Error extracting metadata in docfx project

Question

Despite reading through the docfx documentation, and numerous Github issues, I am unable to get my docfx project working.

Steps taken:

1. Initialise root directory docfx init -q
2. Build from root directory docfx build (no errors)
3. Serve docfx "/user/docfx_project/docfx.json" --serve

Stacktrace:

Error:Error extracting metadata for /user/docfx_project/src/MyApp/MyApp.csproj,/user/docfx_project/src/MyApp.Tests/MyApp.Tests.csproj: System.NullReferenceException: Object reference not set to an instance of an object
  at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.SaveAllMembersFromCacheAsync () [0x00380] in <0779a76130d64c11baa2c8c084cbfe7c>:0 
  at Microsoft.DocAsCode.Metadata.ManagedReference.ExtractMetadataWorker.ExtractMetadataAsync () [0x000c0] in <0779a76130d64c11baa2c8c084cbfe7c>:0 

Environment:

• OSX 10.13.6
• docfx 2.56.2.0
• .NET Core SDK (3.1.401)
• Mono JIT compiler version 6.10.0.104

Project structure:

docfx-project
 |
 +-- _site
 +-- api
 +-- apidoc
 +-- articles
 +-- docfx.json
 +-- images
 +-- index.md
 +-- obj
 +-- src
     |
     +-- MyApp
         |
         +-- MyApp.csproj
 +-- toc.yml

docfx.json:

{
  "metadata": [
    {
      "src": [
        {
          "files": [
            "src/**/**.csproj"
          ]
        }
      ],
      "dest": "api",
      "disableGitFeatures": false,
      "disableDefaultFilter": false
    }
  ],
  "build": {
    "content": [
      {
        "files": [
          "api/**.yml",
          "api/index.md"
        ]
      },
      {
        "files": [
          "articles/**.md",
          "articles/**/toc.yml",
          "toc.yml",
          "*.md"
        ]
      }
    ],
    "resource": [
      {
        "files": [
          "images/**"
        ]
      }
    ],
    "overwrite": [
      {
        "files": [
          "apidoc/**.md"
        ],
        "exclude": [
          "obj/**",
          "_site/**"
        ]
      }
    ],
    "dest": "_site",
    "globalMetadataFiles": [],
    "fileMetadataFiles": [],
    "template": [
      "default"
    ],
    "postProcessors": [],
    "markdownEngineName": "markdig",
    "noLangKeyword": false,
    "keepFileLink": false,
    "cleanupCacheHistory": false,
    "disableGitFeatures": false
  }
}

Halp.

Answer 1

I have relatively little idea why this works, but what it took on my CI machine was export VSINSTALLDIR= - in other words, clearing the VSINSTALLDIR environment variable before running docfx metadata. I believe that affects the version of msbuild (or some msbuild files, at least) that docfx uses.

This is has been a fix for other issues in the past - that one claims to have been fixed a while ago, but the same workaround seems to work for this issue too... at least on my CI machine.