[WebToolsE2E] Blazor WebAssembly Standalone App Fails to Launch in VS Code with CoreCLR Startup Failure and Missing hostpolicy.dll

[joniel-bolocon]

REGRESSION INFO: In May Prerelease 10 (C# Dev Kit 3.31.199 and C# 2.149.34) #3366 will be encountered

INSTALL STEPS

• OS: windows x64
• Install the SDK 10.0.401
• Install VS Code insiders - 1.137.0
• V3 Sept prerelease C#: [2.152.10]
• V3 Sept prerelease C# Dev Kit: [3.33.195]

REPRO STEPS

1. Make sure enable "[Experimental] Enables C# Hot Reload while debugging"
2. Open VS Code > Create .NET Project > Blazor WebAssembly Standalone App
3. Build project successfully. > F5 to run the project

Note:

1. In SDK 10.0.400, the first project will run successfully, however the succeeding WASM project will encounter the issue

ACTUAL
Project will not run and will encounter the following warnings:

Run via "Start new Instance"

Logs:

The target process exited without raising a CoreCLR started event. Ensure that the target process is configured to use .NET Core. This may be expected if the target process did not run on .NET Core.

Run via "Start without Debugging"

Logs:

A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'C:\Users\v-jbolocon\BlazorApp3\BlazorApp3\bin\Debug\net10.0\'.
Failed to run as a self-contained app.
  - The application was run as a self-contained app because 'C:\Users\v-jbolocon\BlazorApp3\BlazorApp3\bin\Debug\net10.0\BlazorApp3.runtimeconfig.json' did not specify a framework.
  - If this should be a framework-dependent app, specify the appropriate framework in 'C:\Users\v-jbolocon\BlazorApp3\BlazorApp3\bin\Debug\net10.0\BlazorApp3.runtimeconfig.json'.

EXPECTED
Blazor WebAssembly Standalone App templates will consistently run successfully

[LittleLittleCloud]

joniel-bolocon I am not able to reproduce this on my environment, it might be flakiness. could you give it another try by reopen VSCode and rebuild project and relaunch,,

[joniel-bolocon]

I check it again with the latest C# Dev Kit internal prerelease [3.33.194] and C# [2.152.5], but this time I was able to repro the issue in the first run, I tried to reopen VS Code and rebuild the existing project and also tried creating a new project but the issue still persists.

I run via "Start new Instance" and "Start without Debugging"

[cyimperial]

Verified this issue repro in CDK (main): 9.9.512-g6b007c and C# (main): 2.152.5

[JakeRadMSFT]

Finding: orthogonal Web Tools/Blazor launch issue.

The reported Blazor WebAssembly launch failure includes no CoreCLR event, a missing hostpolicy.dll, and a framework-less runtimeconfig. It was verified on C# Dev Kit main 9.9.512-g6b007c with C# 2.152.5; later reports reproduced it on first run even after reopening, rebuilding, or creating a new project.

This is a Web Tools/Blazor launch issue, separate from C# Doctor SDK selection. It needs focused standalone Blazor WebAssembly debug and no-debug launch coverage around runtimeconfig generation.

vs-green PR !779247 does not fix this issue.

Cowritten with Copilot

[joniel-bolocon]

The issue was still repro, using the latest prerelease of C# [2.152.10] and C# Dev Kit [3.33.195]

Updated the bug content and title accordingly, as this issue now reproes in the first try

[LittleLittleCloud]

Would you share the C:\Users\v-jbolocon\BlazorApp3\BlazorApp3\bin\Debug\net10.0\BlazorApp3.runtimeconfig.json and your .csproj? Here's what I got

<Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <OverrideHtmlAssetPlaceholders>true</OverrideHtmlAssetPlaceholders>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly" Version="10.0.11" />
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.DevServer" Version="10.0.11" PrivateAssets="all" />
  </ItemGroup>

</Project>

{
  "runtimeOptions": {
    "tfm": "net10.0",
    "includedFrameworks": [
      {
        "name": "Microsoft.NETCore.App",
        "version": "10.0.10"
      }
    ],
    "wasmHostProperties": {
      "perHostConfig": [
        {
          "name": "browser",
          "host": "browser"
        }
      ]
    },
    "configProperties": {
      "Microsoft.AspNetCore.Components.Routing.RegexConstraintSupport": false,
      "Microsoft.Extensions.DependencyInjection.VerifyOpenGenericServiceTrimmability": true,
      "System.ComponentModel.DefaultValueAttribute.IsSupported": false,
      "System.ComponentModel.Design.IDesignerHost.IsSupported": false,
      "System.ComponentModel.TypeConverter.EnableUnsafeBinaryFormatterInDesigntimeLicenseContextSerialization": false,
      "System.ComponentModel.TypeDescriptor.IsComObjectDescriptorSupported": false,
      "System.Data.DataSet.XmlSerializationIsSupported": false,
      "System.Diagnostics.Metrics.Meter.IsSupported": false,
      "System.Diagnostics.Tracing.EventSource.IsSupported": false,
      "System.GC.Server": true,
      "System.Globalization.Invariant": false,
      "System.TimeZoneInfo.Invariant": false,
      "System.Linq.Enumerable.IsSizeOptimized": true,
      "System.Net.Http.EnableActivityPropagation": false,
      "System.Net.Http.WasmEnableStreamingResponse": true,
      "System.Net.SocketsHttpHandler.Http3Support": false,
      "System.Reflection.Metadata.MetadataUpdater.IsSupported": false,
      "System.Resources.ResourceManager.AllowCustomResourceTypes": false,
      "System.Resources.UseSystemResourceKeys": true,
      "System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeSupported": true,
      "System.Runtime.InteropServices.BuiltInComInterop.IsSupported": false,
      "System.Runtime.InteropServices.EnableConsumingManagedCodeFromNativeHosting": false,
      "System.Runtime.InteropServices.EnableCppCLIHostActivation": false,
      "System.Runtime.InteropServices.Marshalling.EnableGeneratedComInterfaceComImportInterop": false,
      "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization": false,
      "System.StartupHookProvider.IsSupported": false,
      "System.Text.Encoding.EnableUnsafeUTF7Encoding": false,
      "System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault": true,
      "System.Threading.Thread.EnableAutoreleasePool": false,
      "Microsoft.AspNetCore.Components.Endpoints.NavigationManager.DisableThrowNavigationException": false
    }
  }
}

