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前一篇提到webhost.createdefaultbuilder(args)方法创建了webhostbuilder实例，webhostbuilder实例有三个主要功能 1、构建了iconfiguration实例和基础环境配置，2、构建了iservicecollection服务，也就是依赖注入的容器，3、创建了webhost实例，这个webhost就是我们的接收请求的第一个管道，其中暴露出来的主要方法build，请看源代码：
我要评论 1 public iwebhost build()
2 {
3 if (!_webhostbuilt)
4 {
5 _webhostbuilt = true;
6 aggregateexception hostingstartuperrors;
7 iservicecollection servicecollection = buildcommonservices(out hostingstartuperrors);
8 iservicecollection servicecollection2 = servicecollection.clone();
9 iserviceprovider hostingserviceprovider = _003cbuild_003eg__getproviderfromfactory_007c13_0(servicecollection);
10 if (!_options.suppressstatusmessages)
11 {
12 if (environment.getenvironmentvariable("hosting:environment") != null)
13 {
14 console.writeline("the environment variable 'hosting:environment' is obsolete and has been replaced with 'aspnetcore_environment'");
15 }
16 if (environment.getenvironmentvariable("aspnet_env") != null)
17 {
18 console.writeline("the environment variable 'aspnet_env' is obsolete and has been replaced with 'aspnetcore_environment'");
19 }
20 if (environment.getenvironmentvariable("aspnetcore_server.urls") != null)
21 {
22 console.writeline("the environment variable 'aspnetcore_server.urls' is obsolete and has been replaced with 'aspnetcore_urls'");
23 }
24 }
25 addapplicationservices(servicecollection2, hostingserviceprovider);
26 webhost webhost = new webhost(servicecollection2, hostingserviceprovider, _options, _config, hostingstartuperrors);
27 try
28 {
29 webhost.initialize();
30 ilogger<webhost> requiredservice = webhost.services.getrequiredservice<ilogger<webhost>>();
31 foreach (igrouping<string, string> item in from g in _options.getfinalhostingstartupassemblies().groupby((string a) => a, stringcomparer.ordinalignorecase)
32 where g.count() > 1
33 select g)
34 {
35 requiredservice.logwarning($"the assembly {item} was specified multiple times. hosting startup assemblies should only be specified once.");
36 }
37 return webhost;
38 }
39 catch
40 {
41 webhost.dispose();
42 throw;
43 }
44 }
45 throw new invalidoperationexception(resources.webhostbuilder_singleinstance);
46 }
这个方法中有几个笔记重要的部分，下面我给标记一下
调用buildcommonservices私有方法主要是注入一些程序所需的基础组件，请看源代码：
1 private iservicecollection buildcommonservices(out aggregateexception hostingstartuperrors)
2 {
3 hostingstartuperrors = null;
4 _options = new webhostoptions(_config, assembly.getentryassembly()?.getname().name);
5 if (!_options.preventhostingstartup)
6 {
7 list<exception> list = new list<exception>();
8 foreach (string item in _options.getfinalhostingstartupassemblies().distinct(stringcomparer.ordinalignorecase))
9 {
10 try
11 {
12 foreach (hostingstartupattribute customattribute in assembly.load(new assemblyname(item)).getcustomattributes<hostingstartupattribute>())
13 {
14 ((ihostingstartup)activator.createinstance(customattribute.hostingstartuptype)).configure(this);
15 }
16 }
17 catch (exception innerexception)
18 {
19 list.add(new invalidoperationexception("startup assembly " + item + " failed to execute. see the inner exception for more details.", innerexception));
20 }
21 }
22 if (list.count > 0)
23 {
24 hostingstartuperrors = new aggregateexception(list);
25 }
26 }
27 string contentrootpath = resolvecontentrootpath(_options.contentrootpath, appcontext.basedirectory);
28 _hostingenvironment.initialize(contentrootpath, _options);
29 _context.hostingenvironment = _hostingenvironment;
30 servicecollection servicecollection = new servicecollection();
31 servicecollection.addsingleton(_options);
32 ((iservicecollection)servicecollection).addsingleton((ihostingenvironment)_hostingenvironment);
33 ((iservicecollection)servicecollection).addsingleton((microsoft.extensions.hosting.ihostingenvironment)_hostingenvironment);
34 servicecollection.addsingleton(_context);
