升级至.NET 8.0.200后Blazor WebAssembly无法定位App.razor

将Visual Studio升级至17.9.0版本、项目升级至.NET 8.0.200后，Blazor WebAssembly项目无法运行。在Program.cs的代码builder.RootComponents.Add<App>("#app");处，出现错误：

error CS0246: The type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?)

尽管App.razor与Program.cs处于同一命名空间中。尝试添加app.razor.cs后项目可编译，但加载完成后显示空白页面，且无任何错误提示。

App.razor 代码 #

@using MyApp.Client;
<CascadingAuthenticationState>
    <Router AppAssembly="@typeof(App).Assembly">
        <Found Context="routeData">
            <AuthorizeRouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
            <FocusOnNavigate RouteData="@routeData" Selector="h1" />
        </Found>
        <NotFound>
            <PageTitle>Not found</PageTitle>
            <LayoutView Layout="@typeof(MainLayout)">
                <p role="alert">Sorry, there's nothing at this address.</p>
            </LayoutView>
        </NotFound>
    </Router>
</CascadingAuthenticationState>

Program.cs 代码 #

using Blazored.Toast;
using Microsoft.AspNetCore.Components.Authorization;
using Microsoft.AspNetCore.Components.Web;
using Microsoft.AspNetCore.Components.WebAssembly.Hosting;
using Polly;
using MyApp.Client.Helper;
using Polly.Contrib.WaitAndRetry;
using MyApp.Shared.Models;
using Microsoft.JSInterop;
using System.Globalization;

namespace MyApp.Client
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            var builder = WebAssemblyHostBuilder.CreateDefault(args);
            builder.RootComponents.Add<App>("#app");
            builder.RootComponents.Add<HeadOutlet>("head::after");
            builder.Services.AddBlazoredToast();
            builder.Services.AddSingleton<EventService>();
            builder.Services.AddScoped<AuthenticationDataMemoryStorage>();
            builder.Services.AddScoped<BlazorSchoolAuthenticationStateProvider>();
            builder.Services.AddScoped<ClipboardService>();
            builder.Services.AddScoped<AuthenticationStateProvider>(sp => sp.GetRequiredService<BlazorSchoolAuthenticationStateProvider>());
            builder.Services.AddHttpClient("HttpClient",
            httpClient =>
            {
                httpClient.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress);
            })
            .AddTransientHttpErrorPolicy(policyBuilder => policyBuilder.WaitAndRetryAsync(Backoff.DecorrelatedJitterBackoffV2(TimeSpan.FromSeconds(1), 5)));

            builder.Services.AddScoped<IUserService, UserService>();
            builder.Services.AddScoped<IHttpService, HttpService>();
            builder.Services.AddScoped<ILocalStorageService, LocalStorageService>();
            builder.Services.Configure<AppSettings>(builder.Configuration.GetSection("AppSettings"));
            builder.Services.AddBlazorDialog();
            builder.Services.AddAuthorizationCore();
            builder.Services.AddLocalization();
            var host = builder.Build();
            CultureInfo culture;
            var js = host.Services.GetRequiredService<IJSRuntime>();
            var result = await js.InvokeAsync<string>("blazorCulture.get");

            if (result != null)
            {
                culture = new CultureInfo(result);
            }
            else
            {
                culture = new CultureInfo("en-US");
                await js.InvokeVoidAsync("blazorCulture.set", "en-US");
            }

            CultureInfo.DefaultThreadCurrentCulture = culture;
            CultureInfo.DefaultThreadCurrentUICulture = culture;
            await host.RunAsync();
        }
    }
}

解决方法 #

1. 修正App.razor的命名空间声明 #

在App.razor顶部添加@namespace MyApp.Client（可保留原@using MyApp.Client）。@namespace是声明组件自身所属的命名空间，仅用@using无法让编译器识别组件属于当前命名空间：

@namespace MyApp.Client
<CascadingAuthenticationState>
    <!-- 原有组件内容 -->
</CascadingAuthenticationState>

2. 清理编译缓存并重建项目 #

• 手动删除项目目录下的bin和obj文件夹
• 执行dotnet clean命令清理项目
• 重新执行dotnet build或在Visual Studio中点击"生成解决方案"

3. 检查app.razor.cs的代码隐藏类（若保留） #

如果添加了app.razor.cs，确保它是正确的部分类实现，且命名空间与Program.cs一致：

namespace MyApp.Client;
public partial class App : ComponentBase
{
    // 仅添加必要的组件逻辑，避免冲突
}

4. 排查空白页问题 #

• 打开浏览器开发者工具（F12），切换到"控制台"标签页查看是否有JavaScript错误
• 在Program.cs中添加日志输出，帮助定位渲染问题：

  var builder = WebAssemblyHostBuilder.CreateDefault(args);
  builder.Logging.SetMinimumLevel(LogLevel.Debug); // 添加这行
  

5. 验证项目文件配置 #

检查项目的.csproj文件，确保TargetFramework和Blazor SDK配置正确：

<Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
  <!-- 其他依赖项配置 -->
</Project>

内容的提问来源于stack exchange，提问作者Taufiq Abdur Rahman