Content and BundleResource items are resolved using the defining project's location

[rolfbjarne]

Project file:

<Project Sdk="Microsoft.NET.Sdk.Razor">
  <PropertyGroup>
    <TargetFramework>net9.0-ios</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
  <ItemGroup>
    <SupportedPlatform Include="browser" />
  </ItemGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.Web" Version="9.0.0" />
  </ItemGroup>
</Project>

Building results in:

$ dotnet build /bl
Restore complete (3.6s)
    info NETSDK1057: You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  RazorClassLibrary failed with 4 error(s) (0.7s)
    wwwroot/background.png : error
      The path '../../../../../../../../../Users/rolf/test/bugs/RazorClassLibrary/wwwroot/background.png' would result in a file outside of the app bundle and cannot be used.

    wwwroot/exampleJsInterop.js : error
      The path '../../../../../../../../../Users/rolf/test/bugs/RazorClassLibrary/wwwroot/exampleJsInterop.js' would result in a file outside of the app bundle and cannot be used.

    Component1.razor : error
      The path '../../../../../../../../../Users/rolf/test/bugs/RazorClassLibrary/Component1.razor' would result in a file outside of the app bundle and cannot be used.

    _Imports.razor : error
      The path '../../../../../../../../../Users/rolf/test/bugs/RazorClassLibrary/_Imports.razor' would result in a file outside of the app bundle and cannot be used.


Build failed with 4 error(s) in 6.7s

Complete test project: RazorClassLibrary-9528657.zip

Binlog: msbuild.binlog.zip

The reason this happens is that:

1. The Blazor SDK adds default Content items.
2. We compute the LogicalName metadata for these Content items using the defining file's location, computing the relative path of the Content item relative to the defining file's location.
3. Since the defining file in this case is inside the Blazor SDK, the computed relative path ends up being something ridiculous like '../../../../../../../../../Users/rolf/test/bugs/RazorClassLibrary/_Imports.razor' (from above).

The fix seems trivial: compute the LogicalName metadata using the project file's location instead.

This would be a rather big breaking change however, so it shouldn't be done until .NET 11 (or alternatively as an opt-in in a .NET 10 service release, then make it opt-out in .NET 11, and potentially remove the opt-out option completely for .NET 12).

In the meantime, the workaround is to add this to the project file:

<ItemGroup>
    <Content Update="@(Content)" IsDefaultItem="true" />
</ItemGroup>

Ref: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2576178

[javiercn]

I think if there's anything we need to adjust, we will do it here

[BethMassi]

@javiercn @rolfbjarne Just want to add that this does not happen in .NET 9 when building on Windows. We will need to document this somewhere because users upgrading from .NET 9 to .NET 10 may not know what is happening. .NET 9 on MAUI will be out of support in May 2026 and in MAUI hybrid apps, it's common to reference Razor Class Libraries. Multi-targeting them allows you to provide different UI for different devices.

Also, it seems like it should be taken care of more broadly than just the MAUI webview because it happens no matter what the head project is -- it doesn't have to be a MAUI app. I repro'd it with a simple console app.

[rolfbjarne]

I'll have a look and see if I can figure out what changed.

[rolfbjarne]

I think I found a palatable workaround, it's in progress right now.