35 iconfigurationbuilder configurationbuilder = new configurationbuilder().setbasepath(_hostingenvironment.contentrootpath).addconfiguration(_config);
36 foreach (action<webhostbuildercontext, iconfigurationbuilder> configureappconfigurationbuilderdelegate in _configureappconfigurationbuilderdelegates)
37 {
38 configureappconfigurationbuilderdelegate(_context, configurationbuilder);
39 }
40 iconfigurationroot configurationroot = configurationbuilder.build();
41 ((iservicecollection)servicecollection).addsingleton((iconfiguration)configurationroot);
42 _context.configuration = configurationroot;
43 diagnosticlistener implementationinstance = new diagnosticlistener("microsoft.aspnetcore");
44 servicecollection.addsingleton(implementationinstance);
45 ((iservicecollection)servicecollection).addsingleton((diagnosticsource)implementationinstance);
46 servicecollection.addtransient<iapplicationbuilderfactory, applicationbuilderfactory>();
47 servicecollection.addtransient<ihttpcontextfactory, httpcontextfactory>();
48 servicecollection.addscoped<imiddlewarefactory, middlewarefactory>();
49 servicecollection.addoptions();
50 servicecollection.addlogging();
51 servicecollection.addtransient<istartupfilter, autorequestservicesstartupfilter>();
52 servicecollection.addtransient<iserviceproviderfactory<iservicecollection>, defaultserviceproviderfactory>();
53 servicecollection.addsingleton<objectpoolprovider, defaultobjectpoolprovider>();
54 if (!string.isnullorempty(_options.startupassembly))
55 {
56 try
57 {
58 type startuptype = startuploader.findstartuptype(_options.startupassembly, _hostingenvironment.environmentname);
59 if (introspectionextensions.gettypeinfo(typeof(istartup)).isassignablefrom(startuptype.gettypeinfo()))
60 {
61 servicecollection.addsingleton(typeof(istartup), startuptype);
62 }
63 else
64 {
65 servicecollection.addsingleton(typeof(istartup), delegate (iserviceprovider sp)
66 {
67 ihostingenvironment requiredservice = sp.getrequiredservice<ihostingenvironment>();
68 return new conventionbasedstartup(startuploader.loadmethods(sp, startuptype, requiredservice.environmentname));
69 });
70 }
71 }
72 catch (exception source)
73 {
74 exceptiondispatchinfo capture = exceptiondispatchinfo.capture(source);
75 ((iservicecollection)servicecollection).addsingleton((func<iserviceprovider, istartup>)delegate
76 {
77 capture.throw();
78 return null;
79 });
80 }
81 }
82 foreach (action<webhostbuildercontext, iservicecollection> configureservicesdelegate in _configureservicesdelegates)
83 {
84 configureservicesdelegate(_context, servicecollection);
85 }
86 return servicecollection;
87 }
这个方法中其中就包含了注入iconfiguration的代码和构建iservicecollection容器的代码，当然还有一些其他的主要中间件注入啊iapplicationbuilderfactory，包括httpcontextfactory等等都在这个方法里面被iservicecollection容器管理，具体请看下图
这里讲清楚之后，我们直接进入usestartup方法中，这个方法是注入了startup的实例，请看主要源代码
1 public static iwebhostbuilder usestartup(this iwebhostbuilder hostbuilder, type startuptype)
2 {
3 string name = startuptype.gettypeinfo().assembly.getname().name;
4 return hostbuilder.usesetting(webhostdefaults.applicationkey, name).configureservices(delegate (iservicecollection services)
5 {
6 if (introspectionextensions.gettypeinfo(typeof(istartup)).isassignablefrom(startuptype.gettypeinfo()))
7 {
8 services.addsingleton(typeof(istartup), startuptype);
9 }
10 else
11 {
12 services.addsingleton(typeof(istartup), delegate (iserviceprovider sp)
13 {
14 ihostingenvironment requiredservice = sp.getrequiredservice<ihostingenvironment>();
15 return new conventionbasedstartup(startuploader.loadmethods(sp, startuptype, requiredservice.environmentname));
16 });
17 }
18 });
19 }
上面只是被iservicecollection所管理 并没有真正起作用，真正调用的是main方法中的build().run()，这里的build就是上面那个iwebhostbuilder中的主要的构建方法，而run则是正式启动应用程序和调用startup中的configureservices方法
请看代码：
分享program类的过程大致就到这里了
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