiceProvider(ICollection<ServiceDescriptor> serviceDescriptors, ServiceProviderOptions options)
{
    // 创建一个根级别的服务引擎作用域
    Root = new ServiceProviderEngineScope(this, isRootScope: true); 
    // 获取服务引擎用于解析依赖关系
    _engine = GetEngine();
    // 访问器,动态创建服务
    _createServiceAccessor = CreateServiceAccessor;
    // 缓存已经解析出来的服务实例(线程安全)
    _realizedServices = new ConcurrentDictionary<Type, Func<ServiceProviderEngineScope, object?>>();
    
    // CallSiteFactory用于创建和缓存服务的调用站点（ServiceCallSite）
    CallSiteFactory = new CallSiteFactory(serviceDescriptors);
    // 添加内置的服务
    CallSiteFactory.Add(typeof(IServiceProvider), new ServiceProviderCallSite());
    CallSiteFactory.Add(typeof(IServiceScopeFactory), new ConstantCallSite(typeof(IServiceScopeFactory), Root));
    CallSiteFactory.Add(typeof(IServiceProviderIsService), new ConstantCallSite(typeof(IServiceProviderIsService), CallSiteFactory));
    
    // ValidateScopes属性为true，表示需要验证服务范围
    if (options.ValidateScopes)
    {
        _callSiteValidator = new CallSiteValidator();
    }
    
    // ValidateOnBuild属性为true,需要检查所有服务是否能够成功创建
    if (options.ValidateOnBuild)
    {
        List<Exception>? exceptions = null;
        foreach (ServiceDescriptor serviceDescriptor in serviceDescriptors)
        {
             ValidateService(serviceDescriptor);
        }
    }
}

internal abstract class ServiceCallSite
{
    protected ServiceCallSite(ResultCache cache)
    {
        Cache = cache;
    }
    // 服务类型
    public abstract Type ServiceType { get; }
    // 实现类型
    public abstract Type ImplementationType { get; }
    // 调用链类型(Scope、Singleton、Factory、Constructor、CreateInstance等)
    public abstract CallSiteKind Kind { get; }
    public ResultCache Cache { get; }
    // 是否需要捕获可释放资源，类似IDisposable接口
    public bool CaptureDisposable =>
        ImplementationType == null ||
        typeof(IDisposable).IsAssignableFrom(ImplementationType) ||
        typeof(IAsyncDisposable).IsAssignableFrom(ImplementationType);
}

private void ValidateService(ServiceDescriptor descriptor)
{
  // 这个方法中出现了循环依赖和多构造函数
  ServiceCallSite callSite = CallSiteFactory.GetCallSite(descriptor, new CallSiteChain());
  if (callSite != null)
  {
      // 这个方法中进行依赖校验
      OnCreate(callSite);
  }
}

internal ServiceCallSite GetCallSite(Type serviceType, CallSiteChain callSiteChain) =>
    _callSiteCache.TryGetValue(new ServiceCacheKey(serviceType, DefaultSlot), out ServiceCallSite site) ? site :
    CreateCallSite(serviceType, callSiteChain);

private ServiceCallSite CreateCallSite(Type serviceType, CallSiteChain callSiteChain)
{
    var callsiteLock = _callSiteLocks.GetOrAdd(serviceType, static _ => new object());
    // 保证对CallSite缓存的线程安全。由于多个服务之间可能存在依赖关系，因此需要确保同一时间只有一个服务的CallSite被创建和缓存
    lock (callsiteLock)
    {
        // 哦吼，出现了 检查是否存在循环依赖关系，以避免产生无限递归调用
        callSiteChain.CheckCircularDependency(serviceType);
        // 依次尝试创建精确类型、开放泛型类型和IEnumerable类型的CallSite对象，并返回第一个成功创建的对象
        ServiceCallSite callSite = TryCreateExact(serviceType, callSiteChain) ??
                                   TryCreateOpenGeneric(serviceType, callSiteChain) ??
                                   TryCreateEnumerable(serviceType, callSiteChain);

        return callSite;
    }
}

public CallSiteChain()
{
    _callSiteChain = new Dictionary<Type, ChainItemInfo>();
}

public void CheckCircularDependency(Type serviceType)
{
    if (_callSiteChain.ContainsKey(serviceType))
    {
        throw new InvalidOperationException(CreateCircularDependencyExceptionMessage(serviceType));
    }
}

private ServiceCallSite CreateConstructorCallSite(
        ResultCache lifetime,
        Type serviceType,
        [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicConstructors)] Type implementationType,
        CallSiteChain callSiteChain)
    {
        try
        {
            // 将当前服务加入callSiteChain，以便后续的依赖解析过程中使用
            callSiteChain.Add(serviceType, implementationType);

