DotNet 6 Preview 4: CS8785: Generator 'RazorSourceGenerator' failed to generate source

Describe the bug

After upgrading to Net 6 Preview 4 from Net 5, all my projects were successfully built except for one with cshtml and razor files. It was working fine under Net 5 but fails to build under Net 6.

The only useful piece of info I could find is from the build output:

CSC : warning CS8785: Generator 'RazorSourceGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'AggregateException' with message 'One or more errors occurred. (Specified argument was out of the range of valid values. (Parameter 'index'))' 

It does not reference any concrete code file or specific error it encountered in there.

I get the same error whether I build in VS or from dotnet CLI.

Strangely, intellisense in VS does not report any errors for entirely .razor-based components. That is I can easily reference my .razor-based components in .cs code files w/o any issues. Yet, build fails saying that exactly those components / namespaces are undefined (presumably because during build generator fails to generate cs code for those, while it works during design/editing in VS).

Also, in VS there is no output of Microsoft.NET.Sdk.Razor.SourceGenerators: no .cs files are visible under that node.

Exceptions (if any)

CSC : warning CS8785: Generator 'RazorSourceGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'AggregateException' with message 'One or more errors occurred. (Specified argument was out of the range of valid values. (Parameter 'index'))' 

Further technical details

• ASP.NET Core version:
• Include the output of dotnet --info

.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.4.21255.9
 Commit:    950e4949a7

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17134
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100-preview.4.21255.9\

Host (useful for support):
  Version: 6.0.0-preview.4.21253.7
  Commit:  bfd6048a60

.NET SDKs installed:
  2.1.520 [C:\Program Files\dotnet\sdk]
  5.0.300-preview.21258.4 [C:\Program Files\dotnet\sdk]
  6.0.100-preview.4.21255.9 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.0-preview.4.21253.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.24 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.28 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.0-preview.4.21253.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.15 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.0-preview.4.21254.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

• The IDE (VS / VS Code/ VS4Mac) you’re running on, and its version: VS 2019 16.11 Preview 1

[GordonRudman]

Please Please Please fix this as soon as possible.

Over the last few months I’ve wasted many DAYS already because of this issue (since preview 3 launched) - Basically every time I try update VS2019 or .net 6. I’m currently stuck on VS2019 v16.9.7 and .net 6 preview 2 specifically because of this issue, because all the newer versions of .net 6 and VS2019 are incompatible with Blazor (even using the standard templates inside VS0219).

Please also make the error messages more useful - they are completely useless and actually counter productive at the moment. Please also make it throw a proper error instead of warnings - The current behaviour is dangerous! I’m really hard pressed to think of examples where I want a warning instead of an error message when the code I’m expecting to get generated doesn’t. Because the code doesn’t get generated it just leads to confusing errors further down the chain.

This is just one of the many warnings I’m getting because of the RazorSourceGenerator: CSC : warning CS8785: Generator ‘RazorSourceGenerator’ failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type ‘InvalidOperationException’ with message 'TargetPath is not specified for additional file ‘C:!\RazorGeneratorErrors1a\BlazorApp1\App.razor.’

The following repo has a simple Blazor server website in it. It works perfectly in VS2019 v16.9.7 .Net 6 Preview 2, but doesn’t even compile in any of the newer versions of VS2019 with .net 6 (preview 2, 3, or 4). It does however unfortunately compile and get deployed when using the CLI tools (as they treat the problems as warnings instead), but results in a broken website being deployed 😦 and then even more cryptic error messages to wade through on Azure: https://github.com/GordonRudman/RazorGeneratorErrors1

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