netcore3.0 IServiceCollection 依赖注入系统（三）

上篇重点讲了netcore依赖注入系统是如何解析服务的

再回顾一下

ServiceProvider类继承IServiceProvider接口，实现IServiceProvider接口的GetService方法

ServiceProvider又是根据ServiceProviderOptions类的Mode属性创建对应的IServiceProviderEngine实例（真正的解析服务）

IServiceProviderEngine的实现有DynamicServiceProviderEngine、ExpressionsServiceProviderEngine、ILEmitServiceProviderEngine、RuntimeServiceProviderEngine

上面的实现类都继承自ServiceProviderEngine抽象类

ServiceProviderEngine抽象类的CallSiteFactory对我们注册的服务IEnumerable<ServiceDescriptor>进行转换，转换成对应的ServiceCallSite对象。

ServiceProviderEngine抽象类的属性RuntimeResolver（CallSiteRuntimeResolver类型）也能解析服务。

DynamicServiceProviderEngine、ExpressionsServiceProviderEngine、ILEmitServiceProviderEngine、RuntimeServiceProviderEngine实现了ServiceProviderEngine抽象类的RealizeService方法解析服务

总结：ServiceProvider是服务的核心。

IServiceCollection的扩展方法：

/// <summary>
    /// Extension methods for building a <see cref="ServiceProvider"/> from an <see cref="IServiceCollection"/>.
    /// </summary>
    public static class ServiceCollectionContainerBuilderExtensions
    {
        /// <summary>
        /// Creates a <see cref="ServiceProvider"/> containing services from the provided <see cref="IServiceCollection"/>.
        /// </summary>
        /// <param name="services">The <see cref="IServiceCollection"/> containing service descriptors.</param>
        /// <returns>The <see cref="ServiceProvider"/>.</returns>

        public static ServiceProvider BuildServiceProvider(this IServiceCollection services)
        {
            return BuildServiceProvider(services, ServiceProviderOptions.Default);
        }

        /// <summary>
        /// Creates a <see cref="ServiceProvider"/> containing services from the provided <see cref="IServiceCollection"/>
        /// optionally enabling scope validation.
        /// </summary>
        /// <param name="services">The <see cref="IServiceCollection"/> containing service descriptors.</param>
        /// <param name="validateScopes">
        /// <c>true</c> to perform check verifying that scoped services never gets resolved from root provider; otherwise <c>false</c>.
        /// </param>
        /// <returns>The <see cref="ServiceProvider"/>.</returns>
        public static ServiceProvider BuildServiceProvider(this IServiceCollection services, bool validateScopes)
        {
            return services.BuildServiceProvider(new ServiceProviderOptions { ValidateScopes = validateScopes });
        }

        /// <summary>
        /// Creates a <see cref="ServiceProvider"/> containing services from the provided <see cref="IServiceCollection"/>
        /// optionally enabling scope validation.
        /// </summary>
        /// <param name="services">The <see cref="IServiceCollection"/> containing service descriptors.</param>
        /// <param name="options">
        /// Configures various service provider behaviors.
        /// </param>
        /// <returns>The <see cref="ServiceProvider"/>.</returns>
        public static ServiceProvider BuildServiceProvider(this IServiceCollection services, ServiceProviderOptions options)
        {
            if (services == null)
            {
                throw new ArgumentNullException(nameof(services));
            }

            if (options == null)
            {
                throw new ArgumentNullException(nameof(options));
            }

            return new ServiceProvider(services, options);
        }
    }

BuildServiceProvider方法根据注册的服务创建ServiceProvider对象，此时我们可以从ServiceProvider获取到我们注册的服务了

我们看下简单的使用场景

public interface IUserService
    {
        string GetName();
    }

    public class UserService : IUserService
    {
        public string GetName()
        {
            return "test";
        }
    }
class Program
    {
        static void Main(string[] args)
        {

            var services = new ServiceCollection();
            services.AddTransient<IUserService, UserService>();
            var provider = services.BuildServiceProvider();
            var userService = provider.GetService<IUserService>();
            var result = userService.GetName();
            Console.WriteLine(result);


            Console.Read();

        }
    }