            // 获取所有构造函数
            ConstructorInfo[] constructors = implementationType.GetConstructors();

            ServiceCallSite[] parameterCallSites = null;
            
            // 0个构造函数
            if (constructors.Length == 0)
            {
                throw new InvalidOperationException(SR.Format(SR.NoConstructorMatch, implementationType));
            }
            // 1个构造函数
            else if (constructors.Length == 1)
            {
                ConstructorInfo constructor = constructors[0];
                ParameterInfo[] parameters = constructor.GetParameters();
                if (parameters.Length == 0)
                {
                    return new ConstructorCallSite(lifetime, serviceType, constructor);
                }

                parameterCallSites = CreateArgumentCallSites(
                    implementationType,
                    callSiteChain,
                    parameters,
                    throwIfCallSiteNotFound: true);

                return new ConstructorCallSite(lifetime, serviceType, constructor, parameterCallSites);
            }
            
            // 多个构造函数如何选择，终于等到你，还好我没放弃0.0
            Array.Sort(constructors,
                (a, b) => b.GetParameters().Length.CompareTo(a.GetParameters().Length));

            ConstructorInfo bestConstructor = null;
            HashSet<Type> bestConstructorParameterTypes = null;
            for (int i = 0; i < constructors.Length; i++)
            {
                ParameterInfo[] parameters = constructors[i].GetParameters();

                ServiceCallSite[] currentParameterCallSites = CreateArgumentCallSites(
                    implementationType,
                    callSiteChain,
                    parameters,
                    throwIfCallSiteNotFound: false);

                if (currentParameterCallSites != null)
                {
                    if (bestConstructor == null)
                    {
                        bestConstructor = constructors[i];
                        parameterCallSites = currentParameterCallSites;
                    }
                    else
                    {
                        // Since we're visiting constructors in decreasing order of number of parameters,
                        // we'll only see ambiguities or supersets once we've seen a 'bestConstructor'.
                        //由于我们以参数数量递减的顺序访问构造函数，

                        //只有在看到“最佳构造函数”后，我们才会看到歧义或超集。
                        if (bestConstructorParameterTypes == null)
                        {
                            bestConstructorParameterTypes = new HashSet<Type>();
                            foreach (ParameterInfo p in bestConstructor.GetParameters())
                            {
                                bestConstructorParameterTypes.Add(p.ParameterType);
                            }
                        }

                        foreach (ParameterInfo p in parameters)
                        {
                            if (!bestConstructorParameterTypes.Contains(p.ParameterType))
                            {
                                // Ambiguous match exception
                                throw new InvalidOperationException(string.Join(
                                    Environment.NewLine,
                                    SR.Format(SR.AmbiguousConstructorException, implementationType),
                                    bestConstructor,
                                    constructors[i]));
                            }
                        }
                    }
                }
            }

            if (bestConstructor == null)
            {
                throw new InvalidOperationException(
                    SR.Format(SR.UnableToActivateTypeException, implementationType));
            }
            else
            {
                Debug.Assert(parameterCallSites != null);
                return new ConstructorCallSite(lifetime, serviceType, bestConstructor, parameterCallSites);
            }
        }
        finally
        {
            callSiteChain.Remove(serviceType);
        }
    }

private void OnCreate(ServiceCallSite callSite)
{
    _callSiteValidator?.ValidateCallSite(callSite);
}

public void ValidateCallSite(ServiceCallSite callSite)
{
    Type scoped = VisitCallSite(callSite, default);
    if (scoped != null)
    {
        _scopedServices[callSite.ServiceType] = scoped;
    }
}

protected override Type VisitScopeCache(ServiceCallSite scopedCallSite, CallSiteValidatorState state)
{
    // We are fine with having ServiceScopeService requested by singletons
    if (scopedCallSite.ServiceType == typeof(IServiceScopeFactory))
    {
        return null;
    }
    // ScopedInSingletonException异常！
    if (state.Singleton != null)
    {
        throw new InvalidOperationException(SR.Format(SR.ScopedInSingletonException,
            scopedCallSite.ServiceType,
            state.Singleton.ServiceType,
            nameof(ServiceLifetime.Scoped).ToLowerInvariant(),
            nameof(ServiceLifetime.Singleton).ToLowerInvariant()
            ));
    }

