.NET 6新特性试用 | 无需配置开发人员异常页

前言

在.NET 6之前，我们需要在“Startup.cs”文件中手工配置开发人员异常页：

if (env.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
    app.UseSwagger();
    app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "WebApplication5 v1"));
}

如果当前环境变量​​ASPNETCORE_ENVIRONMENT​​的值是“Development”，将显示未经处理的请求异常的详细信息：

在.NET 6中，我们发现，无需显式调用​​UseDeveloperExceptionPage​​，开发人员异常页同样起作用。

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

​​为什么呢？​​

原理探究

在https://github.com/dotnet/aspnetcore/blob/main/src/DefaultBuilder/src/WebApplicationBuilder.cs中，我们找到如下代码：

private void ConfigureApplication(WebHostBuilderContext context, IApplicationBuilder app)
{
    ......

    if (context.HostingEnvironment.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    ......
}

internal WebApplicationBuilder(WebApplicationOptions options, Action<IHostBuilder>? configureDefaults = null)
{
    ......

    _bootstrapHostBuilder.ConfigureWebHostDefaults(webHostBuilder =>
    {
            // Runs inline.
            webHostBuilder.Configure(ConfigureApplication);

            // Attempt to set the application name from options
            options.ApplyApplicationName(webHostBuilder);
    });

    ......
}

原来是在构造函数进行了默认处理，也就是在API实现的第一句代码中执行的；

var builder = WebApplication.CreateBuilder(args);

// Microsoft.AspNetCore.Builder.WebApplication
public static WebApplicationBuilder CreateBuilder(string[] args)
{
 return new WebApplicationBuilder(new WebApplicationOptions
 {
  Args = args
 });
}

​​那.NET 6还帮我们默认做了哪些事呢？​​

其他默认操作

继续查看​​ConfigureApplication​​实现，可以发现.NET 6还进行了其他默认处理，比如：

• UseRouting

// If this is set, someone called UseRouting() when a global route builder was already set
if (!_builtApplication.Properties.TryGetValue(EndpointRouteBuilderKey, out var localRouteBuilder))
{
    app.UseRouting();
}
else
{
    // UseEndpoints will be looking for the RouteBuilder so make sure it's set
    app.Properties[EndpointRouteBuilderKey] = localRouteBuilder;
}

而在.NET 5中必须手工配置，否则报错：

• UseEndpoints

if (_builtApplication.DataSources.Count > 0)
{
    // We don't know if user code called UseEndpoints(), so we will call it just in case, UseEndpoints() will ignore duplicate DataSources
    app.UseEndpoints(_ => { });
}

而在.NET 5中必须在​​UseEndpoints​​​方法中才能执行​​MapXXX方法​​：

app.UseEndpoints(endpoints =>
{
    endpoints.MapGet("/", async context =>
    {
        await context.Response.WriteAsync("Hello My IO!");
    });
});

结论

正是因为.NET 6内部帮我们做了大量工作，才使得实现最小Web API成为可能：

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/", () => "Hello World!");

app.Run();

减少初始代码量，这对新手入门无疑是更友好的！

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