问题描述
当本地环境中安装.NET 6.0后，用指令 dotnet new web 或 dotnet new console 生成的项目，使用的都是新模板生成的Program.cs文件。里面去掉了namespace， class 以及main函数的定义。使得代码更简洁。
生成的 Program.cs 代码为：
var builder = WebApplication.CreateBuilder(args); var app = builder.Build(); app.MapGet("/", () => "Hello World!"); app.Run();
与示例代码中所使用的代码结构差距十分巨大(示例链接：https://docs.microsoft.com/en-us/azure/azure-web-pubsub/tutorial-subprotocol?tabs=csharp)：
using Azure.Messaging.WebPubSub; using Microsoft.AspNetCore.Builder; using Microsoft.AspNetCore.Hosting; using Microsoft.AspNetCore.Http; using Microsoft.Extensions.Azure; using Microsoft.Extensions.Configuration; using Microsoft.Extensions.DependencyInjection; using Microsoft.Extensions.Hosting; namespace logstream { public class Startup { public Startup(IConfiguration configuration) { Configuration = configuration; } public IConfiguration Configuration { get; } public void ConfigureServices(IServiceCollection services) { services.AddAzureClients(builder => { builder.AddWebPubSubServiceClient(Configuration["Azure:WebPubSub:ConnectionString"], "stream"); }); } public void Configure(IApplicationBuilder app, IWebHostEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.UseStaticFiles(); app.UseRouting(); app.UseEndpoints(endpoints => { endpoints.MapGet("/negotiate", async context => { var service = context.RequestServices.GetRequiredService<WebPubSubServiceClient>(); var response = new { url = service.GetClientAccessUri(roles: new string[] { "webpubsub.sendToGroup.stream", "webpubsub.joinLeaveGroup.stream" }).AbsoluteUri }; await context.Response.WriteAsJsonAsync(response); }); }); } } }
那么如何来定义 ConfigureServices， 如何来 Configure 中的代码呢？如何来解决 CS8803 : Top-level statements must precede namespace and type declarations
问题分析
这是代码由.NET 5.0 到 .NET 6.0的升级转换问题。
在.NET 6 之前，每一个应用程序都将应用的初始化代码拆分放在 Program.cs 和 Startup.cs 文件中。他们是两个独立的类，而在.NET 6.0中，为了简洁，为了最小化API，把两个类进行了合并。生成新的Program.cs中去掉了命名空间(Namespace, Class定义， Main函数)。使得在启动一个应用时，代码达到最少。 （PS:C# 编译器会自动生成Mian函数)
在查看官方对于5.0 变为 6.0的文档介绍：https://docs.microsoft.com/en-us/aspnet/core/migration/50-to-60?view=aspnetcore-6.0&tabs=visual-studio
- ConfigureServices 函数里面的 services.AddXXXXX()等都可以转换为 builder.Services.AddXXXXX()
- Configure 函数中的内容，可以直接写在 var app = builder.Build(); 代码之后。
所以，本文之前的代码，可以直接转换为：
using Azure.Messaging.WebPubSub; using Microsoft.AspNetCore.Builder; using Microsoft.AspNetCore.Hosting; using Microsoft.AspNetCore.Http; using Microsoft.Extensions.Azure; using Microsoft.Extensions.Configuration; using Microsoft.Extensions.DependencyInjection; using Microsoft.Extensions.Hosting; var ConnectionString = ""; var builder = WebApplication.CreateBuilder(args); builder.Services.AddAzureClients(builder => { builder.AddWebPubSubServiceClient(ConnectionString, "stream"); }); var app = builder.Build(); if (!app.Environment.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.UseStaticFiles(); app.UseRouting(); app.UseEndpoints(endpoints => { endpoints.MapGet("/negotiate", async context => { var service = context.RequestServices.GetRequiredService<WebPubSubServiceClient>(); var response = new { url = service.GetClientAccessUri(roles: new string[] { "webpubsub.sendToGroup.stream", "webpubsub.joinLeaveGroup.stream" }).AbsoluteUri }; await context.Response.WriteAsJsonAsync(response); }); }); app.MapGet("/", () => "Hello World!"); app.Run();
特别注意，在新模板下的代码，非常容易出现：CS8803 : Top-level statements must precede namespace and type declarations 错误。这是因为 .NET 6.0在隐藏了Main函数后，在编译代码时候，会自动补上Main函数，所以在Program.cs 的代码中，如果需要定义其他类，必须放在文件的末尾，不能在文件开头部分和中间。
当Class定义代码放在Program.cs开头部分
当Class定义代码放在Program.cs中间部分
当Class定义代码放在Program.cs结尾部分
更多关于Top-level statements的说明，请见：https://www.cnblogs.com/lulight/articles/16285885.html
参考资料
Tutorial: Publish and subscribe messages between WebSocket clients using subprotocol: https://docs.microsoft.com/en-us/azure/azure-web-pubsub/tutorial-subprotocol?tabs=csharp
Migrate from ASP.NET Core 5.0 to 6.0：https://docs.microsoft.com/en-us/aspnet/core/migration/50-to-60?view=aspnetcore-6.0&tabs=visual-studio
C# 9.0 - Introduction To Top-Level Statements ：https://www.c-sharpcorner.com/article/c-sharp-9-0-top-level-statement/#:~:text=Top%20Level%20statements%20should%20be%20first%20before%20any,Top-level%20Statements.%20That%E2%80%99s%20all%20for%20the%20Top-level%20statement.
当在复杂的环境中面临问题，格物之道需：浊而静之徐