    VisitCallSiteMain(scopedCallSite, state);
    return scopedCallSite.ServiceType;
}

internal object GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
{
    // 使用了ConcurrentDictionary来缓存已经解析的服务
    Func<ServiceProviderEngineScope, object> realizedService = _realizedServices.GetOrAdd(serviceType, _createServiceAccessor);

    OnResolve(serviceType, serviceProviderEngineScope);
    
    // 服务的实际实现
    var result = realizedService.Invoke(serviceProviderEngineScope);

    return result;
}

private Func<ServiceProviderEngineScope, object> CreateServiceAccessor(Type serviceType)
{
    // 取给定服务类型的CallSite对象
    ServiceCallSite callSite = CallSiteFactory.GetCallSite(serviceType, new CallSiteChain());

    if (callSite != null)
    {
        OnCreate(callSite);

        // 服务具有Singleton生命周期，可以优化处理，避免每次获取服务实例时都需要重新创建
        if (callSite.Cache.Location == CallSiteResultCacheLocation.Root)
        {
            object value = CallSiteRuntimeResolver.Instance.Resolve(callSite, Root);
            return scope => value;
        }
        // 服务具有Transient或Scoped生命周期，需要创建并返回一个新的服务实例访问器
        return _engine.RealizeService(callSite);
    }

    return _ => null;
}

public class SingletonService : ISingletonService
{
    private readonly IScopedService _scopedService;

    public SingletonService(IServiceScopeFactory serviceScopeFactory)
    {
        _scopedService = serviceScopeFactory.CreateScope().ServiceProvider.GetRequiredService<IScopedService>();
    }        
}

public class SingletonService : ISingletonService
{
    private readonly IScopedService _scopedService;

    public SingletonService(IServiceProvider serviceProvider)
    {
        _scopedService = serviceProvider.CreateScope().ServiceProvider.GetRequiredService<IScopedService>();
    }        
}

class A {
  final B b;
  A(B b){
    this.b = b;
  }
}

class B {
  final A a;
  B(A a){
    this.a = a;
  }
}

+---------+      +---------+
|    A    |<-----|  B      |
|         |      |  |  +-+ |
|         |      |  +->|C| |
|         |------+---->| | |
|         |      |     +-+ |
+---------+      +---------+

                         +---------+
+---------+              |    B    |
|    A    |<-------------|         |
|         |              |         |
|         |    +---+     |         |
|         |--->| C |<----|         |
|         |    +---+     +---------+
+---------+

class C {
  C(){
  }
}

class A {
  final C c;
  A(C c){
    this.c = c;
  }
}

class B {
  final A a;
  final C c;
  B(A a, C c){
    this.a = a;
    this.c = c;
  }
}

class C : IC
{
    private readonly IServiceProvider _services;

    public C(IServiceProvider services)
    {
        _services = services;
    }

    public void Bar()
    {
        ...
        var a = _services.GetRequiredService<IA>();
        a.Foo();
        ...
    }
}

public static IServiceCollection AddLazyResolution(this IServiceCollection services)
{
    return services.AddTransient(
        typeof(Lazy<>),
        typeof(LazilyResolved<>));
}

private class LazilyResolved<T> : Lazy<T>
{
    public LazilyResolved(IServiceProvider serviceProvider)
        : base(serviceProvider.GetRequiredService<T>)
    {
    }
}

class C : IC
{
    private readonly Lazy<IA> _a;

    public C(Lazy<IA> a)
    {
        _a = a;
    }

    public void Bar()
    {
        ...
        _a.Value.Foo();
        ...
    }
}