[joniel-bolocon]

Hi Xiaoyun Zhang (@LittleLittleCloud), here are the following contents asked for the BlazorApp3 project:

BlazorApp3.csproj

<Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <OverrideHtmlAssetPlaceholders>true</OverrideHtmlAssetPlaceholders>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly" Version="10.0.12" />
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.DevServer" Version="10.0.12" PrivateAssets="all" />
  </ItemGroup>

</Project>

C:\Users\v-jbolocon\BlazorApp3\BlazorApp3\bin\Debug\net10.0\BlazorApp3.runtimeconfig.json

{
  "runtimeOptions": {
    "tfm": "net10.0",
    "includedFrameworks": [
      {
        "name": "Microsoft.NETCore.App",
        "version": "10.0.12"
      }
    ],
    "wasmHostProperties": {
      "perHostConfig": [
        {
          "name": "browser",
          "host": "browser"
        }
      ]
    },
    "configProperties": {
      "Microsoft.AspNetCore.Components.Routing.RegexConstraintSupport": false,
      "Microsoft.Extensions.DependencyInjection.VerifyOpenGenericServiceTrimmability": true,
      "System.ComponentModel.DefaultValueAttribute.IsSupported": false,
      "System.ComponentModel.Design.IDesignerHost.IsSupported": false,
      "System.ComponentModel.TypeConverter.EnableUnsafeBinaryFormatterInDesigntimeLicenseContextSerialization": false,
      "System.ComponentModel.TypeDescriptor.IsComObjectDescriptorSupported": false,
      "System.Data.DataSet.XmlSerializationIsSupported": false,
      "System.Diagnostics.Metrics.Meter.IsSupported": false,
      "System.Diagnostics.Tracing.EventSource.IsSupported": false,
      "System.GC.Server": true,
      "System.Globalization.Invariant": false,
      "System.TimeZoneInfo.Invariant": false,
      "System.Linq.Enumerable.IsSizeOptimized": true,
      "System.Net.Http.EnableActivityPropagation": false,
      "System.Net.Http.WasmEnableStreamingResponse": true,
      "System.Net.SocketsHttpHandler.Http3Support": false,
      "System.Reflection.Metadata.MetadataUpdater.IsSupported": false,
      "System.Resources.ResourceManager.AllowCustomResourceTypes": false,
      "System.Resources.UseSystemResourceKeys": true,
      "System.Runtime.CompilerServices.RuntimeFeature.IsDynamicCodeSupported": true,
      "System.Runtime.InteropServices.BuiltInComInterop.IsSupported": false,
      "System.Runtime.InteropServices.EnableConsumingManagedCodeFromNativeHosting": false,
      "System.Runtime.InteropServices.EnableCppCLIHostActivation": false,
      "System.Runtime.InteropServices.Marshalling.EnableGeneratedComInterfaceComImportInterop": false,
      "System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization": false,
      "System.StartupHookProvider.IsSupported": false,
      "System.Text.Encoding.EnableUnsafeUTF7Encoding": false,
      "System.Text.Json.JsonSerializer.IsReflectionEnabledByDefault": true,
      "System.Threading.Thread.EnableAutoreleasePool": false,
      "Microsoft.AspNetCore.Components.Endpoints.NavigationManager.DisableThrowNavigationException": false
    }
  }
}

Added a note in the bug, when using SDK 10.0.400, the first WASM Project run successfully without errors, however the succeeding projects will encounter the issue

[v-korosales]

Verified with SDK 10.0.401 + C# DK V3 Stable 3.40.202 + C# 2.151.28 (public prerelease)

Observed the same behavior noted previously:
The first Blazor WebAssembly Standalone project runs successfully and continues to work across multiple debug sessions. However, succeeding newly created Blazor WebAssembly Standalone projects repro with the CoreCLR startup failure / missing hostpolicy.dll error.

Workaround: Close and reopen VS Code, then debug the affected project.