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.